Technical Report 540

SOURCEBOOK OF TEMPORAL FACTORS AFFECTING

o INFORMATION TRANSFER FROM VISUAL DISPLAYS

R. Sekuler and P. D. Tynan 1

Northiwestern University

2.

•: • ~ ~~R. S. Kennedy L :3

Canyon Research Group 3 -

SYSTEMS RESEARCH LABORATORY

U.S. Army

Research Institute for the Behavioral and Social Sciences

June 1981

Approved for oublic release; distrioution unlimited.

S2 Q56 .
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U. S. ARMY RESEARCH INSTITUTE

FOR THE BEHAVIORAL AND SOCIAL SCIENCES

A Field Operating Agency under the Jurisdiction of the

Deputy Chief of Staff for Personnel

L. NEALE COSBY

I JOSEPH ZEIDNER Colonel, IN

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i• to the Department of the Army

I• Northwestern University

Vi

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Technical Report 540 /) 1 1 22L... 4. TITLE (and Subt~tle) S. TYPE OF REPORT & PERIOD COVERED

SOURCEBOOK OF TEMPORAL FACTORS AFFECTING Final

INFORMATION TRANSFER FROM VISUAL DISPLAYS

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R. Sekuler & P. D. Tynan N61756-76-M-5961

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Northwestern Psychology Department University Canyon Westlake Research Village, Group CA 2Q162722A777

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Institute with the U.S. Naval Pacific Missile Test Center, Point Mugu,

California and U.S. Naval Biodynamics Laboratory, New Orleans, Louisiana.

I1. KEY WORDS (Continue an reveree side If necessary md identify by block mnmber)

Temporal factors Vision Illusions

Human Engineering Design Criteria Vection Dynamic Visual

Visual Displays Motion Perception Acuity

Military Standards Flicker Display Design

Contract Sensitivity Brightness Enhanrmint ,patjo Temnnr-1

AMrACr AC1- .,.rerse eb N esiy -- Identify by block ntombe,) Interactions

This report collects in one document the important research literature on

temporal factors in vision. Over 350 scientific articles are cited herein and

this regporesents approximately 10 percent of the data base which was consulted.

The liter-ature searched was comprised of the following: 1) several thousand

articles (under the general rubric temporal factors and infnrmation processing)

from existing reprint files of the authors and others; 2 Ergonomics Abstracts. --

DD I , 3 EDWTooI ,ov ol is IsoSETE Unclassified

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Unclassified SECURITY CLASSIPICATION OF THIS PAGE(Wh, Data h•,nto) .. 0. (conttnueo)

Psychological Bulletins, Psychological Reviews and Human Factors for the last 12

years;,j. a listing from two automated look-up syste-m-s (Psychological Abstracts

1967-present and National Technical Information System 1964-present). An

integrative review of the literature is provided and three chapters are included

which deal with application of these findings to display design. The subject

matter is preception of temporal events--specifically motion perception (real

and apparent) and flicker/flash sensitivity. A small chapter covers some

temporally based phenomena which distort or degrade perception. Features of

these phenomena may be observed in visual displays. Only studies which report

findings which are robust enough to be expected to be important outside the

laboratory are included. Where suggicient data were available, equations are

provided to the engineer for the calculation of design criteria (e.g., periphera"

motion threshold, contrast thr•sholds, contrast thresholds and age, etc.).

Where gaps exist in our scientific knowledge, recommendations are provided for

applied research. General guidelines are offered for incorporating design

criteria into Military Standard 1472 for perceptions due to temporal events.

iiUnclassified

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I

Tochnical Report 540

SOURCEBOOK OF TEMPORAL FACTORS AFFECTING

A INFORMATION TRANSFER FROM VISUAL DISPLAYS

R. Sekuler and P. D. Tynar,

Accession'For Northwestern University

NTSR. S. Kennedy4 IIC TA" Canyon Rescarch Group

SBY-••-• tr Submitted by:

Av~U~ ~ codes Milton S. Katz, Chief Av- .,-, t:/or TRAINING TECHNICAL AREA

____ -Approved by:

Edgar M. Johnson, Director

SYSTEMS RESEARCH LABORATORY

.• U.S. ARMY RESEARCH INSTITUTE FOR THE BEHAVIORAL AND SOCIAL SCIENCES

5001 Eisenhower Avenue, Alexandria, Virginia 22333

Office, Deputy Chief of Staff for Personnel

Department of the Army

i I U.S. NAVAL PACIFIC MISSILE TEST CENTER

* i Point Mugu, California

U.S.NAVAL BIODYNAMICS LABORATORY

New Orleans, Louisiana

June 1981

Army Project Number Individual Training Technology

20162722A777

Approved for public .•uis, distribution unlimited.

A iiM
ARI Research Reports and .Technical Reports are intended for sponsors of

R&D tasks and for other research and military agencies. Any findings ready

for implementation at the time of publication are presented in the last part

of the Brief. Upon completion of a major phase of the task, formal recommendations for official action normally are conveyod to appropriate military

agencies by briofing or Disposition Form.

I *i . i

t i

LI

ra

i lv

I
FOREWORD

This Technical Report, "Sourcebook of Temporal Factors

Affecting Information Transfer from Visual Displays," is the

result of a cumulative eftort made between Naval Air Systems

Commands NoL•hwestern University and the Army Research

Institute for the Behavioral and Social Sciences (ARI).

In this technic.A report, the basic and applied science

literature are surveyed for data to aid human factors engineers in understanding temporal events and how these events

may affect the visual response to dl.splayed information.

Through the main efforts of Dr. Robert Sekuler and Dr.

Paul Tynan from Northwestern University, IL, this research

was completed under Army Project Number 2Q162722A777.

(JO EPH Z ID~ R

Tehnicole rector

V
Authors' Preface

When one compares vision and audition as input pathways for pro- cessing information one is impressed with the spatial sensitivity of

the eya and with the temporal sensitivity of the ear. In military

workplaces vision is the predominant display mode. Thus, it is not

L surprising that the framers of Military Standard 1472, (including A and

B versions) emphasized this fact by concentrating almost exciusively on

the spatial coding mechanisms of the visual system (size, location,

contrast, etc.) and ignored temporal cues (E.g., motion). Indeed,

except for ". .. flashing lights.. ."virtually ino mention is made in

Military Standard 1472 of how one could code information temporally in

order to improve the extraction of information from displays. However,

starting with Bcynton's (1961) seminal paper calling attention to the

temporal capacities of the visual system, a large literature has evolved. Prior to that time little work was performed except for

studies of motion perception, flicker, and curiosities in vi.ion (e.g., Mach bands, brightness enhancement, Crawford effect, etc.) Since then

symposia have been held and books of readings have been compiled.

Thus, we believed that the time was right to acquire in one place all

the research data associated with temporal factors in vision, particularly with reference to how these data impact on display design.

However, the task was obviously too ambitious because, as we progressed, more and more phenomena were discovered which qualified for 17

the denomination "temporal factors" in vision. Therefore, it became

necessary to delimit what we meant by "temporal factors". We have

chosen to report only on motion perception, flicker, flash acuity, and

a rept'esentative collection of phenomena that distort perception. An

attempt was maae in the report to avoid studies that were more closely

related to throughput than to input, but this approach has resulted in

omissions. We feel that the literature is sufficiently large and

growing fast enough so that a sequel could update the existing area,

and add a few items. Specifically, the distortion chapter could be

amplified to include all "factors which inhibit seeing" (under which

Mach bands, masking, metacontrast, and the Crawford effect might fall).

Issues such as "sensory and environmental interactions", "eye movement

versus image movement", "simultaneity and numerosity", are also candidates for a future revision. Additionally, specific subtopics could

most likely include reports concerning effects nemed for: Pulfrich, Parks, Troxler, Bezold-BrUcke, Sherrington, and others. Plus, s5ccadic

and blink suppression, binocular rivalry, alphanumeric recognition,

stabilized retinal images, phantom images, vection, and directional

specificity in scanning, could be included. Moreover, future documentaries should attempt to divide the literature into the separate and

critical issues of enhancement (or degradation) criteria for training equipment versus design criteria for equipment which is to be ope:-ated

by skilled persons. (Only the latter is implied in Military Standard

1472B.)

The present sourcebook is a self-conscious attempt to address the

difficult issue of Technology Transfer and is a first sLep in the

temporal factors content area. To the extent that it is found useful

to the design engineer, it will have fulfilled its purpose. To the

vii

PA pig. =~wN 

u
extent that it does not, it s 'hould be improved in future revisions.

The best places tot the design engineer to look are in the first three

chapters, "Applications and Recommendations for Applied Research",

"Extrapolations from Laboratory Data", and "Toward a Military Standard."

vii
q¶

TABLE OF CONTENTS

Chapter Title 1a2e

Introduction 1

1. Applications and Recommendations for Applied Research 4

2 Extrapolations from Laboratory Data 10

3 Toward a Military Standard 16

4 The Absolute Threshold for Motion Perception 18 •I5 Suprathreshold Motion Perception '13

6 Illusions of Motion 30

Motion Aftereffect 30 ilApparent Motion 33

Induced Motion 38

7 Motion Perception in the Periphery 43

8 Flash Sensitivity 56

9 Flicker Sensitivity 67

10 Suprathreshold Flicker Perception 78

11 Brightness Enhancement 90

12 Effects of Flash and Flicker on Visibility 96

13 Effects of Movement on Visibility 105

Dynamic Visual Acuity 105

Non-Acuity Measures 111 I

Research in Applied Settings 112

14 Temporal Events Causing Distortion 125

Glossary 142

Appendix A 145

Appendix B 146

Appendix C 147

Bibliography 148

Author Index 171

Subject Index 175

ix
PIT I I I______ . -

Introduction

In this report the basic and applielJ science literatures are surveyed for data to aid human factors engineers in understanding temporal

events and how temporal events may affect the visual response to displayed information. By this means, we hope to provide informaktion

which will aid design engineers in improving display design. To our

knowledge there has been no attempt to review these literatures on a

broad enough scale to produce a docdment thit would be generally useful

to anyone involved in display design. However, the topic of "temporal

factors" is sufficiently broad in scope that one document can hardly

provide complete, encyclopedic coverage.. We have therefore concentrated on factors that we felt may impact on the design of visual . L displays with the hope that in many cases a human factors engineer will

be able to use the information presented in this text directly to guide

display design. In other cases, the information may not be presented

in sufficient detail, and either original sources will have to be

consulted by the engineer and/or integration may be required.

The usefulness of this report depends on the task that the designer faces. In many cases, the information he needs is simply not

available. The reason for this is two-fold. First, most scientists

engaged in "pure" research are primarily concerned with testing theories about how the Visulal system interacts with a set of display variables. Most researchers are more interested in the changes in observers responses with changes in display parameters than in the

quantitative description of the responses themselves. The researcher

is often not interested in the variation of the response from observer

to observer. The second part of the problem arises because applied

researchers generally focus only on rather specific, applied problems.

For instance, a researcher interested in the perception of television

systems may only use displays of about the same size, luminance, number

I- of lines on the raster, and flicker rate as are possible with a particular television system. Thus, the research would not be very useful to

a designer trying to optimize the visibility of warning beacons, or

even other television systems.

This report combines many data sources in an effort to develop

general rules of perception that may be useful to human factors engineers. To accomplish this, only certain types of articles were considered. If a paper did not directly study the temporal variable or

its effect, but only used temporal variation to examine some other dimension of perception, the paper was not used as a reference. For

example, iri many studies the stimulus is presented for a brief duration

Lonly to p luce errors in identifying the stimulus. The error lev"!

can then be che dependent variable used to measure some other variabi:,,

such as a drug effect. Some effects have inspired so great a volume of

literature that not even a majority can be directly included in this

survey. Ari example of this is the motion aftereffect. In such cases,

only those articles which provide a good description of the basic

phenomenon, or may be directly of interest to the human facto,,-s engineer, have been included.
In order to be included in this report, a scientific finding hadI

to satisfy three criteria. First, the effect had to be "robust", that

is, strong enough that it would be noticed in an industrial or military setting. If an effect could only be teased out statistically under the

best of laboratory conditions, it was not included. Second, the effect

had to be observable in "normal" (non-pathological) conditions. Finally, we insisted that the stimulus conditions producing the effect not

be so exotic that they would never be seen outside the laboratory.

These criteria were selected to maximize the report's interest to the

human factors engineer.

Because it is hoped that this book will be used by design engineers seeking solutions to problems, the solution section appears

first. However, students will probably prefer to read the literature

review sections first. The report consists of three basic parts: The

first part is on the application of data to p-actical problems. The

second part is on the perception of tem~poral events. t'alf of this part

is concerned with the perception of motion, the other half with the

perception of flashes and flickering. Th'% third part includes the

effects of temporal events on distortions, illusions, anid degradations

of displays. The relative size of each part reflects the amount of

information available for each. Most of the report is concerned with

the second part, simply because so much work has been done in these

areas that is important in display design. The first part, on applications, was the least adequately treatet' in the literature. These

three parts of the report are described in more detail as follows:

1) Application. Articles in this part show the application~ of

temporal effects information to industrial a3nd military problems,

whether to avoid distortions in a display, to -ýmrrove visibility and

detection, or to code information. Examples include temporal cues for

radar displays, flashing indicators of all kinds, the perception of CRT

displays, and the design of aviation instruments. In addition, thfý'e

is a summary of important instances which have been mentioned in the

other two sections. Also included is a summary of the important "bare

r spots" in the literature and comments on what additional research would

be most useful for the design engineer. The authors also suggest

changes in the section of Military Standard 1472 regarding the uses of

visual indicators, as well as speculate upon the possibility of using

temporal events as a medium for conveying information to the operator.

k2) The perception of temporal events. One goal of this report is [ to suggest ways temporal events -could be used to convey information

from machine to man. The first step is to understand the factors

influencing the perception of temporal events themselves. Articles in

this category delineate the conditions under which motion, flicker and

other transients are perceived and the discriminability of changes

along these dimensions. Examples of variables included in this category are the spatial characteristics of the stimulus, eye movem'ents,

adaptation, and factors influencing the after effects to temporal

stimuli.

3) Distoytions, illusions, enhancement and degradation due to

temporal events. If a temporal event causes a misperception o., illu2
slon in some stimulus dimension, other than its clarity or visibility,

that effect is included in this category. Hue shifts, spat-ial distortions such as changes in perceived curvature, line length, and spatial

frequency are examples. Also included are the effects of temporal

events on the clarity, visibility and recognizability of a stimulus.

Examples include dynamic visual acuity, brightness enhancement, masking, and sequential blanking effects.

i3
CHAPTER 1

r Applications and Recommendations for Applied Research

In each chapter of this report, comments will be made on the

display application of specific perceptual phenomena, when it seems

appropriate. An attempt will be made to integrate some of these suggestions into more general recommendations on how the interface between

man and machine may be improved and expanded. Because of data gaps in

both the pure and applied literatures, some of these recommendations

are admittedly speculative. The authors have enough confidence in

* other recommendations, however, to suggest changes in the Military

Standard. These will be reiterated at the end of this section. In

U addition to the summary and analysis of the review, a chapter on how

laboratory data may generally be interpreted for useinapedst

This review covers the basic phenomena and psychophysical data on

moinperception (absolute notion threshold, differential motion

thresholds, perceived velocity, etc.). Perhaps the most important

parmeerfor the human factors engineer to consider when applying

these data is the amount of time the operator will spend looking at any

one display element. ntne h ecpino no otion as measured by absolute motion thresholds follows different empirical rules

depending on whether the exposure duration of the display is greater

than or less than 1 sec ("inferred" and "directly sensed" motion).

Ford, White and Lichtenstein (1954) and White and Ford (1960) found

that the average amount of time an operator observes any particular

area of a display is about .25 to .33 sec. This puts absolute motion

thresholds in the "directly sensed" range, where reference lines have

little effect and motion thresholds are greatly dependent on the lumiinance of the target. If the human fý.ctors engineer demands the utmost

performance from the operator on this task, and thus the advantages of

"inferred" motion perception, he would have to rearrange his entire

display configuration so that the operator would not have to constantly

take his eyes off the motion display to monitor other instruments.

1hese changes in fixation are c~alled "switching" by Valerie (1960), who

felt that it hindered performance on certain types of delicate tracking

tasks. His solution was to provide extra information in the peripheral

field of vision coded by the luminance of LED's. Indeed, this approach

reduced "switching" and improved tracking performance.

Another problem -is taking too short a "glance" at a moving target.

r As shown by Runeson (1974, 1975) and others, short exposure durations

lead to various misperceptions in perceived velocity. In Runeson' s

case, the perceived velocity of an object was overestimated, reducing

* an observer's ability to predict the collision of two moving objects.

An even more severe problem is one of providing information about

the acceleration of some element in the observer's environment. While

an observer can detect as little as a 14% change in the velocity of an

object if this change occurs sudel, with a gradual acceleration, an

object must change position b0efore motion is detectable.

4
The solution to this problem may require re-thinking how an instrument should code the state of the world. When considering television or other CRT systems, for instance, one usually thinks that the

more resolution, either spatial (number of raster lines), or temporal (frames per second), the better. The problem of representing acceleration, however, may require a reduction in temporal resolution, In this

case, acceleration may be easier to detect if it is quantized in

units of approximately .5 sec. The target would move at a constant

velocity for the next .5 sec. Therefore, if each jump were greater than

14%, acceleration would be detected. On the other hand, if presented

more faithfully (updated many times per second), motion may not be

detected. Schmerler (1976) found that acceleration detection improved

when he "blanked" the target for part of its path. However, quantizing

the motion may be preferable to intermittent presen-tation of the

target, because flashing the target would degrade visual prediction of

collision and other psycho-motor tasks. Of course, whether quantizing motion produces similar problems remains to be investigated.

The classic motion "illusions" are reviewed for the clues as to

how they might help or hinder the man-machine interface. For instance,

I, the motion aftereffect (MAE) could produce a problem in displays like Kone of the aviation displays described by Kitchel and Jenney (1968) as

shown in Figure 1. Part of this display is a moving dashed line;

upward movement of the line indicates that airspeed is less than that

commanded by the pilot, while downward movement indicates that airspeed

is greater than desired. The "ideal" state is a stationary line. If

air speed were consistently less than that commanded (upward moving)

then, when airspeed is finally correct, and the line is stationary, the

motion aftereffect may actually make it appear to move downward, creating the false impression that aircraft had "overshot" the target speed. A slight change in fixation could eliminate the illusion, or

the MAE might be eliminated entirely by adjusting the line's motion.

Another chapter section of the report deals with apparent motion

itself (i.e., the movement of one object, between two positions). The

most important finding of our review is that the data of the "classic"

apparent motion literature does not allow one to predict the perception of other apparent motion displays. Notably, these other displays involve: (1) the motion of a cluster of many objects between two posi- tions, like the simultaneous motion of a swarm of insects, and (2) perhaps more importantly, the movement of one object across many posi- I.. tions, as in the simulation of continuous motion by motion pictures, television, and computer driven displays. Unfortunately, the informa- tion about 'the latter ýituation is sparse and contradictory at present.

Yet, precisely this sort of information will be invaluable to future

disp~lay designers.

adTo summarize and perhaps oversimplify the literature on induced

adrelative motion: (1) when judging the movement of objects, it is

important to have a large stable frame of reference; and (2) when more

than one object moves, the objects may interact if they are close to

one another. The display in Figure 1, for instance, has many other

contours besides the moving dashed i ne. However, the addition

of other moving'dashed lines (perhaps to indicate relative airspeed of

5
I.

• AIRSPEED INDICATOR

I

Figure 1. CRT aviation display (from Kitchel & Jenney, 1968)

6
enemy aircraft) would not be advisable; one of the lines might induce

movement into its otherwise stationary mate.

One chapter of this report is devoted entirely to peripheral

motion perception, and in the chapters on flash and flicker perception,

* information on perception in the peripheral field of vision was included whenever possible. We have already alluded to one reason for

using this part of the visual field: to reduce the number of separate

fixations the operator must make in the course of monitoring his environment. Reducing the number of fixations also improves performance

when the operator must search for a target embedded in a complex pattern such as the noise on a radar screen, or in a pictorial representation of terrain, as might be the case when using a television monitoring system.

When compared on most tasks the peripheral field of vision seems

inferior to central vision. One exception to this is the detection of

temporal events. In fact, at low and moderate levels of luminance, the i.~1 periphery is sometimes more sensitive than the fovea (for instance

* Welde and Cream's (1972) observation that television monitors may

flicker when viewed peripherally). Even at high levels of luminance,

the observer is able to respond to fast moving peripheral objects as

quickly as to those in central vision. In one way, however, the periphery is too sensitive to movement. Movement of even one object over

an extended distance may cause linearvection, an illusion in which the

observer feels that he and his immediate environment are moving. Such

an illusion could cause considerable disorientation to the pilot of an

aircraft who must rely on visual sensations to assist in flying the

plane. ýor this reason, it might be preferable to include informationI

to the periphery via flashing lights, or spatially localized motion

such as the rotation of a small contour.

The chapters on flash and flicker sensitivity, provide guidelines

on how to decide if a flashing or flickering signal will be detected.

tectioni of a second flash at certain inter-flash intervals, that is, a

double flash could be less effective than two widely spaced single

flashes, and (2) flicker is often avoided in display systems by modulating the display at rates above CFF, yet amplitude or modulation of I. I frequencies up to 1000 Hz can produce "sideband" frequencies that fall

in the range of visual sensitivity, and cause transient flashes or

flickering. These could be most distracting and confusing when produced in a system that isn't supposed to flash or flicker.

This report comments on the use of flashes and flickering asa

coding mechanism -a way to provide information to the operator of a

machine. Kitchel and Jenney (1968) have reviewed the applied work on

using flicker as a coding mechanism and conclude that the medium is

only suitable as an attention-getting device, or for simple types of

loading (for successful applications of these principles, see Goldstein

& Lamb, 1967; Smith & Goodwin 1971). There are many reasons to agree

with Kitchel and Jenney. Flash durations seem identical when less than

70 msec, and this range probably is greater at lower levels of luminance. Also there is confusion in the literature on perceived "numerA 7
osity" when compared with --ervations on "perceived rate". More

information is needed to determine exactly what the observer is responding to under these different tasks.

Studies by Mowbray and Gebhard (1955, 1960) show that the visual

system is acutely sensitive to differences in the flicker rate of two

stimuli. This suggests that perhaps the key to using flicker as a

coding mechanism is to always provide at least two adjacent flickering

patches; information would then be coded in terms of the size of the

easily recognized difference between the two rates of flicker.

The literature on brightness enhancement of flicker shows that the

parameters that optimize the effect are similar to those that optimize

the threshold detection of flirker. Thus brightness enhancement is an

effect that could be taken advantage of when one is concerned with the

detaction of a flickering target. Detectability will be maximized

under conditions that produce brightness enhancement. How~ever, if the

task is not detection, but based on spatial acuity, then the conditions

that produce brightness enhancement are the worst conditions to choose.

In this case, one should be careful about following Kitchel and

Jenney's advice that flicker should be used as an attention getting

device. In fact, flickering contours might get the observer's attention, but leave him unable to interpret information represented in the

contours.

The possible degradation of visibility by flicker is strongly

dependent on the spatial frequency content of the display and the range

of spatial frequencies is most critical for the observer's task. Both

flicker and movement may enhance the visibility of low/medium frequencies, even though both types of temporal modulation degrade the visibility of very high spatial frequencies. It will be pointed out in the

chapter on "Dynamic Visual Acuity", that the observer's task is very

important. A task such as recognizing letters or symbols may require

the analysis of medium spatial frequencies. This may or may not be

affected by temporal modulation. On the other hand, a task that is

clearly an acuity task will really always suffer from temporal modulation. It is suggested that tasks could be analyzed in terms of

relevant spatial frequencies, so that the display designer would be

able to tell whether temporal modulation will help or hinder the operation.

In reviewing the literature on visual distortions, the authors

enumerated a list of phenomena to be avoided in the design of visual

displays. In addition, speculations as to how some distortions could F be put to good use are also considered.

One major class of distortions arising from the movement of a

target is the misperception of the path of a moving target. In eve'-y

case,~ the distortion is an underestimation of th 'e object's path caused

by the failure of the observer's eye to "keep-up" with the target being

pursued. As a result, a straight-line path seems shorter than it would

be, a circlular path of smaller diameter than it should, and a square

path seems "bowed" inward. One effective cure for this distortion is

to keep the observer from pursuing the target by having him fixate on

8
some stationary point on the screen. However, another kind of distortion, the Ansbacher effect, might occur under conditions of fixation,

since its cause is probably masking rather than pursuit error. However, both kinds of distortion are eliminated when the target moves in

a discrete, "jerky" type of apparent movement.

Another problem associated with pursuit eye movements is the

K multiple images that may occur with certain types of motion pictures,

computer driven CR~s, or LED displays.

Some of the distortions associated with flicker are even more

bi~zzare and inexplicable. Flicker may produce the illusion of contours

and colors in large homogeneous fields, or increase the perceived fineness of contours and reduce the leogth of lines in contoured displays.

Flicker can also produce color in a more predictable way in small

achromatic fields, or alter the perceived color of chromatic targets. Thes latereffects, however, are probably not great enough to affect

discrimination in "gross" color coding schemes, like traffic lights.

A few of the illusions we treat might find application as coding

media themselves. For instance the "phantom" gratings discussed in theI

chapter on distortion (Tynan & Sekuler, 1975a), do not seem to interfere with the visibility of real contours, and could therefore, be used

P as "overlay" information on another display readout. Also, if flickerinduced subjective colors could be produced reliably in the observer

population, the subjective colors might be used t~o convey information

on a temporal variable and do so with temporal resolution of better

than one msec.

9

Akb
CHAPTER 2

Extrapolation from Laboratory Data

Most of the perceptual data in this report were collected in

laboratories dedicated to "pure" research on visual processes; a considerable niumber were collected in laboratories interested in perception in the "real" world, especially the interfacing of ;nformation

between man and machine. Few data were collected outside of the laboratory, in the environment similar to which the application of the

data would occur. But wherever the data were collected and for whateve~r reason, they all have several things'in common: (1) many variables were intentionally excluded to make the experiment manageable,

and (2) when thresholds were measured, they almost invariably may be

based on a criterion of seeing, detecting or identifying the signal of

interest 50% of the time.

For the human factors engineer, detecting the the signal 50% of

the time may not be good enough - the observer should be detecting it

all the time, or as close to 100% as is possible. In addition. the

6k signal; this may not be true in the applied setting. Other variables

such as heat stress and oxygen deprivation are also likely to affect perceptual performance in the field.

r To produce acceptable performance levels in the field, signal

levels must be greater than those suggested by basic vision research.

Although not much work has been done concerning the method for "correcting" laboratory data for extrapolation to likely field conditions, t

enough research has been completed to provide some rough guidelines. 1

The rest of this chapter attempts to summarize these attempts to develop correction factors for field conditions.

Table 1 is from Johnston, Cole, Jacobs and Gibson (1976), and

-; summarizes correction factors collected by Taylor (1964b) for contrast

threshold of small targets. The first set of factors is called "proVbability conversion factors" and is to be used as follows. Suppose

that data have been collected for the 50% probability of seeing criterion. After determining the energy or contrast level that permits

the target to be detected 50% of the time, one merely multiplieý this

energy or contrast value by 1.91 to produce a stimulus that the obwould be visible 90% of the time. Teichner and Krebs (1972) found that

k the same correction could be made for luminance thresholds of targets

presented against a dark background. In fact, this type of correction

may be made for many types of threshold data.

But these constants are only a, portion of the total correction 1

that may be necessary to assure adequate performance by the observer in

the situation of interest. The second part of Table 1 lists factors

that correct for the observer's knowledge or certainty about some

target properties. It could be used for example, to extrapolate from

an experiment in which there is no uncertainty about where the target

10
TABLE 1

A.

Contrast Threshold Correction Factors

Probability

(Detection' Multiply laboratory

Desired Threshold Value by

0.50 1.00

0.90 1.50

0.95 1.64

0.99 1.99

B.

Corrections for Various Sources of Target Uncertainty

Factors to be applied when the observer does (+) or does

not (-) have knowledge of various target properties.

Sourze of Uncertainty

Correction Location Onset time Size Duration Factor

+ + + + 1.00

+ + + 1.40

+ + - 1.60 + - + 1.50

+ -- - 1.45

-+ + + 1.31

Note: If observer vigilance is required outside the laboratory,

multiply laboratory threshold values by 1.19

If observers outside the laboratory are naive rather than

trained, multiply laboratory threshold values by 1.90

(after Johnston et al., 1976)

I . ... • ' "... . . • - ••
would appear, to predict performance in a field situation in which the F I observer only knows the location of potential signals to within 4 deg.

~ I In order to correct for the spatial uncertainty, one should multiply

the experimentally determined contrast or target energy by 1.31. Other

multipliers are used if: (1) the observer only knows to within a few

se'!onds when the target will occur, or, (2) if several sizes may

appear but the observer does not know which, or, (3) if the target may

appear for one of several durations, and this information is unknown. Taylor warns that these factors are based on rather incomplete data and

that this is why correction factors are not available for all possible

combinations of these four variables. Unfortunately, at roresent, these

are the best available data.

LTaylor also adds a few important correction factors. "Vigilance

required" (derived from Jerison & Pickett, 1963) is a correction factor

to be applied if, in the field setting, the target will appear only

once or twice during a period of 20 min. The use of thc "practice

effects" factor (from Taylor, 1964b) depends on whether the observer is

naive or well-trained. An example from Taylor's paper (1964b) illustrates how these factors may be combined.

L At this point, it is well to give an example of how a field factor

is determined for a real case, and how it may be used to arrive at

a realistic estimate of observer performance under field conditions. Let it be assumed that an observer must confidently detect

the occurrence of a stimulus of known duration and size but of

unknown location within a circular display area with a diameter of

8 deg. The target will be present at infrequent intervals, say

once ;,,very 15 min or so; and he can be allowed to miss only 5% of

the occurrences. He is new to the task, and our problem is to

arrange the contrast of the target so that this 95% criterion will

be met. We begin by consulting the laboratory data, which t'cl) us

that, for our target size and duration and for the prevailing

adapting luminance, the required contrast for 50% correct discrimination by practiced observers in a forced-choice experiment

was found to be 0.0061. To correct, respectively, for confidence

level, unknown location, vigilance, and lack of training we multiply this contrast value by 1.64, 1.31, 1.19, and 2.00, i.e. by

5.12. The needed target contrast, therefore, is 0.031 for our

F problem.

[ More recently, Sekuler and Ball (1977) and Ball (1977) found that

uncertainty about the direction and speed of moving targets affected

both the luminance threshold for detecting the targets, end reaction

time to the same targets at suprathreshold luminances. The observer

requires twice as much luminance to attain the same detection performance when he is uncertain about which of two directions (90 deg

* apart) the target could move, than when he was certain of the direction. This condition also caused a 15% increase in reaction time to

suprathreshold targets. For a moving target of about 4 deg/sec, an 8

fold difference in speed between the two alternative speeds under

conditions of uncertainty caused a 40 msec increase in reaction time

over reacticn time when speed was known. A 16 fold difference in speed

produced a reduction in detection performance that would require a

doubling of luminance to correct.

12
Besies analmst hes facors ifinte umbr ofdificutie

peculiar to the specific man-machine system may arise. For instance,

several factors affect the functional extent of the viaual field. The

excessive gravitational forces resulting from the acceleration of

aircraft reduces a pilot's vision in the peripheral field; an increase

from 1 to 4 G's reduces the functional diameter of the visual field by

50% (Gillingham & McNaughton, 1977). Also "loading" the center of ~ V vision with a difficult task reduces the size of the functional visual

field (Webster & Haseirud, 1964; Ikeda & Takeuchi, 1975). Moreover,

the magnitude of this visual field shrinkage increases with age

(Layton, 1975). Unfortunately since clinical, perinietric measurements

of the visual field are made without concomitant stress or attention

demands on the observer, such measurements may be poorly correlated

with the size of the field actually useable by an observer in some

non-clinical setting. As a result, clinical tests of the extent of the

observer's visual field may bear little relation to an observer's

ability to use peripheral vision in some applied setting.

P The last problem to consider is variability within a population of

observers. Johnston, et al. (1976) do an excellent job of reviewing2

problems associated with distributions of both color and acuity deficiencies among observers, and how a display designer may take these into

accdunt. Table 2 shows the CIE's recommended correction factors for

acuity as a function of age. Given that a particular target is seen by

50% of 20-year-olds, contrast would have to be increased by 1.6 so

that it could be seen by 70% of 40-year-olds. Variability of visual

capabilities is much less among some populations than among others.

t For example, commercial airline pilots are pre-screened for visual

capabilities much more carefully than automobile drivers. Yet, each

I: screening procedure must allow some degree of leeway, and Figure 2

shows that even a slight visual deficiency may require a substantial

correction factor.

It is important to repeat and emphasize the fact that these correctioni factors and the rules for using them are very rough procedures

and the data are very sketchy and incomplete. The correction for

probability assumes that every procedure used in the laboratory mea- l:sures the 50% frequency of seeing, however, this is not true. The

method of adjustment, for instance~, (the observer controls the strength of the signal anid adjusts iVt until it is just barely detectable) most

certainly measures a frequency of seeing much higher than 50%. Thus, L .the procedures presented here may be overly conservative. Also,

Taylor's technique of serially multiplying correction factors is purely

arbitrary. Hall (1977) provides evidence that the effects of two

sources of uncertainty in her experiments should not be combined in

this manner. If there are few rules for applying detection data in the

field, there are none at all for reaction time. Parametric experiments

will have to be conducted to find appropriate correction factors for

both measures. Certainly, much more emphasis needs to be placed on

finding and testing rules for applying laboratory data to field situations.

13
TABLE 2

Contrast Threshold Correction Factors for Age

and Variability Between Observers

Percentage of the Population to be Included

Average

ae50 60 70 80 90 95

20 1.00 1.09 1.20 1.34 1.56 1.76

25 1.00 1.09 1.20 1.34 1.56 1.76

30 1.03 1.12 1.24 1.38 1.61 1.81

35 1.09 1.19 '131 1.46 1.70 1.92

40 1.17 1.28 1.40 1.57 1.82 2.06

45 1.33 1.45 1.60 1.78 2.08 2.34

' 50 1.58 1.72 1.90 2.12 2.46 2.78

55 1.94 2.12 2.32 2.60 3.02 3.42

LI60 2.30 2.51 2.76 3.08 -3.58 4.05

65 2.606 2.90 3.19 3.56 4.15 4.68

I (after Johnston et al. , 1976)

F1
F

MEAN VISUAL ACUITY

6/45 646 6ý9 6L12 6/I6 649

5.7 cd/m

15 -2 / 5.9 mm pupil

12

81 cd/ti 0 /2 4 o 0.54 cd/m

6.6 am pupil

107 0 0.04 cd/m2

' 10 I7.3 mmpupil

10

0'

- 68

S ---- 7o

IiI _ , i , 1I i I i i . i

0 10 20 30

OCULAR DEFOCUE

(diopters)

Figure 2. Contrast multipliers plotted against defocus for four levels

of background luminance. Visual acuity was recorded for p = 0.5, using

a Landolt C target presented at a low photopic level of illuminance

(from Johnston, et al., 1976).

15
CHAPTER 3

Toward A Military Standard

Military Standard 1472B provides guideli~nes for designing equipL ment so that military personnel can use it effectively. The motto of

the standard regarding visual displays seems to be "keep it simple",

that is, do not create a display that is any more complicated than is

necessary. Few could argue with such an approach. Yet, as military

systems become more complex, and computers are needed to integrate the

information generated by these systems, it will become increasingly

difficult for human operators to "keep up" with the actions of their

machines. The resulting visual displays may be radically different

from the concept of a panel of indicators, the concept on whf-ich much of

the Military Standard is based.

For instance, the Military Standard assumes that the operator is

looking directly at a display when extracting information from it,

except for a flashing indicator which may direct his attention to a

different display. Thus, the Standard is not much help to a display

designer interested in providing the operator with information from

several displays without necessitating his changing fixation.

Even assuming that an operator looks directly at each display, the

Standard neglects much important information on temnporal parameters.

For instance, it may be important for an operator to determine whether

the pointer on an indicator is changing position, yet the Standard

pr~ovides no criteria for rate of movement that wculd insure motion

detection. A guideline similar to Table 1 of this report should be

added. In addition, the Standard should make mention of thresholds for

detecting a, _*atga,_in the velocity of an object moving on a display.

These motion guidelinbs and others apply to CRT displays as well.

Also, information on flicker is almost completely lacking from the

Standard. Paragraph 5.2.2.1.19 suggests that flickering lights only be

used as an attention-getting device, and that a flickering indicator

Kshould only flicker at a rate between 3 and 5 Hz. The data presented

in this report, however, show that desirable flicker rate depends on

the task. If the operator is to detect the flicker, then a rate between 8 and 10 Hz would probably be more effective depending on other

conditions. If the operator must read or perform some acuity task and

the contour information is on the flickering indicator, then a much

lower, or higher rate would be prefereil.

There are also a few "don'ts" that should be considered for inclusion in the Standard. One of these concerns linearvection. This

illusion of self-movement is quite powerful and dangerous, and the

designer should be cautioned against producing motion in the operator's

peripheral field of vision. Another highly reliable effect is the

perception of flashes or transients when the flicker rate of a display,

well above CFF, is suddenly changed either in frequency or amplitude.

Such a situation is quite possible if the flicker rate of a CRT is

16
dependent on the amount of information presented, which may happen with r computer driven displays.

Many of the data reported in this report, however, are not suitable for immediate use as the basis for a Military Standard for two

reasons: (1) many of these visual phenomena have not been tested for

their applicability to field situations, and (2) many of these data are

not very relevant to the general design of displays, yet may be very

important to the designer trying to solve some specific display problem. For instance, the information on observers' ability to make fine

discriminations between two sources of flicker is only useful to someone experimenting with new design ideas. Similarly, the chapter on

distortions may be useful for a designer whose new display "looks

funny". However, it is impractical to list all these possible sources

of distortion directly in the Standard.

- 17
CHAPTER 4

The Absolute Threshold for Motion Perception

An object's "absolute motion threshold" is the minimum speed with

which an object must move in order for its motion to be detectable. [ Although the definition is straightforward, a number of subtleties

attend its measurements. The most important of these subtleties involves the allowable inspection time. For example, even glacial movement could be detected if observed over a long enough period of time.

Obviously, the width of the temporal window used to define the absolute

motion threshold is of practical interest; a pilot has only limited I time to spend observing any one object or instrument waiting to see

motion. As shall be noted, other subtleties of measurement involve

effects of the size of the test object, its luminance, and the presence

and character of possible reference marks in the visual field.

Reviewing the effect of limited observation time upon the percep- tion of motion, Bonnet (1975) supported the conclusion of earlier

investigators: there are at least two types of motion perception,

viz., "inferred" motion and "directly sensed" motion. In the first

case, the motion of an object is inferred by noting that the object's

position, sampled at two or more separate times, has changed. In the

second case, if the exposure is brief and the speed of motion rapid

enough, motion may be sensed directly in a manner analogous to sensing aflash of light. This distinction will be made from time to time

throughout this section.

First, how does one know when one is dealing with inferred motion?

If the observer has unlimited time to view the movying '6ýjct, then the

motion at threshold will be inferred. However, what is the shortest

observation time for which motion perception can be considered inferred? Leibowitz (1955b) varied the luminance and exposure duration

(1/8, 1/4, 1, 2, and 16 sec) of a moving stimulus and found that for

all, except the longest exposure (16 sec), variations in luminance had

a strong effect on motion threshold. At 16 sec, however, it had almost

no effect and the ttlreshold varle-d only slightly around a value of'0.4

min of arc/sec. Leibowitz argued that the luminance-sensitive thresholds reflected the "directly sensed" type of detection but that at long [K durations, the observer merely noted changes in the position of the stimulus with time, and as long as the object was visible, its luminance was not important. johnson and Leibowitz (1976), using many more

stimulus exposure values, concluded that the critical duration between

"sensed" and "inferred" motion was 1 sec. All durations greater than 1

sec had the same threshold for motion: 1.5 min of arc/sec. Below 1

sec, threshold velocity increased with decreased exposure duration such

that the object had to move a constant distance in order to be detected.

The great range of values in the literature for the threshold for

motion is probably in large part the result of the presence and character of reference marks. As far back as 1886, Aubert (as related in

* Graham, 1965) found a ten-fold reduction in motion threshold when

18
reference marks were added to the display. Mates (1969) found the same

results, a gradual lowering of motion threshold occurred as the number

of reference marks was increased from 0 to 16. Her target measured 7

mn x 17 mn and was located about 27 min from an edge in which reference lines were scribed. Leibowitz (1955a) examined the effect of

reference marks while varying exposure duration (1/4, 112, 1, 2, and 16

sec) and found that the marks had little effect at the short durations,

but lowered threshold at long durations. This suggests that reference

marks will only help an observer in detecting "inferred"' motion.

The exact spatial arrangement of the movement display varies from

one study to another, but Kinchia and Allan (1969), and Kinchla (1971)

suggest just how close the reference mark must be to the moving target

to facilitate detection. In their work, a reference mark (a small

spot) had little effect on motion thresholds of a small object if it

was more than 10 deg away; the mark reached near optimal effectiveness

in the 3 to 5 deg range.

Another important variable is the size of the test object. In

this case, however, the results are not as intuitively obvious as for

reference marks, since an increase in the size of an object actually

raises its motion threshold. Brown (1965a) predicted this outcome

because he had found that increasing the size of an object makes it

look slower (although it is well above mction threshold; see the

Suprathreshold Motion chapter). He correctly felt that this phenomenal

slowing would simply extend to the threshold levels of velocity and

that an increase in physical speed would be needed to offset the effect

of increased size. For a square 27.5 min on a side, motion threshold

was 6.6 min of arc/sec; for a 13.8 min square, threshold was 3.61 min

* of arc/sec, and for a 6.9 min square it was 1.89 min of arc/sec. For

every one/half reduction in the stimulus dimensions the motion threshold dropped roughly by a factor of two. Mates and Graham (1970) came

to a similar conclusion when they varied the length of a moving line.

Shortening the line (6.9 min wide) from 58.4 min to 17.2 min lowered

motion thresholds from 6.75 min of arc/sec to 2.3 min of arc/sec, a

roughly proportionate change. On the other hand, Graham (1968) found

that varying line length between 2.8 and 45 min had no effect on motion

4 ,thresholds when exposure duration was less than one second.

So, for long expcsure (greater than 1 second) durations and "inferred" motion, the basic findings are: (1) motion threshold is best

described as the minimum rate cf motion necessary for detection; (2)

as long as the moving object is visible, luminance is not important;

(3) the spatial characteristics of the stimulus (viz., reference marks

and object size) are important. None of this holds for exposure durations of less than 1 sec.

Bonnet (1975) concludes that in the exposure range of about 0.3 to

1 sec, velocity threshold is not a constant, but varies with the duration of exposure. The critical event for motion perception in this

case is the extent. of the movement. Cohen and Bonnet (1972) found that 

J

they could "trade-off" exposure and velocity over an exposure range of

50 to 700 msec and still have a stimulus near thresifold as long as it

moved through a distance of 0.8 min of arc. Johnson and Leibowitz

19
1 1 (1976) found the same relationship holding over a range of 0.1 to 1

sec, although their constant displacement was 1.5 min of arc Leibowitz'

(1955b) data suggest that this displacement value is sensitive to

luminance. It var ies roughly from 0.5 min of arc at his highest luminance of 1591 cd/rn to 5 min of arc at the lowest of .016 cd/rn , although the "constant extent rule is not as clear at high luminances.

Somewhat below 100 msec this constant distance relationship breaks

down. Henderson (1971) claims that the, distance-time relationship in

the lower range now takes the form dt =c, where d is distance, t is

time, c is a constant, and mr is an exponent that varies between 1/2 and

1 depending on background luminance. In other words, for motion to be

perceived, as exposure decreases, distance must increase. At very low

target 

(Bonnet, luminances, 1975, Henderson, 

the task 

1973). 

is really 

At 

one 

this 

of 

level, 

detecting 

a constant 

anything at 

energy

all 1

(dependent on luminance) is needed to detect the object, thus exposure is constant.

Johnson and Leibowitz (1976) found that for exposures below 100

msec, velocicy threshold is constant at 18 min of arc/sec. This means

that distance actually decreases as exposure decreases, the opposite of

Henderson's findings. The authors do not dwell on these results, nor

compare their findings to Henderson, but they do note that since their

display was a stop-go-stop type of movement (that is, the object was

clearly stationary before and after the movement), their observers saw

nothing but the sudden acceleration or jerk of the stimulus. Johnson

F and Leibowitz think that this is qualitatively different from the

experience of motion in the 0.1 to 1 sec range. In Henderson's experiment, however, the obje...t was moving when it appeared and then disappeared so the observers' tasks (at very small exposures) were simply to

detect a streak of light. These two types of displays evidently

stimulate different perceptual mechanisms.

During the discussion of "inferred" motion detection it became

clear that motion thresholds in the "directly sensed" range (less than

1 second) are not affected by either' reference marks (Leibowitz 1955a)

or the size of the object (Graham, 1968). However, it should be noted

that the latter study only looked at the effect of the length of a thin

line, not the size of square or round objects, and their longest line

l ength was stil l11ess than 1 deg.

So far we have been considering foveal vision, but since motion I- thresholds invariably increase as the object is located more and more

peripherally, the retinal location must also be taken into account.

This and other effects of eccentricity on motion perception have been

treated separately in another chapter (see Peripheral Motion Percep- tion). An important point is that reference marks are not effective in

the periphery (Tyler & Torres, 1972).

As a way of summarizing these data, we could consider the task of

a display designer or human engineering specialist trying to decide

whether an object's motion will be seen or not. Table 3 presents a

model for determining changes in motion thresholds that would accompany

changes in an object's peripheral location, size, and proximity

to reference marks. The designer would start with a rough idea of how I i 20
_, O 0

TABLE 3

Calculation of Peripheral Motion Threshold

Peripheral Foveal Target Size 12 2

Threshold = Threshold 7.5125 .45y) X2 ]

(2)

Peri Fov Target Size X d. V(l.45(sini) 2 + (cos. )2

Thresh Thresh 7.5

Equation (1) is used to estimate the threhold for motion percer ion

with a target presented at a retinal locus specified as (x,y) in

degrees visual angle.

Equation (2) is used when target position is specified in terms of

meridian, c , and distance, d, from fixation, in degrees.

Notes: In each equation, the first variable represents the foveal

threshold. If unknown, an approximation might be 5'/sec for

the motion of a small (5' to 10') target which is highly

visible on a photopic background (> .1 cd/m2 ) with exposure

durations > 1 sec. Target size should be expressed in minutes of arc. If reference lines are present, the computed

values should be reduced (see text). If exposure duration is

between 0.1 and 1.0 sec, foveal threshold should be expressed I "in terms of the minimum distance the object must travel to be

seen (1.5 min arc) rather than velocity required for detection of motion. Also, for durations in this range, the

effect of object size and reference lines are reduced.

Unfortunately, present data do not allow us to estimate by

how much. The constant 1.45 in the above equations reflects

the scale difference between horizontal and vertical axes of

McColgin's (1960) iso-response contours; the contrast 1.25 reflects the rate of eccentricity.

21
much time the human operator could devote to looking at the object, for

example, a radar operator may have several seconds. If he has a long

time, (1 second or more) and the object is small but quite visible (5

to 10 min of arc), and no reference marks are present, threshold could

be anywhere from 1 to 5 min of arc/sec. As the size of the object is

Increased, threshold should increase proportionately up to a limit of

one degree (beyond this limit data are not available). If reference

marks are added, threshold could be changed by a factor of 10, depending on the number, spacing and proximity of the points, they should be

at least within 5 deg of the object. If the object is viewed peripherally, threshold should go up by at least a factor of 10 comparing

the fovea to 80 deg in -Lhe periphery. In Table 3 is an attenuation

factor based on the formula for elipses that we have fitted to

McColgin's 

field coordinates 

(1960) data. 

in degrees 

The upper 

as parameters 

model in the 

(fovea 

table 

is 

uses 

0.0), 

the 

the 

visual 

lower

1

one the angle and distance of the object from the fovea.

Our rules cannot be as specific when the exposure duration of the

movement puts it in the "directly sensed" range. 'If exposure duration

is greater than 100 msec it is probably safe to calculate velocity

hased on the rule that the object must travel at least 1.5 min of arc

for motion to be detected. Reference marks probably will not make any I

difference, nor will the object size, at least up to 1 deg. The data

on luminance are sketchy, but from Leibowitz's (1955b) study we can

infer that this displacement value increases by a factor of 10 as the

target becomes 2 dim; most of this change occurs for target luminances

below 1.6 cd/in . It is probably safe to assume, at this time, that the

effect of peripheral location is about the same for this exposure range F as for inferred motion. Below 100 msec exposure, it is difficult to

find a simple rule which will ensure that motion will be visible,

except that below 6 msec it is very hard to produce a stimulus that

appears to be anything more than a noni-moving flashed line or streak

(Henderson, 1973).

The previous conclusions are quite tentative since they assume

that there are no unknown interactions among variables. These figures

are based on careful laboratory measurements and therefore should only

be considered the limit of the human visual system when trying to apply

them to existing situations. For instance, an observer's uncertainty

about which direction an object will move may hamper performance (cf.

chapter on Extrapolations from Laboratory Data).

22
CH.'4TER 5

Suprathreshold Motion Perception

Display-related aspects of motion perception in which both the

target and its motion are highly visible are discussed in this chapter.

Of particular interest are (1) how easily the movement's characteristics are recognized or discrimin3ted, and (2) how a target's speed

may appear to change with time and other variables (e.g., the spatial

structure of the environment). In additon, we shall discuss the relationship of these findings to actual human performance on tasks which

depend on visual information about velocity. Specifically, tracking

performance and observers' abilities to predict the future position of

a moving object will1 be discussed.

One of the most salient features of a target's motion is its

velocity. The first question certainly should be concerned with how

perceived velocity varies with physical velocity. If the velocity of

an object is doubled will it seem like the velocity is doubled? It is

well known that this is not true of many stimulus dimensions; for

instance, doubling the amount of current of an electric shock stimulus

will much more than double the sensation (Stevens, 1962). The relationship between the magnitude of the stimulus parameter and sensation

is often expressed as a power function: I - sensation = (stimulus magnitude)P

where p, is an exponent. If p=1, then the relationship between sensation and stimulus magnitude will be linear; a doubling of magnitude

would produce doubling of sensation. In the case of velocity, the

power law has been found to be stable (Kennedy, Yessinow, & Wendt,

1972; Mashour, 1969; Walker, 1975), although estimates of this exponent

vary between 0.7 - 1.0 (Rachlin, 1966). Unfortunately, these experiments have not used a large enough variety of stimulus conditions nor

enough variety of stimulus conditions to permit easy generalization to

arbitrarily chosen target sizes, contrasts, luminances, etc. But it

can be assumed as a first approximation, non-linearities are minimal in

the relationship between perceived and physical velocity.

The next question is, "How sensitive is an observer to differences

in velocity?" The spatial relationship between two moving objects

which are to be compared is one impoftant determinant of velocity

discrimination. Brown (1961a) in his review of nine papers on velocity

fisrstmisathen "djacet"inguse which thre staretmoves cofogratwilns aThoe

kdirscrimintion "discet"ingui hesmon thetrgee stmulus confgratwilatons. Te

velocity, then suddenly changes to another. The second is the "separate". in which' the targets whose velocities are to be compared are

separated in time and/or space. For instance, an observer may see the

motion of an object on one screen and then look to another screen for

the comparison stimulus; or, if the targets appear on the same screen,

they appear in succession with an appreciable interval between the two.

In the third, "superimposed", configuration the two stimuli move

through the samc space at the same time. This allows one stimulus to

23
actually "gain on" and pass the other. This is also called a motion

parallax display, because it produces a monocular cue for depth (if the

two objects are actually moving at the same speed but one were further

from the observer, it would appear to move more slowly).

Comparing data from the three configurations, Brown (1961a) found

the smallest velocity difference threshold in the superimposed configuration. This is probably because Vernier acuity cues are confounded

with the difference in velocity. With even a small difference in

velocity between two superimposed points, the gap between them will

quickly change and be detected. Next, Brown found that the "separate"

configuration permitted smaller differences to be detected than did the

configuration with "adjacent" stimuli. The reason for Brown's finding

*is not clear and is not in agreement with findings on acceleration

detection which will be reviewed. In fact, Brown apparently did not

recognize that the "separation" condition would probably introduce a

range of possible velocity discrimination performances as a function of the size of the temporal and spaticO separations between stimuli.

This report will consider onlylthe last two configuratio:,s, since

the velocity difference threshold found with the superimposed configuration may depend more on acuity than \on temporal sensitivity per se.

The practical question is, "How doe'\ minimal velocity difference

change, vary with the velocities of the objects?" For instance, could

the "difference threshold" be proportional to velocity (Weber's law)?

Brown concluded that Weber's law roughly holds over a limited range for

test stimuli presented in either "adjacent" •d "separate" configura- tions. For adjacent stimuli, 8 = .138, whvre Aw is the change

needed to detect a difference in velocities a tspeed w. Stated

differently, for a given velocity, w times .138 is the just discriminably different velocity. In the "separate" configur ion, aw = .0769.

The first equation holds over a range of 1 to 200/se For the "separate" configuration, Weber's law only holds well in the range 1 -

10 0 /sec. Brown (1961b) found that Weber's law also held for field

judgments of the speed of planes, he obtained a similar ra io of .085. r Note that Brown's "adjacent" configuration actually allows tle observer

to respond to stimulus acceleration since, at some point along its

movement, the stimulus undergoes instantaneous acceleration.

Other forms of acceleration have also been investigated For

instance, Gottsdanker (1961) noted that one could not predict di~ferS.aoce thresholds for steadily accelerating objects from diffetnce

thresholds measured with iistantaneously accelerating objects (such as

those in Brown's "adjacent" configuration). Rather than the approximately 14% difference needed for instantaneous acceleration, more than

a 200% change is necessary to discriminate a positively accelerating

object from one of constant velocity; about half as much change is ineeded to recognize a negatively accelerating object. Gottsdanker concluded that steady acceleration was inferred rather than perceived

instantly: observers compared the velocity at the beginning of the

movement with velocity at the end. This relationship held for mean

velocities ranging from .96 to 7.7 deg/sec. Exposure duration ranged

from .45 to 3.6 sec. Schmerler (1976) came to the same conclusion from

a very similar experiment.

2424
What is the appearance of a moving object that does not move at a

acceleration constant speed but accelerates or decelerates? Since the difference threshold for acceleration is so high, one would think that

many accelerating or decelerating objects would appear to be moving at

a constant velocity. Indeed, Gottsdanker, in his 1956 review, notes

that observers almost always underestimate the speed of accelerating

objects and overestimate the speed of decelerating ones. They have

difficulty noting that the object is starting to move faster or slower,

i.e., their perceptions do not keep up with the change in the stimulus.

Schmerler (1976) offers additional demonstrations of this hysteresis in

apparent velocity. Johansson (1950) also reports that sinuosidal

movement appears to most observers to be of constant velocity. In

other words, if the position of an object varies as a sinusoidal function of time, its velocity would illusorily appear constant.

The difficulty of discriminating between an accelerating motion

and one of constant velocity can be eased by enriching the spatial

structure of the field through which the object moves. Gottsdanker

(1962) found a 10% increase in the apprehension of a target's acceleration when various reference marks were added to the stimulus field.

Interestingly, it made little difference whether the reference mirk was

a single spot or a rather complex texture. Any single stationary

object seemed to suffice. Similarily, Schmerler found that the detection of acceleration was improved by about 50% by "blanking" the object

for some portion of the middle of its run (in this case the object

entered and left a tunnel). An important question at this point is,

"Does constant velocity movement continue to look constant if viewed

over an extended period?" Runeson (1974) demonstrated that it does

not. Figure 3 has been reproduced from this study. Runeson asked his S~observers to draw graphs depicting the perceived change in velocity of

the moving objects. Stimulus P1 was of constant velocity (dotted

line), yet it seemed to slow down considerably in the first 1 to 2

seconds. Runeson concludes that the stimulus which produces the sensation of constant velocity is one that accelerates quickly in the first

few seconds of presentation and then asympotes to a constant velocity

(see FA 35 of Figure 3). These results agree with other measures by

Runeson in which observers had to select the constant velocity stimulus

from various pairs of these moving stimuli.

In addition to this immediate change in perceived speed, several

investigators have noted a longer term but very substantial change in

the perceived speed of a moving object with time. Goldstein (1957)

used a matching technique to measure the perceived speed of a moving

grating (train of bars moving perpendicular to their orientation) at

various times during the observer's inspection of it. He found no

change up to 8 sec; but between 8 and 30 sec, the perceived speed of

the grating dropped to 30%, beyond 30 sec there was little additional

change. Using a similar technique, Thompson (1976) confirmed this

finding although he did not measure speed until 15 sec of inspection

had elapsed. Incidentally, Goldstein's data for 0 to 8 sec fail to

show the short term effect reported by Runeson. We suspect that

Goldstein's technique of measuring apparent speed (observers moved a

hand-held stylus at a speed which supposedly matched the apparent speed

of the object) would not be sensitive to such a transient effect.

25
P 0.8 F.A 15

P1Al

Al

FA 100 P3

IDq Ait

-------------

FA 35Motion direction

O's drawing

-- -- *.- -Physical velocity

IV

Figure 3. The graphs from one observer (AL) at 20*/sec average velocity. Horizontal axes represent position along the track (s) and vertical axes represent velocity (v). The corresponding physical motion

functions drawn with v as a function of s have been added for compari-

* Ison. Function names and results of cla~ssification are given In each

graph. The graphs are typical, except for the very unusual stepwlise

increase at FA 15. (From Runeson, 1974).I

26
This report has tried to describe the general response of the

human visual system to suprathreshold moving stimuli without considering the complications of variation in spatial configuration, luminance,

or contrast of the moving objects. Since these parameters influence

perceived speed, and the choice of the parameters' values could account

for differences among studies, these influences must bez reviewed before

proceeding to relate psychophysical data on movement to human performance in visio-motor tasks.

Results on the effect of spatial parameters on the perceived speed

of objects have been conflicting. Walker (1975) measured the perceived

speed of rotating disks using a matching technique, and found that an

increase of the coarseness of the pattern on disks (increasing the size

of the dots and their spacing) produced an increase in perceived speed. Atenfold magnification of the pattern produced a 25% increase in perceived speed. Diener, Wist, Dichgans, and Brandt (1976) found the

opposite effect for grating stimuli; decreasing the spatial frequency,

(making the bars wider and spaced further apart) decreased their perceived velocity. He also found that for a single moving bar, a wider

bar seemed to move more slowly than a narrow one. These effects were,

like Walker's, on the order of 25%. Although Diener et al.(1976) used

a much larger display, (160 deg with 6 deg wide bars) their results

agree at least qualitatively with those of Brown (1965a, 1965b) whose

entire display of moving bars measured only about 7 deg across. There

are so many differences between Walker's display and Diener's or

Brown's (e.g., rotary versus linear motion; random dots versus bars;

absence or presence of stationary contours) that it is difficult to

speculate on what might cause these discrepant findings.

The effects of target contrast and luminance are also worth mentioning although their importance is just being recognized. Thompson

(1976) has shown that the contrast of a moving grating dramatically

alters its perceived velocity. The variations in apparent speed with

contrast changes ranged from the marginal, to as large as 40%. For

a two cycle/deg grating moving at four deg/sec, a reduction in

contrast reduced its apparent speed; but when the same grating moved at

speeds faster than four deg/sec, reduced contrast produced the opposite

effect: increased speed. Reducing the luminance also may make an

object appear to move more slowly (Brown, 1965a, 196rb), but no quantitative work has yet been done on this effect.

So far, the basics of suprathreshold motion perception, the ability of observers to discriminate among different speeds. and the appearance of the speed of moving objects have been considered. But the

design engineer will also want to know how these perceptual processes

would affect responses to motion information. For example, how would

the perception of motion determine tracking behavior? How would it

affect an observer's ability to predict the future position of a moving

object and take appropriate action? Obviously, these behaviors are

affected not only by motion perception but by memory and motor control

as well. In this report we will only consider how the motion phenomena, already discussed, could affect these two performance measures.

27
Brown (1961a, 1961b) devotes a large part of his review of the

literature on the velocity difference threshold to tracking and pre- itdictive behavior. His conclusion is that, in general, the two classeF

of measures agree. For instance, difference motion thresholds for

tracking errors increase linearly with the average speed of a moving

target. In addition, the average error in a tracking task is about the

same as for the difference velocity threshold (14%). In a motion

prediction task, Kimball (1970), Ellingstad and Heimstra (1969), and

Gerhard (1959) found a linear relationship between mean velocity and

random error. In the first two papers, the task was as follows: the

observer saw an object moving over a limited path; for part of the path

the object was occluded. The observer's task was -to guess the moment

at which the occluded object would reach some prespecified point.

In Gerhard's study the task was similar except that the object, a

vertically moving point, intersected a horizontally moving one. The

observer tried to adjust the speed of the horizontal point so that the

two points would intersect. All three studies found a linear relationship between speed and error, although error was minimized in Gerhard's

study because the observer could continuously adjust the speed of the

horizontal point. Kimball, and Ellingstad and Heimstra both found

standard deviations of about 3 deg/sec for an object moving at 7 deg/

sec; but their data differ considerably for slow speeds. It is difficult to generalize much beyond qualitative statements, however, because many other variables in the prediction task affect error

(Gerhard, 1959; Kimball, 1970): for example, the distance the target travels, the time for which the target is exposed, the distance and/or f

time over which the target is hidden, and whether the target intersects

with a stationary or moving object.

It is even more difficult to relate the literature on the perceived speed of an object to that on tracking and prediction behavior. j; We will restrict ourselves to the relationship between an object's perceived speed and an observer's over- oi' under-estimations of that speed

as reflected in his tracking or predicting behavior. For instance,

Runeson (1974) claims that the perceived speed of a moving object

decreases dramatically in the first few seconds it is seen. This

implies that, in a prediction task, the length of time the observer is

exposed to the moving object should drastically effect the constant

error in his predictions of when the object will reach a target point.

Also predicted is, that tracking and prediction will be affected by

intermittently presenting the object as it is in motion (in a series of

Wsinr (16)adRsnam(95 hnaloig exposures fontht tt b o dura tinonsl hadeno

shonr(16)adRsnam(95fontht hnalwn exposures tt ecn drathe inousl seen.

effect on prediction performance: the latter using exposures as short

as .25 sec. On the other hand, intermittently presented objects do

appear to move faster than continuously presented ones (i.e., observers

underestimate the time that it will take an object to reach a target if

the object appears intermittently). Returning toGerhard's (1959)

study, he looked at the effect of occluding sections of the path taken

by the vertical point on constant error. When the vertically m1oving

object was not occluded along its path, the observer exhibited no

constant error, but when an occluder was present, hie underestimated the

28
FW ......... ~ ~~~~ f~ ~ h e Ph .h~ pfl' . - . " '~ ,* "'

time that it would arrive at the point of intersection (it looked too

fast). In accordance with the random error data in that study, so too

this effect was small. Similarly, Morin (1956) using a radar display,

found that observers underestimated the arrival time of intermittently

presented moving objects, especially when the objects moved slowly

(about .2 deg/sec). Observers in this case, had to make their predictions during the "fast" phase of their responses to the movement (see

Figure 3). When the object appeared to be of constant velocity (physical ly it s 'tarts off fast, Figure 3), the time at which it was occluded

made little difference. They also made the most accurate predictions

during this condition. Knowledge about the motion illusion did not

F help them correct their errors.

Intermittent light can also severly disrupt motor tracking tasks.

Ailslieger and Dick (1966) assessed the effect of intermittent illumination on a number of tasks related to aviation. The pursuit rotor

task was the only one seriously effected; not only did the flashing

light make tracking very difficult, but observers commented that the

rotor was moving faster than under the steady light condition. Croft

(1971) also noted that it is very difficult to catch a tossed beanbag

under stroboscopic illumination. As with other phenomena in this

section, ample research has been conducted to allow contradictions, but

too few studies have been done to allow them to be resolved. In sum,

it seems clear that many of the variables discussed in this secticn,

spatial environment, luminance, contrast, etc. , could affect the perceived speed of an object in a man-machine situation, which in turn

F could affect an operator's abilIity to parform his assigned task.

Another performance measure important to the design engineer is

the speed with which an observer can react to motion. Since so much of

this literature compares reaction time for foveally and peripherally

viewed displays, the authors have reviewed this literature in the

29
CHAPTER 6

Illusions of Motion

This section will briefly review some ofth bai iluon

assoiatd otin wth pecepion th moionaftereffect, apparent motion, ard induced mto.Tefirst and last o, these are particularly important to the display-designer because they could influence an

operator's motion thresholds and his judgments of an object's speed.

Apparent motion, soeie ald"phi" or "beta" motion, is important

for another reason. It is the illusion of smooth continuous motion

produced when the observer only sees discrete pictures or "frames" of

that motion. Thus this illusory effect is crucial if motion pictures,

television, and computer driven displays are to successfully imitate r real motion.

F ' Motion Aftereffect

If after observing a moving pattern (a train of bars, a rotating F. spoke, a spiral pattern, or moving random dots) for 15 sec or more, the

motion suddenly stops, the pattern will seem to move slowly in the

Lopposite direction. This is the motion aftereffect (MAE). Usually, it

is measured in one uf three ways: (1) the time it takes for the

illusitin to fade away (i.e., the pattern finally appears to stop), (2)

the velocity of the illusory motion, measured either by matching or

magnitude estimation technique (Stevens, 1957), and (3) a nulling

technique in which the pattern its not completely stopped but is moved

slowly in the direction opposite to that of the MAE until the pattern

appears to be stationary. Researchers who have measured more than one

of these properties have usually found them to be highly correlated

(Sekuler & Pantle, 1967).

any pattrned objectio thatirsentis conatinuouscterotios tof the ame part

stheulfirstequestionsil fraised moisn "Watfhaaterisfetics ofltheug amovit

stiulu pattrned responsibletforesethe montinosmton atrfet?' Athoug almospat

of the retina will produce the effect, this chdpter shall try to define

the conditions which are most likely to produce illusory after-motion.

Scott, Jordan, and Powell (1963) found that a velocity of 2 to 4 deg/

sec was optimal for producing the effect with the Archimedes spiral

stimulus (the speed is that of the contour of the spiral along any

radial). Sekuler and Pantle (1967) found a decrease in the speed and

duration of the aftereffect with increased speed during the inspection

period using a disk of rotating radial lines. The linear speed of a

point half-way between the center and the edge ranged f rom 3 to 24

deg/sec. Thus the low end of their range of speeds may already have

*1been optimal, and their data do not disagree with that of Scott et al.

(1963). They also show a slight increase in the duration and velocity

of the MAE with increased inspection time; for 17 sec of inspection,

the MAE lasted about 3.75 sec and for 60 sec of inspection, about 4.75

sec. Holland (1957) found that inspecting a rotating spiral display

f or as i ttl e as 5 sec produced an MAE of 5 to B sec i n durati on.

[I 30
" .1 The absolute speed of the MAE movement itself cannot be known from

Sekuler and Pantle's data because they measured relative speeJs using

C the method of magnitude estimation. Scott et al. (1963), however, r• matched the apparent speed of the MAE to that of a stimulus that was

actually moving, and found the MAE velocity to be about 5 to 6 min of

arc/sec.

The question of how long the MAE lasts is not as easy to answer as

might be expected. One problem is that all the variables so far mentioned have been found to influence the duration of the MAE. The reason

the information on MAE duration is so complete is that duration is the

easiest way to operationally define the "strength" of th'3 MAE. The

observer inspects the moving pattern, then views the stationary one,

and in some way signals when the MAE has completely disappeared.

Holland (!957) found the maximum duration of the MAE to he from 80 to

100 sec. lowever, if one retests the observer later (wit~lout an additional exposure to motion), he may see the MAE again, and again it will

fade, perhaps more quickly.

F The amounL of time that can pass between the inspection period and

the test, and without eliminating the MAE response from the observer,

is another measure of the duration of the MAE (sometimes called the

"long-term MAE"). Masland (1969) was the first to discover the

strength f this effect. He had 121 naive observers inspect a spiral display for 15 minutes and found that he could measure MAEs from 20 to

26 hours later. These results have been replicated by Kaflin and Locke

(1972), and Favreau, Emerson, and Corballis (1972). ihe latter report

that after 24 hours the effect is weak and fades in 5 sec.

Those are the basic findings, however, spatial, luminance and

contrast parameters that effect the strength or porsistence of tne

illusory after-motion will be reviewed again. The most important

spatial requirement for seeing the MAE is that the stationary pattern

fall on the same area of the retina as the moving inspection pattern.

As Sekuler and Partle (1967) varied the amount of the spatial overlap

between the inspection and stationary targets frum 100% to 54%, they

found a dramatic decrease in both perceived velocity and duration of

the MAE. Masland (1969) found that he could not observe his "long- t-rm" MAE unless the inspection and stationary patterns overlapped by

at least 50%. Under these conditions, if the MAE is seen, even that

portion of the test field that falls on a previously unstimulated

retinal area will seem to move (Sonnet & Pouthas, 1972). In addition,

though the inspection and stationary patterns need to be roughly proximate with respect to the visual field, they do not have to be alike.

Grindley and Wilkinson (1953) asked observers to inspect a 60 deg

rotating spiral (inward movement) using a plain white field as the

stationary test stimulus. The plain field now appeared to be textured

and expanding outward and moving closer to the observer although the

borders did not appear to move. Also, th.ý MAE is stronger if reference

contours appear in the visual field. Day a~id Strelow (1971) found the

MAE was seen less frequently when the area surrounding either the

inspection or stationary pattern was itsel' patternless, and even less

frequently when both stimuli had patternless surrounds.

31
Variation of luminance and contrast of the inspection and stationary patterns do not greatly affect the strength of the MAE

(Holland, 1957). Day (1958) created a disk of colored sections to

produce distinct radial contours with minimal brightness contrast. He 1

found that with a 12% difference between the brightness of adjacent

sectors, he was able to generate a motion aftereffect. Note, however,

that a difference of 12% probably represented a brightness difference

which was many times greater than threshold. Keck, Palella, and Pantle

(1976) found that the contrast of the inspection pattern had little

effect on MAE velocity and duration once it was above 3%. However, as

the contrast of stationary pattern was increased from .9% to 10.5%, MAE

velocity and duration decreased by 50%. The strongest MAE, then, is

generated by a high contrast inspection pattern in combination with aI

stationary pattern of very low contrast.

There are a few additional observations that may be important to

the display designer. One is that an eccentrically viewed MAE appears

faster and lasts longer than one produced in the center of the visual

field (Sekuler & Pantle, 1967). This fact may play a role in the

production of MAE in the third dimension following prolonged exposure

to large-field moving optical flow patterns. Such flow patterns would

exist if an on-board observer steadily gazed at the wake receding from

a ship's bow or, possibly, at the display, of a flight simulator.

Also, there is a difference in the strength of the MAE when the

inspection motion is in a direction away from fixation (centrifugal)

rather than toward it (centripetal); the MAE resulting from centrifugal

motion is stronger. This difference increases with increasing eccentricity, reaching a twofold difference at 20 dog of eccentricity

(Scott, Lavender, McWhirt, & Powell, 1966).

Lastly, the MAE is not as strong when the precipitating motion is

a series cJ- discrete displacements rather than continuous motion.

Banks and Kane (1972) generated a MAE using motion picture images with

various frame-like image displacements corresponding to the same con- Vstant velocity. The duration of the MAE decreased with an increase in

displacement size; when the image displacement was 10' or greater,

there was almost no MAE, even though Banks and Kane claim that picture

quality was itself not much changed. Also Anstis, Gregory, Rudolph,

and MacKay (1963) used continuous motion (spiral disk) for the inspection pattern, but stroboscopically illuminated the stationary one.

Flashing the strobe at a rate of 4 to 30 Hz, completely abolished the

spiral MAE and decreased the MAE produced with other types of movement.

The influence of the MAE is something that display designers

should avoid if possible. That the illusion can affect the perception

of slow real movements is evident from the fact that one of the ways

psychologists measure the illusion is by "cancelling' it with real I movement, so that a pattern that is actually moving seems stationary.

Scott et al. (1963) showed that real and illusory after-motion added

L algebraically for very slow real-motion (less than 10' of arc/sec). r However, the data suggest that a single moving target is not likely to

tproduce a consequential motion aftereffect. A display of a large

moving field viewed for 30 sec or more, Could be a problem, but the

32
literature reviewed in this report suggests that one could easily minimize illusory after-motion by varying gaze momentarily or stroboscopically illuminating either the inspection motion or the pattern

affected by the illusion.

Apparent Motion

Apparent motion is the illusion of continuous movement resulting

from the momentary presentation of an object at an orderly set of

locations in the visual field. A typical stimulus used in research

would be the following: a 1 deg square is presented for 10 msec; 100

msec after the square's offset, an identical square appears 4 degrees

to the right of the first one, again for 10 msec. The observer will

have the impression that the square moved continuously rightward

through space (i.e., that the object at some time occupied every location between the motion's beginning and end). The two objects do not

have to be identical; the illusion persists even when the objects have

different shapes and/or colors (see Kolers, 1972; and Graham, 1965, for

reviews). This chapter is restricted to the variables of the duration

of the objects, and the space and time which separate them. In addition to the many variables manipulated in the course of investigating

this effect, many types of apparent movement have been discovered

(Boring, 1942). However, the discussion will be primarily two types,

phi and beta movements. Beta movement, sometimes known as "optimal

apparent mo-vement" is simply the illusion of an object moving through L space, while phi movement is the illusion of movement but without a

clear impression of an object (Spigel, 1965; Graham, 1965; Aarons,

1964).

In his revieW, Graham relates Wertheimer's (1912) rule of thumb governing appearance of two sequentially flashing lights: (1) for

intervals between lights of less than 30 msec, the lights will appear

simultaneous, (2) for the range of 30 to 60 msec the illusion of movement between the lights is optimal, and (3) for intervals greater than r 200 msec, the lights appear successive, with no motion between them.

One will note, however, that this rule probably only holds for relatively brief flashes. Graham also relates Korte's famous "laws" of

apparent motion formulated in 1915. They are a guide to the interplay between the four variables of: (1) the luminance of the target, (2)

their duration, (3) distance between them, and (4) the time between the

offset of the first and the onset of the second target (Interstimulus

Interval or ISl)> For optimal appar~ent movement to be seen:

1. As the luminance of the targets is increased, the space

between them must be increased.

2. As the ISI is increased, luminance must be decreased.

3. As the ISI is increased, distance must be increased.

4. As the ISI is increased, duration of the target must be 1 decreased.

33
I

Therefore, if the influence of apparent motion is to be avoided in a

display rules 1 - 4 must be considered in display design.

in Much research has been aimed at validating or refuting these rules

in qualitative terms; rather than collecting parametric data that would

allow the computation of optimal apparent movement for many display

conditions. Data on luminance are especially lacking. This is true

even in studies undertaken to provide results which would be used by display designers. Obviously, such an omission is quite a handicap.

In the following paragraphs, the authors assume that display (background and target) luminance is constant, at an acceptable level (midphotopic) for good visibility, and that the targets are of sufficiently !• 'greater luminance than their background to insure a contrast of more than 5 times threshold. Such assumptions make it more probable to

concentrate on other, more thoroughly explored parameters of apparent •- i motion.

Korte's laws imply that with all but one parameter fixed, the free

parameter had to assume some particular value in order for the observer

r to see apparent movement. However, Neuhaus (1930, as related to

Kolers, 1972) found that the free parameter could assume a wide range

of values and apparent motion could still be seen. This fact is reflected in Figure 4. The lower set of three curves represents the

boundary between the perception of simultaneity and the perception of

good apparent motion. The upper curves represent the boundary between

good apparent motion and the appearance of successively presented

lights with no motion. Note the unusual label on the ordinate: the

interval from the onset of first target to the onset of second target. This interval, commonly known as Stimulus Onset Asynchrony (SOA), is

equivalent to the sum of the first target's duration plus the ISI.

Korte's fourth law itself suggested the importance of SOA in determin4ng whether apparent motion or successive flashes will be seen. As far

as the design of display systems is concerned, the most significant . aspect of Figure 4 is that for small spatial separations of the lights,

a wide range of SOAs will produce the illusion of motion.

Two other investigators support the perceptual significance of a

constant SOA. Sgro (1963) found a trade-off between duration and

interstimulus interval (ISI) resulting in a constant SOA of about 100

msec. This was true so long as duration was less than 65 msec; above

this duration value, optimal interval asymptoted at a small value.

Similarly, Kahneman and Wolman (1970) found that apparent motion was

optimal at SOAs of 1i0 msec, for durations less than 100 msec. For

longer durations, apparent motion was optimal when the ISI was zero,

that is, the second object appeared at the offset of the first (SOA =

duration).

A perplexing question arises: "How do these general findings and

rules derived from displays consisting of only two targets generalize

to more complex displays?" A display used by Braddick (1973, 1974) and

by Julesz (1970) provides some answers. They presented observers with

two alternating but spatially overlapping fields of dense random dots;

the fields were identical except that a square section in the center of

one was displaced laterally relative to the corresponding section in

34
Ir

S UCC ES SION

LFLASH DURATION

35- 9 g

- I

MOIO L 00

Figure 4. Abscissa: Spatial separation between targets. Ordinate:

stimulus onset asynchrony for a pair of flashed targets. The parameter of the family of curves is target duration. For any target duration, the regon between upper and lower curves defines tne conditions of

onset asynchrony and spatial separation which produce a good apparent

motion, i.e., producing neither simultaneity nor successiveness in the

percepts. (Kolers, 1972, based on data by Neuhaus, 1930).

35
the other field. Dots were removed or added at the borders of the

shifted-area so that each display seen alone looked spatially random

and insured that no contours outlined the displaced square. When the

two frames were presented alternately to the observer at a rate that

usua'lly produces apparent movement, observers saw the square moving

back and forth. But the range of displacements that produced apparent

M~ovement was much less than with the traditional two-object type of

display (Braddick, 1974). The appearance of motion began to deteriorate when the displacement exceeded 5 min of arc, and was completely

gone where it reached 20 min of arc. This is in comparison to the

range Neuhaus used -- 30 minutes to 4 degrees. In addition, Braddick

found no trade-off between displacement and ISI; Korte's third law did

not hold. Braddick concluded that the type of apparent movement foundI with his random dot type display is not the same as that found with a

simple two-object display.

What about the situation in which one object appears sequentially

in many locations? Sperling (1976) has shown that this situation gives

dramatically different results from the simple, two-position display.

Using a computer generated display, he varied the number of points,

their displacement, and duration. He had observers rate on a 10 point

scale the quality of the movement on the displays, zero meaning no

apparent movement and 10, meaning that the apparent motion looked just

like continuous motion. Figure 5 shows his results. The left hand

part of the figure shows Sperling's data for a two position display;

the lines define the boundaries between combinations of displacement

and duration that produced different qualities of motion. For the two

position display, the quality of motion is given by the percent of the

trials when the observer saw some sort of apparent motion. Thus the

conditions enclosed by the line labeled "100" produced apparent motion

on every trial; the conditions between this line and that labeled "50"

produced apparent motion at least 50% of the time. The longest durations and largest displacements produced the best apparent motion. For

the right hand figure, Sperling used the mean quality rating to draw

these "iso-quality" contours. In this case, an intermediate duration

and the smallest displacement produced the best motion. Sperling notes

that the two-position data would not allow accurate prediction of the

conditions that would produce the highest quality apparent motion with

the multi-position case. Kolers (1972), also using a computer generated display, varied the number of positions across his 7.5T deg screen

between 2 and 1024, by powers of 2. For each number of positions, he I; varied the duration of each object to try to optimize apparent motion.

He couid produce good motion with 2, and with 64 or more positions, but

for intermediate values motion was poor. Motion quality in Kolers'

display was not a monotonic function of the number of positions used.

This finding is at odds with Sperling's claim of a monotonic improvement in apparent motion with increasing numbers of display positions.

hisdislayto llo usto esove hisconflict. This is unfortunate

becase typs hes o daa wold indwide application in display

desin. xamlemanycomute or drvenCRT displays use apparent

movement to convey information about real or simulated movement of a

target or targets. Systems designers will be interested in minimizing

(1) the burden which the display places on the computer central proces36j
(a) (b)j

192 -

IiIF

24--

15

0.75 II_ __II

4 1i R4 256 4 15 64 256

At In milliseconds

Figure 5. Judged quality of perceived movement as a function of the

distance Ax and the time At between exposures of the points. Data of

one subject: 10 judgments were made for each A x and At combination.

(a) Two-point experiment. The area designated zero indicates that

generally no movement was perceived (i.e., less than 10% of quality

judgments were greater than zero). The contours indicate boundaries of

area in which 10% - 50%, 50% - 100%, and lastly, in which all quality

judgments were greater than zero. (b) Multipoint data. The number in

the areas designate the median value of the quality judgments for

(Ax, At) within the area. The quality range is 0 (no perceived

movement) to 10 (apparently continuous real movement). (From Sperling,

1976).

37
sor unit and (2) the bandwidth of the signal that drives the display.

1i Such minimization, if it is to be consistent with compelling, high

quality apparent motion on the CRT, requires that the display designer

know in detail the correct interpretation of the Sperling/ Kolers data. I

Induced Motion

Induced motion is defined as the effect which one set of moving

objects exerts on the perceived velocity of another set of moving

objects. Outside the laboratory, at any one time the visual field

usually contains several objects moving relative to one another. As a

result, the conditions which give rise to induced motion are quite

common in everyday perceptional experience. Duncker (1929) was one of

the first to study relative motion. He wondered how, given that there

are just two objects in the visual field, their motions might interact.

If just one of them were moving, would the observer be able to tell

which one, or could the moving object induce motion into the non-movinq

one, producing a perception that was just the opposite of the physic-,

situation? In this simple case, Duncker found that whichever object

the observer fixated would appear to move, regardless of which one

actually moved. Duncker believed that if the peripheral object provided the "frame of reference" defining the stable world, this caused

all perceived movement to be imparted to the fixated one. But fixation

is not always the dominating factor; if one object enclosed the other,

like a picture frame, the visual system interpreted the frame as the

stable reference and the enclosed object as moving, regardless of where

the observer fixated. Duncker darkened the room so that the observer

could only see the large frame with a small dot inside. When he moved

the frame slowly but kept the dot still, the observer perceived the

frame as still and the dot moving. One cannot make a stationary frame

appear to move by moving the inner dot, however. If the room is well

lighted so that other stationary objects are visible, the illusion is

not so compelling; the frame appears to move, but movement is still

L induced into the dot. These observations were later replicated by Brosgole. 'ristal,.and Carpenter (1968), and others.

The available data on the magnitudes of these effects are mdrely Sadequate to demonstrate their strength but not much more of an analytical character. Duncker, using a display consisting of a disk surrounded by a rotating annulus, used a nulling technique to measure the

amount of induced motion in the disk. In other words, he rotated the

disk in a direction opposing the induced motion until the disc appeared

stationary. The linear motion of the outer annulus between the border

and the disk was about 18 deg/sec; the motion of the inner disk needed

to cancel the •induced motion was about 12 deg/sec'. Brosgole, et al.

(1968) u--- a . ind frame arrangement, were able to induce a 2.5 deg

move,.. i. in tV.' :.ýationary center dot, when the frame moved 10 deg.

These values suggest that Duncker's motion induction was stronger than

that by Brosgole, et al. (1968), Unfortunately, the two displays

differ in many ways so that is is impossible to identify a single

cause.

Other resep .,urs have used more complex types of displays with

many moving objects. Because these displays used large uniform fields

38 '
of movement, usually dots, researchers have discussed them in terms of

the motion "contrast" between one part of the visual display and another. This was also applied in the luminance domain. Indeed, an

analogy between luminance and motion contrast effects hris been drawn

;* I whereby an area of unvarying luminance may appear darker if the surrounding luminance is increased, or lighter if the surround luminance

is darkened (Ratliff, 1965). Similarily, Loomis and Nakayama (1973)

found that the perceived speed of an object is reduced if surrounded by

faster moving object and is increased if surrounded by more slowly

moving ones. Using a shadow-casting technique, they generated a field

of moving dots that formed a velocity gradient; dots on the left of the

screen moved more slowly than those on the right, with a gradient of

Velocity in between. Embedded in this field of dots, were two larger

targets, one on the left and the other on the right; these moved at the

same speed. Despite the fact that the spatial separation of the targets was fixed and should have provided a strong clue that they were

indeed moving at the same speed, the left-side target (embedded in the

slower moving dots) seemed to move faster than the right-side target

(embedded in more rapidly moving dots).

Continuing with the analogy to luminance contrast effects, k Holmgren (1973) found an enhancement of the perceived velocity difference at the border between moving and non-moving dots, similar to the

contrast enhancement found at borders of luminance differences. These

luminance effects are called Mach bands (Ratliff, 1965). Holmgren

produced eight rows of dots on a CRT under computer control. The upper

four rows moved horizontally. When a dot reached the end of the

screern, it began again on the opposite side so the movement was endless. Subjects reported that the bottom-most row of the moving dots,

the one bordering cn the stationary rows, seemed to move faster than

the other moving dots. Of the stationary dots, the row bordering on

the moving ones appeared to be moving in the opposite direction. The

perceived velocity of both moving and non-moving dots decreased with

increased distance from the border betweeni them. Walker and Powell

(1974) report a similar result using six rows of moving dots. In this

case all dots moved, the upper three rows at .3 deg/sec and the bottom

three at .6 deg/sec. Walker and PowellI measured the perceived velocities of each row of dots by a matching technique (see Figure 6).

Again, the difference in velocity between the two halves of the display

was enhanced at the border between them. I,' .iAll these motion contrast studies, using complex displays, employed only one or a few display configurations. Tynan and Sekuler

(1975b) measured one variation of this effect quantitatively, and found

that the "motion contrast" rule was not as simple as -it had seemed

previously. They used a center-surround type of display, the motion in

3 1.2 deg x 1.2 deg area was controlled independently of the motion

surrounding it in a 100 x 90 field of moving random dots. Movement was

of the endless type used by Holmgren, but dots in the center of the

display were restricted to that area; surround dots were correspondingly confined to surround. Tynan and Sekuler varied the velocity of

the surround dots and measured the perceived velocity of the center

dots when they: (1) moved in the same; or (2) opposite direction from

the surround, or (3) remained stationary. Figure 7 shows the results;

39
:1 I

1.8-

0.2 lOTTOM TOP _OTTOM TO

MATRIX lw

iI

Figure 6. Perceived velocity nf each of the six rows in the matrix of

dots, with each point representing the average of three observations.

The vertical bars delineate the range of values - spanned by the two

extreme observations in each case. (a) o, The perceived velocity

profile obtaired when velocities of 0.30 deg/sec and 0.6 deg/sec were

assigned, respectively, to the Upper and lower rows in the matrix. The

profile adds confirmation to the illusion reported by several observers; with fas't and slow moving rows presented to separate eyes (the

left and right respectively) simultaneous contrast does not occur. (b) obtained in control -Profiles trials in which all six ro.ws were assigned

the same velocity, being in the one case U.6 deg/sec.(A) and in the

other, 0.3 deg/sec (A). (From Walker & Powell, 1974).

"40
ii

10

OPPOSITE

8

1-4

0 6

•. . SAME

0

4

STATIONARY

0 _ SI I I

0.0 1.4 2.8 4.2 5.6

SPEED OF SURROUND DOTS (*/sec)

Figure 7. Magnitude estimate of center dots' speed as a function of

the speed of surround dots. In all cases, when the center dots moved,

their speed was 2.8 deg/sec. Except where the standard error is too

small to be seen, +1 S.E. has been plotted about each point. Curves

drawn through the data points are the best fitting polynomial functions

of the second degree. (From Tynan & Sekuler, 1975b).

41
for center dots moving in the opposite direction from the surround,

perceived speed increased as the surround speed increased. Al so;

surround motion induced movement in the stationary dots. As expected, I I center dots that moved in the same direction as the surround were

slowed relative to those that moved in the opposite direction. However, the maximum slowing occurred when the surround dots moved at the

same speed L.s the center, not when they moved faster, as expected from

t-he simple "contrast" rule. A more accurate rule would be a "similari ty" law; the closer the surround speed is to the speed of the target, the slower the target appears, the less similar the speed, the faster

the target appears. Changing the direction of the surround dots relative to the center is equivalent to changing the sign of a number when

performing the arithmetic operations; now the disimilarity between the

surround and the center speeds increases monotonically with the magni- tude of the surround speed, hence the perceived velocity of the center

42j
CHAPTER 7

Motion Perception in the Periphery

According to most studies, peripheral motion perception, like that

of form, is inferior to foveal perception. However, the literature on

this topic is sparse and only a few types of measures have been used.

It. is noteworthy that in some cases the periphery i5 not inferior to

central vision. Despite the sparseness of relevant data, a few established facts can be stated with confidence. First, motion thresholds

always rise as the stimulus is placed further and further in the pe- riphery. Gordon (1947), using rotating sectored disks, found that

motion thresholds rose from 6'/sec at the center to 12'/sec at 400 into

the periphery of vision. McColgin (1960), who was interested in aviation applications, used standard aircraft indicators as stimuli. The

indicators either rotated or moved back and forth. He measured thresholds for movement along 12 meridia and with interpolation constructed

iso-sensitivity contours: These are shown in Figure 8. For instance,

the line labeled "2" in Figure 8 connects all points in the visual

field for which movement thresholds would be 2 RPM. Based on the

dimensions of his apparatus, (and assuming speed at the ends of his

pointers) it is calculated that the threshold for rotary motion at 550

along the horizontal radial is about 22'/sec; for linear motion at 570

along the horizontal axis the threshhold is about 28'/sec. McColgin

took no foveal measurements. The eliptical shape of these isograms will be discussed later.

More recently, Leibowitz, Johnson and Isabelle (1972) using a

small, white square, measured a central threshold of 1'/sec and a

titreshold of about 8'/sec at an eccentricity of 800. The effect of

eccentricity was fairly linear between these two points. Later,

Johnson and Leibowitz (1976), using a small dot target, found the

central threshold to be about 1.5'/sec and the threshold at 600 to be

about 5'/sec, but measured no points at other locations.

Finally, Tyler and Torres (1972) measured motion thteshold using a

sinusoidally oscillating line (10° in height). They varied the fre--

quency, amplitudeand position of this line. Their measurement was the

just-detectable amplitude (extent) of oscillation. Tyler and Torres

estimate motion thresholds at about 1/sec in the fovea and 7'/sec at a

point 200 in the periphery. The presence of stationary reference marks

lowered motion in thresholds in the fovea, but not in the periphery of

the visual field.

Except for the McColgin (1960) study, these finiings are very much

in agreement despite the fact that they used widely different stimuli.

(See Absolute Motion Threshold section for a review of effects of

spatial parameters on motion thresholds.) They all agree that the

periphery is less sensitive than the center of vision for detecting

very slow movement. But what of discriminative powers of the periphery

at faster speeds? There is only one study that discussed difference

thresholds .in the periphery of vision. Link and Valerie (1969) found

that the periphery is poorer than central vision at this task also.

43
I '

0

LII

•~8

L6270 l90•

V '180

Figure 8. Perimetric chart showing the absolute threshold isograms

I- (RPM) of rotary motion. (From McColgin, 1960).

44
Like McColgin, they were interested in the applic:ation of these data to

"aviation and used aircraft instrument indicators as their stimuli. The

difference threshold measured (AV/V) increases linearly with eccentricity. Like McColgin, they interpolated between their data points so

that they could construct isograms. The isograms for linear and rotary

motion are reproduced (Figure 9) for these slowest reference speed of

6.340 /sec. The .200 line means that any motion along this line (240

out along horizontal axis) would have to change from the reference

speed to a speed of 7.6 0 /sec (i.e., 6.34 + 0.2 x 6.34*/sec) or greater

in order for the change to be detected.

Although McColgin, Link, and Valerie used different measures or

peripheral motion sensitivity, they agree on several points. First,

all authors found that the decrease in motion sensitivity with increased eccentricity was linear. Thus, they felt justified in linear

interpolation between data points. Secondly, both sets of isosensitivity contours were ellipcical, that is, the periphery is more

sensitive to motion along the horizontal axis than along the vertical axis.* Finally, although it is not mentioned above, neither demonstrated a consistent difference in sensitivity to rotary or linear

motion. Both studies however, found that observers preferred to be

tested with the rotary motion because it seemed more "salient" to them.

Although the center of vision is more sensitive to certain kinds

of motion than is the periphery, both Tyler and Torres, and Leibowitz

agree that as stimulation is delivered to an increasingly peripheral

retinal location, the response to moving targets declines less rapidly

than responses to stationary targets involving spatial information

(e.g., acuity). Leibowitz, et al. (1972) point out that "in the periphery, all visual functions are degraded, but motion suffers the

least".

A striking example of the superiority of motion perception over

acuity in the periphery is a phenomenon called" extended motion" much

studied ny Thorson, Lange, and Biederman-Thorson (1969) and BiedermanThorson, Thorson, and Lange (1971). This was also noted many years

before by Basler (1906/1975). The basic paradigm is as follows: two

distinguished as two dots (less than 30' separation at 200 of eccentricity). If these dots are flashed sequentially (phi movement) or one I.. dot is moved through this distance, the observer sees movement extending over a much greater distance than the separation of the dots. They

report apparent movements of up to 3 deg at a location 10' in the

• periphery.

i I*A similar finding occurs in connection with induced vestibular stiimulation and may have a common cause (cf. Benson & Guedry, 1971) for a

discussion of the latter effects). A review of vertical/horizontal

difference in information processing would have profound importance for

display design.

45
I

9 00

1800 0

",A 2700

900

i 1809°-. - '-... 00

270

Figure 9. Differential threshold C)isograms for (a) linear and

(b) rotary motion at -the slow refererc~e velocity (16.3401sec). (From

Link and Valerie, 1969).

"46
It should be noted that there are much greater differences among

individuals when measuring motion thresholds in the periphery than when

making similar measurements at the center of vision (Johnson 

&

Leibowitz, 1976). Leibowitz, et al. (1972) showed that although central refractive error may be fully corrected, this does not guarantee

that the periphery is not suffering from optical error -- error that

affects motion threshold. In one case an observer with perfect central

refraction had an 8 diopter error 800 in the periphery. When they

corrected the refractive error among their observers for a peripheral

location, and measured motion thresholds, the great individual differences they noted without correction had disappeared.

it has been established t;.at the periphery of vision is less

sensitive than the center to: (1) slowly moving targets, and (2)

small differences in velocity. Another question emerges: for a given

peripheral location, which is more effective, a moving or non-moving

stimulus? Fankhauser and Schmidt (1960) using a luminance threshold

measu'e rather than motion threshold, found that at the center of

vision, a stationary target had a lower threshold than a moving one,

but this reversed in the periphery (i.e., a moving target could be seen

at a lower luminance than a stationary one). Rogers (1972), and

Sekuler and Tynan (unpublished) also have evidence that suggests that the peripheral retina may be responsible for detecting low contrast

moving targets, while stationary targets are detected by centralvision.

Different measures of peripheral capabilities have provided different results. One important but neglected measure is reaction time.

One of the few studies on this subject was conducted by Borkenhagen

(1974). He found that reaction to a small line moving at 1/sec was

400 msec slower at 600 in the periphery than at central vision. However, when he increased the speed of the line to 120 /sec. this difference dropped to only 50 msec. Preliminary experiments by the authors

[ using isotropic noving dots suggest that this difference can drop to

zero, at least at up to 80 in the periphery. Also, Hamerman (1979)

found choice reaction time to acceleration of a dot to be a function of

degree of acceleration and eccentricity (see Figure 10).

The authors have collected reaction time data specifically to

close this large gap in the literature (Tynan & Sekuler, unpublished).

We were concerned with the variation in simple reaction time to motion

onset with position of the motion in the field. Observers were the

authors and a naive volunteer. Targets were random dot patterns produced by a computer on a cathode ray tube. Some 560 dots appeared with

a 9240 diameter circular apperture (constant luminanc1 equals 0.75

cd/m ); incremental luminance of the dots equals 5 cd/m * Viewing was

binocular. On any trial, dots first appeared as a stationery pattern

for a random foreperiod of 1.3 to 1.7 sec. The dots then instantly

accelerated and continued to move upward until the subject depressed 

a

telegraph key signalling that he had seen the moton. The dependent

measure was reaction time; the interval from motion onset to key press.

Speeds tested were 0.25', 10, 40, and 16

0/sec (with speed constant in

each block of 125 trials). A specially designed electronic, blanking

circuit was used to control the region of the cathode ray tube over

47
VISUAL ACCELERATION

.4 DEG/SEC2

L• .8 DEG/SEC'

CONCURRENT ROTARY

S2500 ACCELERATION IN DEG/SEC2 C ,"- 250

o 0 DEG/SEC2

__ o I DEG/SEC2

_-_2 DEG/SEC2

S2000-

LU

1500-

LU

z ,i

_o - ~~// .N

' ii

S4 carI0

S0 20 40 6

i• ECCENTRICITY IN DEG

:- Figure 10. Mean choice reaction time for vertical dot movement at four

!!: . eccentric-ities for 11 pilots. (From Hamerman, 1979).

' ',l48
which dots could be seen. The circuit blanked out dots over the remaining sections of the screen. The authors used three major configurations. Dots could be shown across the entire screen (hereafter "full

field"), only within a center region of variable diameter (hereafter

"patch"), or only within an annular region of variable inner diameter

but fixed outer diameter of 9.41 (hereafter "hole" refers to the inner

diameter of this annulus). Data for, one observer are shown in Figure

0 11; average data for all observers are given in Figure 12. For-all

observers, reaction time to onset of the slowest motion increased with

increasing hole size; but for the three higher velocities the hole size

is without effect. In other words, only for the slowest speed is

reaction time effective as more and more of the central portion of the

s~timulus is eliminated. As the three lower, dashed liioes in Figure 12

show, so long as the speed is high enough, even eliminating more than

50% of the center dots fails to increase the reaction time. One mayI

conclude from these results that as far as the potential for initiating

reactions to motion onset is concerned, the near periphery of the field

(that is up to 80 or 90 of eccentricity) is every bit as good as the

center of vision. One may also conclude that the center of vision is

more important than the periphery when it comes to initiating reactions

to slowly moving targets. A different trend occurs in the patch condition., in which only the central region of the field is stimulated. As

dots are eliminated from the periphery and smaller and smaller patches

are tested, reaction time decreases for the slowest speed, but i s

unchanged for higher speeds. Note that "or the three higher speeds

(10, 40, and 160i'sec) stimulation of the central field and the periphery of the field are equally effective in producing reactions to motion

onset. As Figure 12 indicates (lower. 3 pairs of curves), for any of

these speeds hole (peripheral stimulation) and patch (central stimulation) configurations of comparable area and therefore comparable number

of dots produce equivalent reaction times. This suggests a form of

equipotentially of retinal regions that is not found with the slowest

speed (upper two curves in Figure 12).

There is also preliminary evidence to suggest that peripheral

-iviewing affects the perceived speed of an object. As part of a study

in 1942, Klein reviewed the literature on this problem and found it

conflicting. Some studies claim the stimulus appears to move more

slowly; others claim the opposite. 'Some investigators, (Von Bruecke,

1907; Stevens, 1908 as reported by Klein 1942) found that objects

looked faster in the periphery than 'in central vision. Others, like

Brown (1965a, 1965b) say the target looks slower. Preliminary evidence

by the authors suggests the reason for these conflicts is: presented

eccentrically, targets near the threshold for motion look slower than

foveal targets of the same speed but the apparent speed of faster

moving targets is unaffected by degree of eccentricity.

Using three naive observers, Tynan and Sekuler (unpublished)

measured the preceived speed of moving random dots presented at se'veral

eccentricities. Dots appeared within a 10 wide strip some 80 in

height. The dots 2were 7 times the luminance of a weak constant background (0.75 cd/in ) against which they were presented. Duration of the

movement varied randomly from 1.5 to 2.5 sec; motion was always upward.

Within a strip some 500 dots were constantly present. The strip of

49
/o1

If,

400 P.T.

360!ss .2S•eec

•i..."i •) 300 ••Ie

.2SVaec

o

o260

200 * :=:=...,- -- . &---- •4

HOLE SIZE PATCH SIZE

I'

S~Figure 11. Reaction time to a sudden velocity increment from zero

velocity (Observer P.T.). The abscissa is the diameter of the hole

l (left-hand figure) or patch (right-hand figure) in degrees of visual

i•; 'angle. FF represents full field, i.e., no patch or hole in the dot

pattern. The parameter of the lines is velo'city increment in deg/sec. Marks to the right of each line show the average standard error for

* each point along the line. Histograms at the bottom of the figures

•- .ishow the percent of trials on which the observer failed to detect the

•. Imotion or responded prematurely for the .25 deg/sec, increment condii I tions (Tynan & Sekuler, unpublished). :

"* 5 50

* I 4
NUMBER OF DOTS]

672 490 400 327 245 163 81 0

HOLE

w -d- 0 -.------ PTH

~300-

z

0

200

0 72 60 5 40 -0- 20-0

w at

A I 0

70 460 50 407 305 20 10 0

Figure 12. Reacton time as a function of amount of retina stimulated,

for three observers. The solid lines are data for the "patch"' conf iguration; dashed lines for the "hole" configuration (see text). =.25

"deg/ sec; o = 1 deg/sec;A 4 deg/sec;A 16 deg/sec. (Tynan &

Sekuler, unpublished).

51
moving dots could be presented at various retinal positions separated

by some 7h0, beginning at the fovea and going out to 3Q0 of eccentrici ty. One-half second after determination of one of these strips, a

similar strip appeared, always in the center of the field. The observer used a potentiometer to adjust the speed of this center motion

until that speed seemed to match the speed of the other test motion

that had been presented in one of five positons: center or 71⁄20, l5*,

220, 300 eccentric. The test strip and the adjustable center strip

alternated until the observer was satisfied with the match. At that

point the matching speed was recorded. Test motion was either at

0. 250, 10, 40, or 160 sec. Eight measures were collected from each

observer in each of the 20 conditions. For all observers, when presented at the largest eccentricities, the slowest speed (O.2501sec)

could almost never be matched by the motion of the central strip; on

most trials the eccentrically presented slow motion appeared to be completely stationary. The remaining data are shown in Figure 13 averaged

across all three observers. The ordinate shows the ratio between (1)

Central speed needed to match the test stimulus, and (2) the actual

test stimulus. The parameter of the curves is the speed of test movement. Note that the perceived speed of rapidly moving targets (for

example, 16*/sec) does not change with eccentricity but that the perceived speed of slowly moving targets (1*/sec) drops by more than 60%

as eccentricity increases. An analysis of variance confirmed the significance of this interaction between velocity and eccentricity

One measure of visual perception in which the visual phenomenon is

completely dominated by the response of the peripheral areas of the

visual field is linearvection. This is the sensation of' self-induced

movement in the visual environment.. Our vestibular apparatus can

provide us with information about our bodily acceleration, but not

about constant velocity - for that we are dependent on visual informiation. Thus if an observer is sitting in a large stripped drum and is

rotating relative to the drum, he cannot tell whether it is he or the

drum that is moving and often incorrectly perceives himself as moving [ -~ and the drum as not (Dichgans & Brandt, 1973; Brandt, Dichgans, &

Koeni g, 1973). Once the drum starts moving, the transition from the

perception of drum movement to self-movement takes only two to three

seconds.

This illusion seems to be exclusively the property of the peripheral visual system as the following brief summary of Brandt et al.

(1973) will show: (1) masking up to 1200 of the center of the visual

field hardily effects the strength of the effect, (2) presenting stimulation of up to 300 in width at the center of vision produces very

little effect, and (3) given two areas of motion of equal size, one in

the center and one in the periphery, the peripheral one is much more

effective. In addition, only one side of the peripheral stimulus need it be presented to produce a considerable effect (see Figure 14).

Apparent self-movement in any direction or rotation can be produced by presenting the appropriate motion. Held, Dichgans, and Baueri

tating textured disk. In this case vestibular cues are present pre52
1.

U wUW

(.

C"8

rAu .4 4

a

w;. 6 -*

i I 4 ...

0 7.5 15 22.5 30

SiI ECCENTRICITY

I I

Figure 13. Perceived velocity as a- function of eccentricity. The

parameter of these lines is the velocity of the eccentrically viewed

strip. An ordinate value of one means that the peripheral strip appeared to move at the same speed as the central one; a value less than

one means it looked slower than the central strip (Tynan & Sekuler,

unpublished).

" ', 53
iI

II

|! Egscentrie and Extmentri¢ Mhtlem Foreeptiei

K 5 10 5 I1

O TAU AR I

b '

, h

c@h

d

:n=lO

5 10 I0

MAGNITUDE ESTIMATION

," -' • CIRCULARVECTION (VELOCITY)

!: {, [• CIRCULARVECTION (INTENSITY)

"• Figure columns) 14. and Magnitude the intensity estimates (white of columns) the subjective of circularvection velocity (shaded with

I ~optokinetic stimuli of differing size and location within the visual

field (a--,). Stimuli presented during fixation of a stationary target

• • with masks central masks subtending 600 (b) and 120' (c), and peripheral leaving a central stimulus area of 60' (c) and 300 (c). In

-:£ " iaddition, stimuli were exposed monocularly to the left temporal (g) and

:'• { right nasal half (h) of the visual field with the central area masked. In (i) an opposite optokinetic stimulation of center and periphery is

i average subjective velocity and the average intensity of CV are highly

1 correlated. (From Brandt, Dichgans, & Koenig, 1973).

* 5

i nI
venting continuous self-motion, that is, they did not feel that they

were turning completely upside down. The degree of apparent tilt

varied between 60 and 430 counter to the direction of disc motion. The

optimal speed of rotation was 300 to 40°/sec. Again, by masking the

disk into equal area concentric rings and testing the effect of each

ring separately, they showed that the concentric ring (480 in the pe- riphery) was the most effective. Johansson (1977b) was able to produce a sensation of upward or downward self-motion (what he calls the

elevator effect) by presenting patterns of random moving dots to the

observer's peripheral field of view (450 to 900) via two television

monitors. In this case he allowed the observer a clear view of the

rest of the room, yet the effect was still present; the room also

appeared to move with the observer like an elevator cage. This effect

could be produced in some observers even when the monitors were masked

down to 10 wide strips (a few percent of the visual field). Again,

when the observer feels self-movement, he perceives the dots as stationary. This effect could even be produced in some observers when

only one dot was presented to each peripheral hemifield. As far as the

number of people that experience these illusions, the proportions range

from 100% for most (cf. Brandt et al. , 1973) stimuli to about 50% for

even the meager, one dot display.

This effect is important to the human engineer for two reasons:

(1) linearvection is accompanied by apparent lack of movement of the

inducing stimulus, and (2) linearvection could cause dangerous disorientation in pilots. Moreover, as Johansson (1977b) has shown, even

the presence of stationary reference marks does not inhibit this compelling phenomenon.

In summary then, the absolute threshold for motion decreases with

eccentricity. (See Absolute Motion section for implications for display design.) Eccentric viewing, with speeds above threshold, may not

necessarily result in poorer performance in terms of contrast threshold

and reaction time. This is particularly true in the near-periphery and

if the peripheral motion effect is influenced by mean luminance level

(see chapter entitled Flicker Sensitivity).

Although the data on perceived speed in the periphery are sparse,

it seems likely that the observer will misjudge the speed of an eccentric object, unless he is well practiced *in such observations. Therefore, tasks which involve direct view (e.g., Airborn Target Acquisition) or extraction of peripheral motion from displays (e.g., visually

coupled systems, wrap around simulators) can be expected to produce

large individual differences which could bc cudied for purposes of

training and selection as well as for design criteria.

5

• 55
CHAPTER 8

Flash Sensitivity

In this section the sensitivity of the observer to non-moving

lights that flash or flicker is discussed. The literature review has

been divided into two sections: detection of single versus multiple

flashes and detection of flicker. Probably the most important variable

in detection is the background luminance of the display (called adaptation level because it is assumed that the observer has adapted to the

background luminance). Additional considerations are the spatial

characteristics of the stimulus and retinal location.I

The simplest case to consider is the detection of a small spot of

light (about 10) viewed with the center of vision. Reviews by HerrickI

(1973c) and Roufs (1972) show that most of the data on the detection of

s ingle flashes can be described by a simple rule: with other variables

held constant for all flash durations below some value (called the "critical duration"), a constant amount of enrg is needed if theI

flash is to be just visible. "Bloch's law"' i-s t-e name given to this

observed trade-off between luminance and duration. The law states that

stimuli which represent a constant product of luminance and duration

yield constant visual effects. This reciprocity breaks down at longer

durations; thereafter constant luminance alone yields constant visual

effect, a fact which could be of considerable importance to display designers. This critical duration ranges from 90 to 150 msec, depend- ing on adaptation level of the stimulus (i.e., the background luminance

upon which the flash is super-imposed). From Herrick's data, the authors have derived the following equation which expresses the critical

duration (in milliseconds) as a fmnction of background luminance:

critical duration = -19.75 log L + 63.9, (1)

where Lum is background luminance in cd/ni2

vOf course, the minimum energy needed during the critical duration

is also a function of background luminance. Another equation cdii be

derived from this same study for threshold energy (a product of milliseconds and luminance) as a function of background luminance:

!i.log threshold energy = .648 . log L + .835. (2)

Taking the antilog of the left hand sidei and dividing it by the duration of the flash (if it is less than the critical duration) one derives the threshold luminance of the flash. Dividing the threshold

luminance by the critical duration, one can obtain the luminance needed

to detect a flash of a duration greater than the critical duration

(since threshold luminance is constant for durations greater than the

critical duration)-.

Roufs' (1972) data are similar qualitatively, alt~hough difficult

to compare with Herrick's because the former used the Troland as his

luminance measure which takes account of variation in retinal luminance

56
with variations in pupil size as background luminance changes (see

Appendix A). For this reason Herrick's experiment is closer to the

applied situation. However, Roufs' review of several studies shows

that estimates of the energy needed to detect a pulse may vary by as

much as a log unit between similar experiments. Still, Herrick's data

are at least a rough approximation of flash thresholds. These results

hold for the restricted conditions of this report regarding one stimulus configuration (a target of about one deg in diameter viewed foveally). Baumgardt and Hillman (1961) tested the validity of Bloch's

law for both large stimuli and eccentric viewing. They tested observers with stimuli ranging up to 80 in diameter at a location 200 in

peripheral vision. Bloch's law was still valid even for their largest

target, but their data are not in a form that allows us to calculate

changes in detectability with target size.

L_ Other data are available on the changes in sensitivity that occur

with changes in retinal location (see Haines, 1975 review). In this

area researchers generally used flashes of some long and constant duration. Riopelle and Bevan (1953, cited in Haines, 1975) examined absolute sensitivity at many points throughout the visual field against a

zero luminance background. Their results are presented graphically in

Figure 15 (from Haines, 1975). They used a 10 diameter target of white

light with a duration from 500 to 750 msec. Sensitivity was greatest

at ±50 temporal from the fovea and 150 to 300 nasal. Taylor (1964b)

measured sensitivity against a background luminance of 257 cd/m with

targets up to an eccentricity of 12.50. He varied the size of his

target from one to 120 minutes of arc at a flash duration of 333 msec.

His data are presented in Figure 16. The drop in sensitivity with

retinal location is less for larger than smaller targets due to the

greater spatial summative capacity of the peripheral retina. Figure 17

is a plot by Taylor showing the relationship between target size and

contrast (Appendix B) for a point just'above the fovea. Also reported

in this study are composite data from several studies on the change in

the size-contrast relationship at different background levels. Reproduced in Figure 18, these data were collected using continuously presented rather than flashed targets.

It has long been recognized that a steady dim target presented

to the peripheral retina will tend to become invisible after a few

seconds. This phenomenon of stimulus saturation is known as Troxler's

Effect. More recently though, Singer, Zihl, and Poppel (1977) described a related and more striking form of saturation associated

with flashing rather than steady targets. Their observers fixed 

a

central mark while a small spot was flashed for 500 msec every 4 sec.

After several minutes of repeated presentatiors the observer's threshold for these flashes increased between 0.5 and 1.0 log units. The

area of decreased sensitivity extended spatially beyond and surrounded

the flashed spot (27' to 116'). This adaptation effect increased with

retinal eccentricity, and persisted for 8 to 10 minutes in the absence

of further stimulation. Curiously, this decreased sensitivity could

be eliminated and normal sensitivity instantly restored if a target

was presented to a homologus area of the contralateral visual field.

The reduction in sensitivity could be prevented if the observer

simply moved his eyes towards the target every time it was presented.

57
iI

EJ87 LsW a A 33T,6* 2 .50

p.'

Lo-Ms 1& 0.

1- S 1oa M&T 14" OF4.3 

4.26ieas LOG W 0GA

r 292.6.

247.50 Q~I

X RIGT EYE FIELD

202.50 180 157.5" OF VIEW LMIT j

F i

F!

- I

- Figure 15. Distribution of absolute achromatic retinal sensitivitv for

: !binocular viewing. (From Haines, 1975).

58
S.60

1 38

L0 2 0 6 t0 12

Figure 16. Threshold contrast as a function of retinal position and

targe~. size for binocuiar photopic vision. Four observers, for which

the plotted average values represent 0.50 probability of detection in

a 11yes-no" experiment. Each data point is based upon 2400 observations. (From Taylor, 1964b).

59
1000

100

01

.001 .01 0.1

THRESHOLD CONTRAST (A) for P 0 -.50

B

Figure 17. Determination of the target size and threshold contrast

* dependency made under controlled conditions of central foveal fixation

Iand an invariant target duration of .33 sec. The data represent

averages from five observers who made a total of 45,000 observations,

using 18 target sizes. (From Taylor, 1964b).

60
SPLITFF

__FIELD

Lai

cr o 000 O~1

100 1 0-

I -. 001 .01 .1 11

THRESHOLD CONTRAST

Figure 18. Threshold contrast as a function of the diameter ofa

uniformn circ:ular -target. The assumption is made that asymptotic values of contrast would be reached in the case of a "split-field" (i.e., with

a target of infinite radius). Each curve refers to a different adapta- tionir luminance, expressed here in footlambe ýts. (The value in footlaniberts times 3.426 yields the value in cd/rn. (From Taylor, 1964b.) L _ _ _61
Although Singer et al. (1977) proposed ar, intriguing and complex physiological explanation for their results, it may be that the effect is

simply another manifestation of the fact that continued attention to 

a i Icentral fixation point reduces peripheral sensitivity. (Webster &

Haselrud, 1965; Ikeda & Takeuchi, 1975.) Therefore, when peripheral

stimuli are to be detected while attention is focused elsewhere care

must be taken to avoid the consequences of the Troxler effect.

In the case of visual effects which are produced by two or more

flashes presented to a single retinal locus in rapid succession, 

a

single rule cannot be adduced. Specifically, the effect which a 'Flash

of light produces in the visual system does not end when the light is i extinguished. Ikeda (1965) and others believe that the neural circuits

of the visual system "ring" after being stimulated by a flash, like 

a

bell or an electronic circuit may react when a pulse increase of voltage is applied to it. Figure 19 shows Ikeda's ccnception of the dampened oscillations that may follow a flash. These curves are derived by

stimulating the eye with two (12.5 msec) pulses of light separated by

varying intervals, t, shown on the abscissa.

On the ordinate 0.3 indicates complete summation of the two

flashes, as would be expected if there are zero intervals between them; r• an ordinate value of zero indicates no summation (i.e. the two flashes

together are only as dete'table as one flash alone). The (a) curves

are foý increments of light and the (b) curves for decrements (i.e.

diminishing). Where either curve reaches its minimum, the visual

system behaves as though there were inhibitions between the responses

to the two flashes; the combined effect of the two flashes is less than

the effect of either flash alone. Uetsuki and Ikeda (1970) produced

similar data for different background luminance levels. The interval

at which this minimum detectability occurs changes with background

luminance to over 100 ansec at low levels.

Herrick (1974) found that most of the literature he reviewed 

]

showed a similar U-shaped relationship between the inter-flash interval

and detection. Moreover, this relationship was relatively independent

of flash duration. Perhaps of more interest to design engineers is the

period for which a second flash will join with a preceding one. Roufs

(1973) using 2 msec flashes, found that they would sum completely as

long as the interval between them was not greater than one third of the

critical duration found for his single flash experiments at the same

mean luminance. Similarly, Herrick (1973a) finds this "two-flash

critical interval" for 5 msec flashes to be less than one half of the

critical duration for a single flash (Figure 20).

Herrick (1973a) studied the effect of trains from 2 to 12 flashes.

Each flash was 5 m~ec duration, but the inter-flash interval was varied. Herrick defined a quantity which he called the "critical duration". For a given number of flashes, this is the longest duration

over which threshold is determined by constant energy. As is usually L .the case, duration is measured from the onset of the first flash to the

offset of the last flash. For example, using a display consisting of

three 5 msec flashes, if the "critical duration" was 70 msec, one could

deliver the flashes with 5 msec between each and total duration would

I 62
SZ•.• 0(a)

0.3

0.2 A

\0

S~+ ,

0.1

50 100 msec

Interva' 1 0.3 (b)

S0.2

''

0 I , , "-p• O.. _•v p-. p p

50 100 msec

Intervul 1

Figure 19. Summation index (a, ) as a function of the interval between I; two flashes of 630 nm for two adapting levels from the subject Ml.

Crosses for 328 trolands and open circles for 61.2 trolands. Panel a

shows data for summation of two incremental pulses; panel b shows data I..for sununation of two decremental pulses. (From Ikeda, 1.965.)-

63 (b)
I i

.,! I~00 ,,,,"

~~12

too ,,I

•i g >60 \

I 20-

2 - .0 0+ .-1000+ 

ii

-II

~~64

, 0- 2.0 - 1.0 0.0 +1.0 + 2.0

•..i• 1LOG BACKGROUND LUMINANCE (mL)

•• i Figure 20. Critical duration as a function of background luminance.

• .1 The number next to each curve gives the number of identical 5-msec

:•' I flashes. The term T_ refers to the critical duration of a variable-

:i I duration, single-flasr experiment. (From Herrick, 1973a).

i 64
equal 25 msec; or with 20 msec between each flash for a duration of 55

msec. All that matters is that total duration does not exceed 7C msec. fl Figure 20 from Herrick (1973a) shows a set of curves for this critical

duration as a function of flash number and background luminance.

Herrick also found that the energy constant associated with each critical duration was independent of the number of flashes, including single

flashes, so that equation 2 may, be used to calculate this energy value.

Also, at any background luminance, the interval between flashes that

makes them least effective in stimulating the eye, is independent of

the number of fTashes.* Figure 21 lists these intervals for the background luminances at which Herrick made measurements. The threshold

luminance necessary to see the flash at this "least effective" interval

is also independent of flash number, but is dependent on background

luminance. The relationship between the threshold luminance and background luminance is provided by the following equation, derived from

Herrick's data:

SThreshold luminance = .296L + .9508, (3)

where L is background luminance in cd/m2.

The response of the visual system to multiple flashes in the periphery is similar to that at the fovea, except that the loss of sensitivity with increased inter-flash interval is monotonic rather than

U-shaped. Herrick (1973b) presented flashes of about 10 in diameter,

at a location 14.20 in the periphery against a completely dark background. Critical duration was about 70 msec for two10 msec flashes

and 100 msec for multiple flashes. Flash duration had no effect over

the range of 5 to 20 msec. Van den Brink and Bouman (1954) compared

critical duration for 2 flashes presented at the fovea and up to 100

into the periphery and found no change in critical duration. They also

found that critical duration was independent of changes in spatial

parameters of their targets.

I

*It should be noted that while this flashing light can be useful to

compel attention, it may also be employed to "fool" the eye - for

exarple when countermeasures are necessary for purposes of security.

65
80

LI

40

20 I

-. S 0 .5 1.0 1.5 2.0

Background Luminance

(log cd/ m2)

Figure 21. Inter-flash interval providing the highest threshold as a

function of background luminance. (Calculated from Hlerrick, 1973a). I __ 66
CHAPTER 9

[ Flicker Sensitivity

After examining the visibility of flashes of light and short flash

trains, the next logical step is to review data on the visibility of

lights wkiich flicker continuously. Although research on visual flicker

has been a scientific endeavor for over 200 years, most of this research has unfortunately been concerned with just one particular phenomenon -- Critical Flicker Fusion (CFF). This is a measure of the

temporal frequency at which a modulating light will appear to flicker,

or the frequency at which the flicker appears to fuse. Ginsburg (1970)

compiled a bibliography on CFF covering the period 1953 to 1968 and

numbering 1293 articles. This information is certainly useful to the

designer who wishes to present information intermittently, but have it

appear flicker free, as in television and film systems.' However, for

other applications (e.g., warning lights) the designer may want the

flicker of the light to be seen and desires to know how much i5ihodulate a light of a given temporal frequency to insure that the modulation is perceptible or maximally identifiable. For example, to keep

secure an electronic data link, it might be possible using flicker to

recombine with the eye, a coded interrupted signal. This latter problem will be discussed, along with the critical flicker fusion literar ture, to summarize what is known about the effects of display size,

color, area, etc. on general flicker sensitivity.

The first issue concerns how sensitive an observer is to different

frequencies of flicker. Kelly (1961a, 1961b) created what he called

the "standard observer" by combining flicker sensitivity data for eight

observers. He used a Ganzfeld (full field) light and varied the amount

and frequency of its miodulation. The waveform of the modulation was

sinusoidal (Figure 22) and modulation (m~) was between 0 and 1.* M--1

means that the peak luminance of the light is twice the mean luminance,

and that the minimum of the wave is zero luminance. This is the maximum amount of modulation; m =0 means that the light is on steadily at

[the mean luminance. Kelly's data are plotted in Figure 23. The left

hand ordinate is in terms' of m. The observers are most sensitive

(i.e. , the lowest value of mn needed to see the flicker) at a frequency

between 10 and 20 Hz (not 4-6 Hz as suggested in MIL STD 1472B).

Observers are less sensitive at frequencies above and below this range.

At approximately 70 Hz, maximum mn is required for flicker detection.

The variance among observers is greatest at 16 Hz, near peak sensitivity. When a single observer is equally sensitive to two frequencies,

one high and one low, variance in his threshold would be substantially

greater for the lower than for the higher frequency.

1 Lma Lmi hr ia aiu uiac n Lina max

Lx+Lmin Lmin minimum luminance of the waveform

67
i

I 2vI

2w

l w I

I ,2mL- .. -- " --

" L .L I W

Time(t) WII

S.3L

Figure 22. Stimulus waveform used by Kelly and others. This waveform

is the sum of a sinusoidal component, of amplitude ml.., and a constant

component, L. The time-average luminance, L, is often called the

"Z "adaptation- level", and the dimensionless ratio, m, is called the

"modulation". (From Kelly, 1972).

P j68
.00l iI

I.

,O6 KELLY .006- D*A)

•M "IMO0AN t Ia Oin

:0 t

I 0 e0

II'e

2 5 10 20 50 2 S 10 20 60

FREQUENCY ("a) FREQUENCY (it)

Figure 23. Photopic modulation sensitivity data. Th curves on the

left were obtained with a large flickering field; those on the right

with a small flickering spot on a steady surround (Kelly, 1961;

deLange, 1958, as reproduced in Kelly, 1972).

69
It is felt that the most important display parameter from a design

standpoint is mean luminance; which controls the adaptative stat.e of

the eye. Two rather complete sets of data on this are pres, ited in

Figure 23 (from Kelly, 1972). The left-hand data are from Kelly

(1961a) for a 650 diameter target; the right-hand data are from deLange

(1958) with a 2* target surrounded by a steady field of the same mean luminance as the target. There are four important characteristics of

these data. (1) At low frequencies, threshold modulation is constant

across mean luminance (except for the lowest luminancpi level). This

means the data obey the Weber-Fechner law, threshold = K. From a

slightly different perspective, the threshold amplitude of the modulation (m) increases proportionately with mean luminance. (2) at low

temporal frequencies, the visual system is less sensitive to the larger

target than to the small one; less than 2 percent modulation is needed

*to see deLange's target while more than 6 percent is needed 'for

Kelly's. (3) Peak sensitivity occurs at a higher temporal frequency for the large than for the small target. (4) At frequencies greater than 10 or 20 Hz, target size is not as important, though critical

flicker frequency is higher for the larger targets (this reflects the

7 Granit-Harper law, to be discussed later).

The display designer probably does not come across many problems

involving the sinusoidal modulation of light; most real-world problems

are of square or rectangular modulation ("on" or "off"). However, the

reason Kelly, deLange, and others are so interested in sensitivity to

sinusoidal modulation is that one can predict threshold sensitivity for

almost any temporal waveform shape from the sinusoidal data. The

design engineer considers the eye in the same way an electronics engineer considers an electronic audio amplifier. Knowing the "frequency response" of the amplifier allows the engineer to calculate the re- [. sponso of the amplifier to a complex sound such as speech or music.

This - the main application of the mathematics of the Fourier analysis. (A simple treatment of this can be found in Bracewell, 1965).

Since this Fourier approach permits generalizations from one set

of conditions to another, an example of its application is considered

in this section. Consider the response of an observer to a train of

square or rectangular flashes, with flash duration not necessarily

equal to the inter-flash interval. According to the Fourier theory, we L .can consider the flash-train's waveform as equivalent to the sum of

many sinusoidal waveforms of different frequency, modulation, and

phase. One of these, the first component is the same frequency as the

flash-train and has a greater modulation than the other sinusoidal components of the flash-train's Fourier representation. Moreover, the

component most deeply modulated, determines threshold. The equation

for this modulation (mi) is:

ms'= sin (TR) * m (4)

TrR f(4

Where R is the pulse-to-cycle fraction, the on-time of the flash di'•

vided by the sum of on-timt and off-time, and mf is the modulation of

the flash-train. The flickering light can now be treated as a sine

wave of modulation m, and its expected threshold read from Figure 23,

using the average luminance of the flash-train as mean luminance. For

70

iJ
an example of these types of calculations see Kelly (1961a, 1961b).

This will only be approximate since the designer will probably not have

a light of exactly 2 or 65 degrees diameter and may not use units of

retinal illuminance (see Appendix A).

These linear models of flicker perception also can account for a

phenomenon which is counter-intuitive. A light may be flickering at a

frequency far above CFF, yet if the frequency suddenly changes, the

transition may be seen (Sen, 1964; Forsyth & Brown, 1961). Bird and

Mowbray (1973) examined this phenomenon parametricaqly using a two

degree diameter target at a mean luminance of 398 cd/m , with the light modulated at 88%. Their data are summarized in Figure 24; the dashed

lines are predictions based on their linear model. If the period of

F the first flash-train is 2 msec (500 Hz), then looking at the right

most line, the period of the second flash-train that will produce a

I Lvisible transient 100% of the time is 6 msec (167 Hz).

Levinson (1968) found that transients could also be produced by a

sudden change in the modulation of a light flickering above CFF. Under

conditions which produce a CFF of 50 Hz, his observers could see a 20%

change in modulation at a frequency of 100 Hz. This phenomenon is also

accounted for by linear models. The important point, however, is that

the design engineer should not assume that a display will be flicker

free just because it is operating at frequencies far above CFF; frequency or amplitude modulation may produce visible "side-band" frequencies. Additionally, vehicular motion may potentiate these effects.

Kelly (1974) used an adaptation procedure to isolate the spatiotemporal sensitivities of chromatic mechanisms which may well correspond to the early processing stages of the color system. Because the

spectral sensitivies of the three chromatic mechanisms overlap so much,

it is unlikely that many non-laboratory displays would actually mimic

the effects which Kelly observed. Still, the differences in spatio- temporal responses of the three isolated mechanisms (shown in Figures

25 and 26) can be used to advantage in display design. In particular,

the limited temporal and spatial resolving capacity of the bluesensitive mechanism should be taken into account if one seeks a display

capable of evoking high resolution responses in either domain.

Other variables that affect an observer's sensitivity to flicker

will be considered in detail. Since contrast sensitivity functions

analagous to those which deLange and Kelly collected for foveal vision

are lacking, the authors have to rely on a flawed indicator of flicker

sensitivity, the CFF frequency. The higher the CFF, the more sensitive

the observer is to high frequency flicker under the specified conditions. Unfortunately, this simple assertion is based on some tenuous

assumptions. 'If one seeks to know the highest frequency at which flicker can be seen under some particular set of conditions, CFF i~s

quite appropriate. But there is a less-than-perfect correlation between the peak of temporal modulation sensitivity function and the

cut-off frequency (CFF) for that function. This imperfect correlation

means that one can not use CFF measured under some conditions to predict the temporal frequency which, under those same conditions, would

yield the most pronounced sensation of flicker (i.e. the peak frequency

71
PIUZO-JW U IMMz

L

0(0,, 2000 ud)

4I

-II //

/" 1t- t-// (0- - Iu

, / I/ /

L I //

// -1/

J/ . 1. / I/V /

0 2 4 1S

t2

I'

J

Figure 24. Partial visibility ranges representing frequency of seeing

measurements for period-jumps t 1 -t,. Theoretical lines of constant sensation (sub- and supraliminal). (from Bird & Mowbray, 1973).

i I72 S iI
I I I I

0.002

r 0.005

0 0

S I0.05 -

S0 .0

0.1

0.2

ICL

0.1 -1

A: 0.5

1 2 5 10 20 50

TEMPORAL FREQUENCY (Hz)

Figure 25. Flicker sensitivities of color mechanisms with optimum

grating patterns: 3 c/deg for green (squares) and red (circles); 1

c/deg for the blue (triangles). Retinal luminance equals 10 td.

Filled symbols show comparable data points transferred from Figure 26.

(Fro.m Kelly, 1974).

73
•! I I ILI

0.002

I I - 0.005 I

S0.01

~0.05---

0.i --

0.02

0.5

0.2 0.5 I 2 5 10

FgrSPATIAL FREQUENCY (cycles/degree)

Figure 26. Contrast sensitivities of color mechanisms with optimal

flicker frequencies: 8 Hz for the green (squares) and red (circles);

1.5 Hz for the blue (triangles). Filled symbols show comparable data

points transferred from Figure 25. (From Kelly, 1974).

74 Lz_ j
of the temporal modulation function). Nor, could one use the CFF to

estimate any other desired point on the temporal modulation function.

For example, the fact that different sets of conditions yield quite

different CFFs, does not guarantee that if one presented the two conditions, each modulated at one half its own CFF, the two rates of

flicker would appear to be discriminately different.

Despite its weaknesses, CFF is often the only available datum we

have on some parameter's effect on temporal responses. For example,

consider the effect of the size of a flickering object in central

vision. In general, CFF increases linearly with an increase in the log

of an object's area (the so-called Granit-Harper Law). Traditionally,

area has been expressed as mm on the retina, but one can convert this

measure to diameter in degrees (assuming a circular target) as follows:

diameter = arc tan (.0676 A), where A = mm2 on retina. (5)

Kugelmass and Landis (1955) summarized the data of six experiments

that tested the validity of this law and found that it held well for a

range of stimulus diameters of from 1' to 5* of visual angle, corresponding to a range of CFFs of 15 to 45 Hz. Roehrig (1959a, 1959b),

however, found that the law held well for diameters up to 300. He also

tried to find the limit on CyFs by using the largest target (49.60) and

highest luminance (1875 cd/mi) that his apparatus could produce. Under

these conditions he recorded CFFs ranging from 80 to 107 Hz for five

observers.

However, it can be seen from Kelly's data that CFF is influenced

by mean luminance; the Ferry-Porter law states that CFF increases

linearly with log luminance. In order to predict flicker fusion frequency, one needs to take both laws into consideration simultaneously.

Kugelmass and Landis (1955) found that there is an interaction between

r, area and luminance; the slope of the Ferry-Porter equations change with

changes in area of the target. However, Foley (1961) obtained a good

fit to his'data using an additive rather than an interactive model. We

L have followed suit and used an additive model based on Roehrig's data

which covers an even greater range of target areas than does Foley's.

Thus a rough prediction of CFF can be made using the equatton:

CFF =27.66+6.25 (log A) + 14.7 (log L)

2 2 where L is luminance in cd/m2 and A is area in mm on the retina.

It would be interesting to verify this relationship between area

and luminance at different retinal locations. However, to the authors'

knowledge, no one has tackled so large a task. But there is some

partial evidence on the effect of retinal location. Ettinger (1959)

found that when targets are larger than 1.50, the periphery of the

visual field (150 to 350) tends to become more sensitive relative to

the fovea. Brooke (1951)' measured CFFs at several retinal locations

and at several 2 levels of luminance for a 20 spot. He found that at

high (3000 cd/mr ) luminances, log CFF fell nearly linearly from 50 to

20 Hz as the spot was moved horizontally from zero to 40* from the

retina. At .3 cd/m however, CFF was constant across the retina. At a

75

I , _, ;__ •
lower luminance, .03 cd/m, at all locations 100 or more from the

fovea, he measured higher CFFs than the foveal location, approximately

13 Hz vs. 7 Hz. Hecht and Verrip (1933), as reported by Pirenne (1962)

found very similar data, also for a 20 target. For example, with a

luminance approximately equal to 0.4 cd/m4 (actually reported as one

Troland retinal illuminance) they found CFF equal across the retina.

Brooke (1951) reported the same outcome for virtually the same luminance, 0.3 cd/m . Hylkema (1942) has shown that the difference between foveal and peripheral CFF diminishes with advancing age. This factor

may be of more importance for display design in the private sector than

in the military however.

Welde and Cream (1972) have shown that the sensitivity of the

peripheral retina to flicker must be taken into consideration in any

applied setting. They were interested in the perception of flicker in

television screens and asked observers to report on the flicker of 

a

television screen seen in the periphery. The screens always flickered

at the standard 60 HZ interlaced rate, subtended about 300 of t~e

visual field and had mean luminances ranging from 10.3 to 30.8 cd/m

Flicker was most easily seen with fixations 300 to 600 of angle from

the television set. Note that the periphery of the retina is more

sensitive to flicker than the fovea in this study even though luminance

is about a log unit higher than in the previous studies. This difference is probably due to the greater retinal size of the television

targets.

The spatial composition of the flickering display is another important parameter. Using a modulation sensitivity measure, Kelly (1959) found that the observer's sensitivity to low rates of flicker

can be increased by the presence of sharp contours in the visual field. For example, blurring the edges of a 30 spot maximum reduced sensitiv- ity to the spot to low frequency flicker by one half; the same manipulation did not affect high frequency response, including CFF. Using a

16' x 8° flickering, bar-pattern, Kelly (1972) found that low frequency sensitivity was most enhanced when the spatial frequency of the grating

was two cycles per degree of visual angle. Similarly, Waygood (1969) found that the detection of very slow continuous changes in luminance (6 to 4% per second) was enhanced by the presence of contours.

Returning to Welde and Creams' 1972 work on television flicker,

one should note that they found no effect of variations in the spatial "parameters of the images (image complexity). This agrees with Kelly's

general findings of little effect of spatial parameters on the CFF.

Likewise, color seems to have little effect on CFF over a range of

luminances. Giorgi (1963) reviewed the literature on the effect of

color on CFFs and found contradictory evidence -- some researchers

found an effect, but many did not. In a very carefully controlled

experiment, Giorgi did find an effect of color but it was very weak,

and probably had not risen above experimental variance in many other

experiments. However, Hecht and Shlaer (1936) found that while there p.. seemed to be no effect of color over most of the useful luminance

range, as the luminance values begin to encroach on the scoptic range

76

__ 

*
2., ,i (less thgn 3 cd/m2 ) color does begin to have an effect. So, at about

.03 cd/m , a green object produces a 10 Hz greater CFF than a red one.

Finally, brief mention should be made concerning the effect of

observer variables on CFF, as this has been an area of great interest

both to clinical and human factors researchers. Many observer variables have been studied, such as age, intelligence, fatigue, stress,

emotional state, etc. (see Ginsburg's bibliography, 1970). However,

age produces probably the most dramatic effect on CFF. Landis and

Hamwill (1956) reviewed the literature on the correlation between age

and CFF and found values ranging from r = 0 to r = -.74. Misiak's

(1951) work is often cited because of his large numDer of observers

(320) and substantial age range (7 to 89 years). Although he found a

significant correlation (-.55) between age and CFF, Misiak's range of

mean CFFs (for each age group) was only 43 to 36 Hz, and he says "that

there were CFFs at age 82 as high as at age 7, and there were CFFs at

age 7 as low as at age 80." This is to be expected from Kelly's

(1961a, 1961b) data which show that ± 1 S.D. spanned a range of 8 Hz,

yet-the effect of age in Misiak's study was only 7 Hz. Very probably

if frequency is employed for coding purposes in visual displays this

CFF test should be part of medical screening in the same way that other

visual acuity tests are conducted.

Weale (1963) reviewed the work on CFF in senescent observers and

proposed that some 70% of the age-related decline in CFF was due to

simple optical factors. It is well established that with advancing age

the crystalline lens "yellows" and loses some of its exquisite transparency; it is also well established that pupil diameter decreases with I

age (senile miosis). Combined, these factors would produce a substantial decrease in retinal illumination, a decline which the Ferry-Porter

Law (see above) would translate into a drop in CFF.

77 A'-- - m
CHAPTER 10

Suprathreshold Flicker Perception

The previous chapter reviewed the literature on the "thresholds"

for the detection of temporal events: the threshold luminance increment needed to see a flash, the modulation of background luminance

necessary to see a flickering light, and in the case of CFF, the fre.-

quency of a flickering light at which one can barely see flicker. This

section will review research on the perception of temporal events which

are suprathreshold. Given that some flash or flicker train is clearly

visible, what does it look like and how does the observer respond to

it. Among other things this section shall examine the perceived duration of a flash, the perceived rate of flickering lights, how well one

can discriminate among flicker frequencies, and how rapidly one can

react to temporal events.

Fundamental to all these concerns is the correspondence between

r !the perceived duration of a flash of light and its actual duration.

The imprecision of this correspondence is well established and, in

particular, the shorter the duration of some flash, the less well does

its perceived duration correspond to the actual duration. Thus no

matter how short a time the light is on, it will appear to be on for

some irreducible minimal amount of time. Efron (1967) relates an 1887

paper by Charpentier that demonstrated this clearly. Charpentier asked

observers to discriminate between a 6 msec flash and another flash

whose duration varied from trial to trial. Observers could not tell

* •the difference between the lights unless the variable one was at least

66 nsec in duration or longer. From this and other data Efron reasons

that short lights always appear to be about 50 to 70 msec in duration.

In related work, Haber and Standing (1970) measured the perceived

duration of flashes by asking observers to adjust the onset of a click

sound to appear simultaneous with light onset. They also had them

match the click to the perceived off-set of the flash. The difference

in time between these two clicks is an indirect measure of the percei'ved duration of the flasP. The stimulus was a matrix of black

letters on a white (16 cd/m ) background about 50 square. The flash

could be preceded by an adapting field of the same luminance, and could

be, followed by the same field. The results are shown in Figure 27 for

"r .the conditions determined by the presence or absence of the adaptation

field (from Haber & Hershenson, 1973). If perceived duration always

equalled the actual duration, the points would fall on the straight

diagonal lines. But perceived duration was usually longer, a fact

which Haber and Standing attributed to perception persistence after

flash off-set. In support of this hypothesis, Haber and Standing were

[. .able to eliminate the difference between perceived and physical duration by presenting a dense masking pattern immediqtely after the offset of the flash. This operation could be expected to eliminate persistence. As Figure 27 shows, persistence was greatest when the flash

occurred in the dark (over 400 msec for a 20 msec flash) and was re- F duced to 175 msec when the flash occurred in the light.

78
TVI THIRATIfON nO T IRi PRENTS/137

onset

cdick

A

Offset click

IOZZIZ'

700

400

300 E • Dark adlapted

€ 200

b" 100 o

S 0

a 70nl

Sol'

50n400

I--

SSoo , -

S200 Light adapted

* 100

0oO 200 300 400 500 700 1000

L

Figure 27. Schematic representation of the presentation sequence of a

light pulse with a click heard near its onset or near its offset (top)

and the resulting mean interclick interval as a function of exposure

duration of the pulse in both a light and dark adapted condition (from

Haber & Hershenson, 1973).

79

11J " : ' , m •- rO NA
In an ingenious experiment, Eriksen and Collins (1967, 1968)

examined the behavioral consequences of this post-stimulus persistence

of perception. Eriksen and Collins flashed a quasi-random dot pattern

for 6 msec and then, after a variable interval, presented a different

dot pattern for 6 msec. If the dot patterns have been Dresented simulI

taneously, somz of the dots would have formed a 3-letter nonsense

syllable. The observer's task was to identify the nonsense syllable

from the two temporally separated components. As the interval between

the flashes was increased from zero to 100 msec, performance decreased

steadily; for longer intervals still, performance was nearly constant.

It seems then that the observers were able to combine the persistence

of the first flash with the presentation of the second one to perceive

the combined pattern. The background luminance for one of these

studies was the same as for Haber and Standing (16 cd/mr). Considering

the difference in method, the correspondence between the two is quite

good.

The work by Eriksen and Collins shows that though observers can

discriminate separate temporal events, that is, see flicker, they can b

still integrate the information from several flashes. Analagously, 

a

television raster display might paint its picture at such a slow rate

that flicker would be perceived, yet the observer could still tell what

the image was. Indeed, Welde and Cream (1972) found that flickering in

!a television image did not seem to disturb the observer's perception of

the image depicted on the screen. However, precisely this same persistence might' make it difficult to discriminate between flashes of light

on the basis of duration. This recalls Efron's claim that observers

can not discriminate among flashes less than 60-70 ms. Haber and

Standing's data indirectly suggest that this may be true of even longer

flashes, depending on light adaptation conditions. Note that in the

dark adapted condition, Haber and Standing's curve for flashes ranging

up to 300 msec is quite flat. This suggests that all these flashes

r appeared of equal duration, 400 msec. Of course if observers directly r compared these flashes they* might be able to discriminate between them

based on other cues, such as brightness differences. However, the data

suggest that flash duration may not be a good way to convey information

to an observer, at least for durations less than 500 msec.

"For many applications, the display designer may not care how long

a flash seems to last, but only how rapidly an observer can respond to

- lthe flash. It would be helpful to have general rules relating reaction

time to changes in the parameters of a flash. The first rule one might

consider is Bloch's' Law: below some critical duration there is 

a

trade-off between duration and luminance of a flash so that reaction

time is determined by the energy delivered by the flash. Both Raab and

Fehrer (1962), and Grossberg (1968) have examined this question and

come to the, conclusion that unlike threshold visibility, reaction time

does not follow Bloch's law. Raab and Fehrer found that if such 

a

trade--f-f existed, it held only for flauhes less than .5 msec and for

flash luminances greater than 100 cd/m (against a dark background).

Grossberg (1968) used even shorter durations, but found that reaction

time was dependent on a combination of energy and luminance. In both

cases, *as luminance increased it quickly became the single controlling

1~ 80
-- .

parameter, evidently because an observer reacted not to the entire time

interval of the flash, but only to its onset.

Bartlett and MacLeod (1954) were interested in how reaction time

depended on background luminance, flash luminance and retinal location.

Reaction times were measured using a small spot (20'). located either at

the fovea or 110 20' above the fixation mark. The flash was always 575

msec. Figure 28 shows data from two subjects at zero background luminance and two retinal locations as a function of flash luminance. Note

that reaction time to dim flashes is faster in the i.eriphery at this

low background luminance level. This was true for ooth dim and dark

backgrounds but not for the brighter one. Bartlett and MacLeod fitted

a family of curves to their data of the form:

+ k (7)

Reaction Time 1

B log (I/Io)

where B, k, and I. depend on field luminance. Io is a unit of "effecr! tive" field luminance; it was adjusted for different retinal positions and subjects. The values for these parameters are provided in Table 4, taken from their paper.

We can compare Grossberg's, and Bartlett and MacLeod's data only

at zero background luminance as Grossberg did not use any other value.

Reaction time to Bartlett and MacLeod's spot remains at approximately

200 msec until flash luminance drops below .• log cd/m ; then it increases to about 500 msec at -1.5 log cd/im. Grossberg's reac ion

times (for a 500 msec flash) increaped from 280 msec at .5 log cd/m to

approximately 500 at -2.3 log cd/m . Unfortunately, Grossberg did not

use a wide enough range of flash luminances to estimate the luminance

value at which his rection time would asymptote.

The effect of shortening flash duration can be seen by comparing

Grossber•'s data with those of Raab and Fehrer. At a luminance of .57

log cd/mr (the only flash luminance value that overlaps the range used

by Grossberg), Raab and Fehrer's range of durations was 20 to 2 msec

and reaction time decreased monotonically from 280 msec to 220 ov r

this range. For Grossberg's flash luminance value of .46 log cd/mr

reaction time decreased from 360 to 290 msec over the same range of

flash duration. The absolute change in reaction time is about the

same. Grossberg's reaction times do not change for durations about 20

nmsec but one cannot look for the same trend in the data by Raab and

Fehrer because they did not use any durations above 20 msec. However,

only Raab and Fehrer use high flash luminances. Their curves showing

reaction time as a function of flash duration are asymptotic for flash

luminances greater than 2 log cd/m.

Haines 41975)2oresents data for reaction time to a small (45') dim

(2.29 x 10 cd/mr), white spot, flashed for 50 msec. He tested 72 Slocations in the visual field and plotted his data in terms of iso-response curves (Figure 29). These data suggest that the observer may

respond faster to a peripherally viewed spot than one at the center of

the visual field, but do not suggest as great a difference in reaction

81
S14

K I M Rnd 8

S0 - Periphery

.V - Fovea

.2

-3 -11.3

LOG FLASH LUMINANCE

Figure 28. Reaction time and 'log flash luminance for the dark-adapted

eye. Each curve is based ci means of both subjects; one curve representing peripheral and the other curve foveal data. (From Bartlett &

MacLeod, 1954).

82
H i TABLE 4

Constants employed in fitting hyperbolic functions

to latency-flash functions of subjects B and M

for peripheral (P) and foveal (F) stimulation

The constants are defined in the text.

Subject B Subject M

Field P P F,

(log mL)

luminance B logIO K B logIq K B .logI K B loQI, K

,0o 0.002 -4.7 140 -. 005 -2.9 157 0.002 -4.4 105 0.007 -2.7 119

-0.4 0.008 -1.4 190 0.008 -2.0 179 -. 007 -1.3 145 0.009 -1.8 145

+1.6 0.021 +0.6 200 0.028 0.0 194 0.014 +0.7 160 0.028 0.2 155

"I83

II

* *

L8
.. "4

WHITE

2.29 x 10- cd/cm2

270 m•/

1 • 50"

FIELD OF VIEW ""

BINOCULAR 1800

MONOCULAR

Figure 29. Iso-response time zones for the binocular visual field and

white stimuli. (From Haines, 1975).

V __84
times as do the data of Bartlett and MacLeod. It shows however that

observers respond to flashes in the horizontal meridian faster than to

flashes positioned along the vertical. A similar asymmetry between

sensitivity and motion difference sensitivity (see related chapters of this report). This knowledge can be easily employed in to improve

detectability of information coded in visual displays.

Tolhurst (1975) suggests that the spatial characteristics of an

object may be important for reaction time, especially when the visibility of the object is marginal. He measured the reaction time to

flashed gratings presented about the fovea. The contrast of the patterns was very near threshold. When the spatial frequency of the pattern was low (0.2 bars/deg; bars of 2.5 deg width) the observer reacted

to the transients in the flash, i.e., either to the sudden on-set or

off-set. When the spatial frequency was higher (3.5 bars/deg; bars of

8.5' of arc width), reactions occured at anytime during the duration of

the pattern. This was also true of the low frequency patterns if the

on-set and off-set of the flash was gradual. This implies that for low

spatial frequencies, the visual system is sensitive to temporal transients, but for high spatial frequency patterns, it integrates contrast

r over time. This suggests that for experiments using square or circular

targets, the size of the target could affect reaction times.

Short flashes may seem much longer than they really are, because

the perceived flicker rate of a flashing light may be less than its

act~ual rate and the persistence of one flash would interfere with the

perception of the next. Therefore, there seems to be a maximum number

of flashes per second that we can perceive regardless of the frequency

of the target or its CFF. This limit is estimated to be surprisingly low: about 6 to 8 Hz (Cheatham & White, 1952; Forsyth & Chapanis,

1958; White & Eason, 1966).

r Bartley (1938) noted that just below CFF the perceived rate of a

flickering light always seemed to be the same. Since a dim light has a

lower CFF than a bright one, the dim light's actual flicker frequency

would be less than that of the bright one; yet the two would seem to be

equal. This means that close to CFF the perceived rate of a light t increases with a reduction in luminance. LeGrand (1937, as reported by

LeGrand 1968) found that a 70 flickering light appeared to flicker at a

reduced rate as it was viewed with eccentricities increasing from 150.

In this case, the background was dark so the luminance may have been

more effective at the more peripheral location, which would bring this

observation into line with Bartley's law concerning brightness enhancement.

However, these findings do not agree with measures of perceived

numerosity. In this case, an o srver had to count or guess the number

of flashes presented in a short train of flashes. One would think that

these results would be in accord, ' qualitatively, with changes in the

V perceived rate of a continuously flickering light, but they are not. [ White & Eason (1966) measured perceivied numerosity as a function of a

number of flashes of light (presented at a frequency of 2J Hz) for

X three light adaptation levels (34.26, 342.6~, and 3426 cd/in ) and at

85
retinal eccentricities up to 700. They found that in the first 300

msec of the flash-train (6 flashes), the number of flashes seen was

affected by adaptation and retinal location (lower background and

greater eccentricity meant fewer flashes were seen). For the rest of

the flash-train, flashes were seen at a rate of 6 to 7 Hz. White and

Eason report that observers did notice a dramatic change in perceived

rate as retinal location was aried, yet this did not affect numerosity

Tju- ments. They offered no explanation for this paradox.

Forsyth and Chapanis (1958) performed a similar experiment except

that they varied frequency and location. Their target way a circle .50

diameter and had an incremental luminance of 5663 cd/mn against a 70

cd/m background. They also found that the limit of perceived rate is

6 to 8 Hz, but their curves show perceived number of flashes as a

function of actual number to be linear (Figure 30). They feel that

this is due to averaging across many observers (n=36). Table 5 provides the slopes of these lines for every condition of location and

frequency, and completely describes their data. For a flash-train of

20 flashes, an increase in retinal location amounts to a maximum loss of 3 to 4 flashes.

The difference in the reported experiences of perceived rate

versus perveived numerosity suggests that care must be taken in transferring this information into design criteria. It is felt that the

display designer should consider carefully the exact nature of the

information that an operator will need to extract from a flashing

indicator before applying the data from either of these approaches.

Previous research shows that no matter how high-the actual flicker

frequency of the light may be, the maximum rate of flicker that an

observer can perceive is 6 to 8 Hz. Yet observers are quite sensitive

to small differences in frequency, according to a series of studies

done by Mowbray and Gebhard. Depending on the particular conditions

used, they found from 280 (Mowbray & Gebhard, 1955) to 375 (Gebhard,

Mowbray, & Byham, 1955) just noticeable differences over a range from 1

to 45 Hz. The latter report that this just noticeable change (A F)

never exceeds 6 Hz (this at 22.5 Hz). Mowbray and Gebhard (1960)

examined the fovea and peripheral locations up to 300. As soon as the

.5' diameter spot was positioned outside of the fovea (in this case 5°+

of eccentricity), A F no longer peaked at 22.5 Hz, but decreased

monotonically from about 2.5 Hz to 2 Hz over a range of frequencies

from 5 to 35 Hz. Differences between the A F from experiment to experiment, may be due to differences in luminance and spot size. The high

discriminative ability, 375 just-noticeable-steps, found by Gebhard,

Mowbray and Byham, (1955) was probably due to their experimental

method: two frequencies were presented successively with no interval

between, thus the observers would have seen transients as the frequencies were switched (see Flicker Sensitivity). However, this does

not account for the high difference sensitivity found by Mowbray and

Gebhard (1960). Since this latter study allowed a one second interval

between presentation of the to-be-compared frequencies.

86
K16 -RETINAL DISPLACEMENT 4Q0

14 * 2.5 /SEC,

-5.0/SEC.

r~I & 15.0/SEC.

10-

LUJ

LU

2 4 6 O8 10 121416 18 20

Figure 30. Mean judged number as a function number of flashes presented at a retinal placement 40' temporally from the fovea. (From

Forsyth & Chapanis, 1958).

87
TABLE5

Slopes of the Regression Lines Relating

Mean Judged Number to Presented Number

Retinal

Displacement 2.5 5 10 15 22.5 30

00 .947 .648 .507 .397 .306 .266

2.50 .911 .641 .478 .376 .282 .250

'50 .902 .632 .470 .363 .272 .242

10' .878 .625 .443 .358 .267 .245

200 .846 .605 .432 .335 .258 .239

400 .795 .584 .412 .311 .231 .210
I With suprathreshold responses to flashes, the design engineer will

be interested in reaction time measures. Unfortunately, we have only been able to find one researcher who examined the difference between

reaction time to the onset of a steady light and a flickering one.

I Gerathewolbl (1953, 1954) found that reaction to a small spot in a 6.4

to 9 cd/mr field was faster when the light flickered than when it was

steady, but only when contrast was low (.19, as opposed to 1.0 and

74.2, see Appendix A). Interpretation of these data is complicated by

the fact that the subject was engaged in a complex psychomotor task and

in addition was uncertain as to the location of the target light.

i

89 IL __
CHAPTER 11

Brightness Enhancement

Generally, the brightness of a single flash is greatest when its

duration is between 50 and 100 msec. This phenomenon is termed the

Broca-Sulzer effect. For a flickering light, brightness is greatest

when flicker frequency is between 3 and 15 Hz. This brightness is

greater than when the light flickers at a frequency above fusion (its

average or Talbot brightness). The enhanced brightness of a light

flickering at below-fusion frequency is kno;,,n as the Brdcke-Bartley

effect.

All the data to be reported in this section were collected in one

of two ways: either the observer rated the brightivess of the I IihL

(magnitude estimation), or he matched the brightness of a flashed or

flickering light to a standard light whose parameters were constant.

In the second case, the duration, size, etc., of the standard light is

arbitrary and varies from experiment to experiment. Thus, either

technique presents relative and dimensionless values that can show some manipulation has increased or decreased brightness by some proportion relative to another condition.

Some authors have already considered a few facts relating to the

increase in a flash's brightness as a power function of its luminance

(Anglin & Mansfield, 1968; Mansfield, 1973). In addition, below some

critical duration, one can trade duration for luminance to maintain a

given brightness (Bloch's constant energy law). The relationship takes

the form:

Brightness = k'Dp (8)

where E is energy, and k is a scaling factor that varies with changes

in conditions. The value of p nermally is 0.5, but for a point source,

p grows to 0.94. Although brightness changes with target location

(fovea and'.O temporal), size (.05 to 40) and wavelength, the proportionai increase in brightness with energy remains unchanged for durations below 100 msec. That is the duration at which the Broca-Sulzer

effect Dccurs. This agrees at least qualitatively with Raab (1962) and

Osaka (1977).

For a flash of duration longer than that which is optimal for

Broca-Sulzer effect, (see Figure 31), the relationship takes the form:

Brightness = k'Lp (9)

Where L is luminance. The exinent p, in this case, is slightly in

excess of 0.3. Again this exponent is also invariant with location,

size, and wavelength. Under these conditions, the exponent for point

sources declines to 0.5. Varying the size, wave-length, and retinal

location of flashes has little effect on the exponent in the

brightness-luminance relationship (Mansfield, 1973).

90
9 . . . 0 4 0. 604

r+ 1 I I I I

++ '.- ,. .

ti I1

PW P94,

"etmain B ightes aI s fucino -- lahdrto a ifrn

A, 1~

l umigure levels for a07 foveal t argeto ht. Ea c poight

represen~ts the average of at least 10 observers' estimates. Standard

S•errors are indicated by the vertical bars. The solid lines drawn

•+ . through the points represent Vual-luminance contours and the parameter

;, .indicates the luminance (cd/m ). (B) Brightness maximization. Dura-

: tion of the brightest flash as a function of luminance for a 0.720

• "foveal target of white light. Each point represents the average of 17

" ~observers' settings with standard errors indicated by the vertical

:• • bars. The line was fitted by least squares (from Mansfield, 1973).

91

•. +• • • lql ~~~~~~'e'- S . :q1 ' I JlL|5I • ••"•'•• I III I
t

Regarding the Broca-Sulzer effect itself, Mansfield also found

that the duration producing the greatest enhancement changed with flash

r luminance (L), and in this case fit the equation:

optimal duration = k' Lq (10)

Again, for an extended source q varied between -. 30 and -. 32, but for a

point source equaled about 0.48. Figure 31 provides data for one set

of conditions, a white foveal target of 0.720 subtense. In this case

k = 278 and L = -. 34. Of course, the data could also be described by

the equivalent formula:

optimal duration = -195.1 (log L) + 353.6 (11)

which is the form used throughout most of this report.

One can see from Mansfield's data (Figure 31) that the optimal

duration may be fairly long for a very dim flash on a dark background.

White, Collins and Rinalducci (1976) report a similar effect for a dim

light in the periphery (a 10 target at 70 eccentricity). Optimal L. duration ranged from 200 msec for one observer to 400 for the other.

The previous experiments which we have reported were all performed

against zero background luminance. With the few dim backgrounds used

in those studies the Broca-Sulzer effect did not become evident until

test flash luminance was appreciably greater than the background luminances. White et al., suggest that if background luminance is greater

than zero, the target must be 1 to 1.5 log units above its own contrast

threshold for the Broca-Sulzer effect to exist.

aeOsaka (1977) provides some evidence that these enhancement effects

are stronger in the periphery than in central vision. Under his exk perimental conditions (zero background luminance), he did not observe

the effect in the fovea, but d'd in the periphery. The luminances used

in this study, .86 to 8.6 cd/m may have been too low to see the effect

at t~e fovea. Osaka also observed that at a flash luminance of 2.7

"cd/m , a peripheral flash looked brighter and "crisper" than a foveal

one. In addition, Osaka found that brightness increased by .1 log unit

(about 26%) as target size was increased from 16 to 116 min of arc in I.. diameter.

This agrees with Marks' findings (1968, 1971) for flashes of fixed

duration. When background luminance was zero (Marks, 1971), a flash

seemod brighter in the periphery than in the fovea. When photopic V[ background luminance levels were used (Marks, 1968) brightness decreased steadily with increased eccentricity. The effect varied from

condition to condition, but, on the average, perceived brightness

decreased .2 or .3 log units as eccentricity increased from 00 to 600.

Thus peripheral placement of instruments would be of some advantage

under low luminances and foveal would be better for bright backgrounds.

The Broca-Sulzer effect seems heavily dependent on the spatial

parameters of the stimulus. Arend (1973) found that he coulO eliminate

enhancement completely by blurring his 1.50 target with +3 diopters of

92
defocusing. (This eliminates much of the high spatial frequency content from the display). Similarly, Kitterle and Corwin (1919) found

that enhancement of the apparent contrast of a flasihed grating pattern

was greatest at the highest spatial frequencies he usea. Unfortunately, his range of spatial frequencies (.25 to .73 cycles/deg) does

not provide information on what the optimal spatial frequen-y may be,

or what the least effective one may be.

Under the proper conditions, a flickering light may appear brighter than a steady light of the same average luminance. As mentioned earlier) this is generally called the BrUcke-Bartley elfect.

According to the review of the effect by van de Grind, GrUsser, and

Lunkenheimer (1973), it occurs at temporal frequencies ranging from 3

to 15 Hz. The effect is most pronounced in the mid-photopic jange. In

fact, Bartley (1951) found that for luminances below 377 cd/m , brightness was actually less than that of a steady light of the same average

luminance. Ball and Bartley (1966) found that the o~timal luminance

range for their configuration was 323 to 2150 cd/m . The optimal

frequency for producing the effect however, changes with mean luminance. Figure 32 shows data from Rabelo and GrUsser (1961) as reproduced by van de Grind et al. Optimal frequency grows with an increase

in mean luminance. This was also found by van der Horst and Muis

(1969). Wasserman (1966) found that optimal freqpncy was highest and

enhancement greatest at a mean luminance of 109 cd/m.

All these studies have used on-off stimuli (100% modulation)

usually produced by interrupting the projecting light beam with a

shutter or rotating sectored disk. Glad and Magnussen (1972) Losed le~s

than complete modulation (80%) and measured the enhancement of the dark

cycle of a flickering light. They found that darkness is enhanced 2 "n-a

manner similar to brightness enhancement. Modulating c 323 cd/rn , 1*

diameter spot of light, they found that it appeared to be half as

bright during the dark cycle as a steady light of the same luminance.

This effect peaked at 3 Hz. Magnussen and Glad (1975a) found that for

identical displhy conditions, brigftroess and darkness enhancement

occurred at the same frequency.

Since both brightness end darkness enhancement occur together, it

may be more sensible to think of the pair as enhancement if perceived

modulation (or temporal contrast). Marks (1970) asked observers to

estimate the temporal modulation of a flickering 2.10 target. He found

a modulation enhancement effzct that peaked in the 5 to 10 liz range for

conditions of high mean luminance and low modulation. His optimal frequencies are probably higher tnan Magnussen and Glad's because of

his larger target size. Optimal frequency was much 3ower for uther

conditions, but due to the restricted range of frequencies he used,

this cannot be specified.

There are important interactlons between spatial and tempor3l

parameters in the BrUcke-Bartley 'effect. The well-known effect called

induced brightness or contrast enhancement (the brightness of a spot is

enhancea if surrounded by a darker area and vice versa) is itself enhanced by f~ickering. Magnussen and Glad (1975b) flickered the surround of a steady, small (10) patch and measured the "induced" bright11 93
r - 150

F 0I

!C

Flickerrqond y (I-i:

100

V4

F.

[0

[&&.

size 6. cm, 30° 49 co 0

[ 1 5 10 15 20 25 30 35 so

Figure 32. BrUcke-Bartley effect obtained with rectangular flicker

st-imuli (L-D-ratio 1:1). Mesopic level of adaptation. The subjective

brightness (relative luminance of the steady light, which matches the

brightness of the flicker StiMUIlus) is plotted on the ordinate, the

frequency of the flicker stimuli on the abscissa. Stimulus diameter

3039s . Three different levels of mean luminance (from Rabelo &

GrYisser, 1961, rep-oduced by van de G,'ind, Grisser, & Lunkenheimer,

94
ness changes. The flickering surround induced modulation in the center

of the opposite phase; when the surround was brightest, the center was

darkest, and vice versa. The effect was about four times greater than

"static" induced contrast. Maximal contrast occurred at frequencies

ranging from 2 to 4 Hz depending on display luminance. This effect is

probably related to Kitterle and Corwin's spatial contrast enhancement

found for flashes.

Similarly, Reiole (1973a, 1973b) found that the area of enhanced

contrast at the border between large light and dark areas (Mach Bands)

car be extended by flickering the display. The brightness enhancement

of the light area covered 10 min of arc during steady illumination.

',ien the light area flickered from 10 to 18 Hz, this area covered

anywhere from 50 to 150 min of arc.

95 IA
CHAPTER 1.2

Effects of Flash and Flicker on Visibility

So far, in this report the data on the detection of flashes and

flicks-ring lights and the effects of temporal variation on the supr&-

threshold brightness of targets have been reviewed. This chapter

concerns the effects of flashes and flicker on spatial discrimination.

Much of this work is concerned with acuity measures and asks, "How do

flash and flicker affect an observer's ability to see thin lines, or

small gaps between objects, when the display is nf high contrast (ftr

example, a reading ta.k)?" Depending on the target size, contrast and

conditions of visibility, flickering or flashing a target may make it

either harder to see, easier to see, or produce no change at all.

First, the effect of presenting an acuity target as a flash (i.e.,

for a short duration) is considered. As with the detection task,

reducing the duration of a target never roves its visibility.

However, simple and useful roles describe the debiitation effect of . shortening tns flash. The most useful of these is similar to Bloch's

Law (see chapter entitled Flash Sensitivity), whereby an intensity-time

trade-off governs observers' performances on acuity tasks.

Kahneman and his co-workers have studied thee phenomenon for

several years. Kahneman and Norman (1964) tested the applicability of

B~och's Law for both briahtness and acuity performance with a 60 x 20

pattern of squares and dots (mean luminance was 2 cd/mr). This was

similar to the findings of other investigators (see chapter entitled

Brightness Enhancement). For the acuity task, however, critical duration was much longer, 200 to 316 ms. These findings have been replicated by Kahneman (1966) and Kahneman, Normnan and Kubovy (1967) using

Landolt C acuity targets.

Baker (1969) also found that time-intensity reciprocity held for

an acuity task, but only under a limited set of conditions. He varied

the size of his black and white letter targets (I or 1.45 min of arc)

and also the level of performance demanded from the observers. His

targets consisted of five letters, but the observer could be instructed

to recognize either just one, two, three, four, or all five of them.

He found that reciprocity held only for low levels of performance, and

only for a low range of luminances of his targets.

Baker expressed this reciprocity relationship by the equation:

log Itk = C (12)

when k = -1, reciprocity holds; when k is between -1 and zero, the

duration (t) needed to perform the acuity task decreases too slowly to

keep energy cunstant as the luminance (I) is increased. As the task was

made more difficult, k increased to -. 6 for both letters and Landolt C

stimulP. As luminance was increased, k shifted between -. 25 and -. 40

(depending cn task difficulty) for the larger letters. For the smaller

letters, k :hifted to 0.45 and was independent of the task difficulty.

96
The obtained values of C (the luminances needed fcr any given duration)

are peculiar to the targets used, However, depending on the difficulty

of the discrimination, it seems that for acuity tasks above a minimum

value of luminance, time is the most important factor.

Baron and Wescheimer (1973) support this no, ion. They foiind that

at relatively high luminances (127 to 509 cd/m ) the time needed to

perform a Landolt C acuity task was about AO0 msec., and relatively independernt of luminance. The critical duration for the detection of his

targets, using the same apparatus, was 80 ms.

Brown and Black (1976) used a somewhat different stimulus configuration. With gratings, they measured the luminance increment needed to

detect its orientation for different background luminances and spatial

freqcencies. As background luminance was varied from zero to 3.183

cd/m , critical duration decreased from 250 to 126 msec. This was at

spatial frequencies between 3 and 6 cycies/deg; at frequencies both

above and below these, the critical duration was less. The range of

spetial frequency to which the human visual system is most sensitive

for flashes of long duration also happens to be 2 to 6 cycles/deg (see

Figure 35).

In describing the visibility of a target flashed just once, the

authors start with a simple idea that the less time one has to see the

target, the harder it is to perform an acuity task. But the situation

is not as simple for a repetitively flashed (i.e., flickering) target.

Whatever the flicker frequency is, both time-averaged mean luminance

and the total stimuli duration can be kept constant. But one wonders

whether subjective effects, like the enhanced brightness at certain

flicker rates (BrUcke-Bartley effect), might affect acuity performance. The answer from Bartley and his co-workers is that it does. In fact,

conditions which produce subjective brightness enhancement actually

degrade acuity. Bartley, Nelson, and Soules (1963) found this when

testing observers with Snellen E targets (contrast = .5; see Appendix

A) using several frequencits and pulse-to-fycle (PCF) fractions (or

duty-cycles) at a mean luminance of 149 dc/m". Acuity was the worst in

every case that generated brightness enhancement. Bourassa and Bartley

(1965) did a more extensive experiment in which the task was to detect

a gap between two square objects, 30.5' and 76' in width. Figure 33

shows their results. The greatest loss of acuity occurred under the

conditions that produced the brightest display. Thus while acuity of a

target presented steadily or as a single flash, improves with increasing luminance, acuity is degraded when the subjective brightness of a

flickering target is increased.

Gerathewohi id Taylor (1953) have also found that flicker did not

help observers to read a special acuity chart. This chart was 50 lines

of black print on a ackgr~und that became progressively darker frol

top to bottom;' 1 x 10 cd/m for line one down to only 1.8 x 10 cd/m

at line 50 Of course, the contrast ot the text al~o dropped as one

reads down the page. They found that at 9 and 15 Hz, flickering

neither improved nor degraded performance. However, they did not check

to see if theIr conditions also produced brightness enhancement.

dr,
II

I.,

I

M -0a WMa Low #Amti hos m to L

I S."tfl t I o /i tol C ir Io

i,.Figure 33. Curves of brightness enhancement, reduced brightness, and

• visual resolution for each of two pulse-to-cycle fractions under simii)lar conditions. (For curves with open circles, pulse-ý.o-cycle fraction

!' I equals one fourth; for curves with crosses, pulse-to-cycle fraction

}- ,equals three-fourths. The effectiveness (enhancement or its opposite)

•i 1 is given in percents, with, brightness of the steady target as a base.

•- Visual resolution also is relative with the separation of the compoP nentc,J of the steady target as a base; therefore values progressively

:•. above zero represent wider separation and reduced acuity or resolution.

• •Units are given in, seconds of arc). 98I (From Bourassa & Bartley, 1965.)

2I 
These acuity tests, however, assess only one aspect of visual

function, the ability to see fine detail at high contrasts (fine lines

or gaps, or for grating targets, gratings of high spatial frequency). (cf. J. L. Brown (1965) for definition). What about the ability to see

larger objects unoer conditions that hamper visibility, for instance

low contrast? This shall be termed a visibility rather than an acuity task. It happens that flicker can improve performan:ce for this typz of

task (Kelly, 1969; Sekuler & Tynan, 1977). As an example, see Fiyure 34 from Sekuler and Tynan. They measured contrast threshold as a 

I function of spatial frequency for two rates of flicker. Lower contrast

thresholds correspond to higher sensitivity levels. The jagged lines

reflect the fact that their task was a tracking procedure; as they

changed spatial frequency, the observer instantly adjusted contrast so

that the pattern was just visible; to avoid clutter, only the midpoints of these tracks are plotted for the other function. It is clear

that the 6 Hz rat2 of flicker improved the visibility of the low

spatial frequency gratings relative to the very slow .3 Hz flicker

rate. However, for the high frequencies, those which assess acuity, sensitivity was unaffected by flicker rate. Sekuler and Tynan did not

state whether brightness enhancement occurred for their display.

A similar result occurs when measuring contrast sensitivity for

various durations of a single flash (Nachmias, 1967, Tynan & Sekuler,

1974). Figure 35 is from the latter paper. As expected, sensitivity

decreases as flash duration is decreased. However, the low frequency

patterns suffer less loss, resulting in the elimination of the lowfrequency roll-off of these sensitivity functions found for long duration flashes. Thus, at short durations, large targets are the most

visible. At longer durations, medium size targets are the most visible.

Flicker may also enhance the visibility of targets obscured for

* reasons other than low contrast. Kirkwood (1968) ncticed that blurry images on a large uniform background tended to disappear after a few . seconds. His patterns were quite large and varied in complexity (Figure 36). It should be noted, however, that the actual patterns were

blurreJ. Kirkwood tried to prolong their visibility by using inter- mittent light. Figures 37 and 38 show that the lower the rate of

r flicker, the longer they were visible. The more complex figure*s benefitted the most.

Alexander (1970) also tried to improve the visibility of fading images. Nonsense figures embedded in a blurry, matted, noise background faded in approximately one second. Flickering the entire scene

at a rate of 4 or 8 Hz improved the visibility of these figures.

Conceivably, this factor could be employed for certain symbols used

with tactical information display (TID's) or heads up displays (HUD's.

where critical information could be flickered at frequencies determineid

Soptimum for seeing.

" ~99 M
.0.3

C

S. I a J : . i I ,I

.0,3 0 e 0

I ~~SPATIAL FREQUTENCY O/dra 2

Figure 34. Contrast-sensitivity function obtained for an observer's

right eye using a target flickered at 0.3 Hz (continuous line) and at

6. 0 Hz (dots). Each dot is the mean of a peak and a trough from the

x,y plotter record (from Sekuler & Tynan, 1977).-

.100

•.L 1 _ q _ ." q ý _ ;
0~ 0

100 I *

90 30

10

0.5 1 2 4 8 1is

SSPATIAL FREOUENCY

(C/T)FG)

Figure 35. Contrast sensitivity as a function of test-grating spatial frequency. Results for test flashes of 16, 90, 512, and 1000 msec are

given by asterisks, propellers, circles, and stars, respectively. The

lowest curve is the least-squares fit to results with l msec flashes.

The model equation was of the form, sensitivity-ke , where F is

spatial frequency. The same curve (with adjustment of the k parameter), has been fitted by eye to data with 90 msec (middle curve), and

to data with the two longest durations (top curve). (From Tynan &

bY. Sekuler, 1974.)

101
I.,

I

A B C

- Ii

I Ii

Ii

I IIJ

[ D E F G

I,

I,

Figure 36. Test patterns used in Kirkwood's 1968 experiment. These

patterns were blurred when presented to the observer.

L02

SI -- -" - -., .-. ,---. h -
*o U I ' U I Ul " " •-i i I I I i i

* so

+

70

50

40 jC

30 

'

20

10 A

10

1 2 5 10 20 so 100

PLICKER RATE cna

Figure 37. Relationship between mtan persistence time and flicker rate

for patterns A, B, C. Standard deviations average 0.38 of mean persistence times shown (from Kirkwood, 1968.)

103
E. 90 .

,,80

70-

~60- K J50

&540

I 30

-20.

1.' 2 00 3 !0 20 50 K~M0

FLICKER RATE CPS

Figure 38. Relationship between mean persistence time and flicker rate I for patterns, D, E, F, G. Standard deviations average 0,38 of mean

persistence times shown (from Kirkwood, 1968.)

104

L ,~
CHAPTER 13

Effects of Movement on Visibility

The effects of movement on the visibility of a target resemble

those of flicker. This equivalence may result from the fact that

spatially repetitive pattern movement can be specified in terms of the

flicker at any one point in the visual field. An important difference

between flicker and movement however, is that due to temporal summation

in the visual system, movement causes apparent bilurriig or "smearing"

of an object. The effect is the same as taking a photograph of a

moving object with too slow a shutter speed (cf. , Brown, 1972a).

Another difference between flicker and movement is that with movement, a target may not remain on the fovea, the region of the retina

r most sensitive to fine detail. Even if the observer pursues the object

with his eyes, tracking errors will introduce both retinal movement and

likely parafoveal viewing of the target.

This chapter is in three parts. The first part will discuss

dynamic visual acuity (DVA), the ability to discriminate fine spatial

detail in a moving target. The second part will deal with the effects

of movement on the visibility of larger objects and for tasks other

than acuity. Lastly, several attempts to measure visibility in applied settings or with displays designed to provide information particularly useful to t;ie human factors engineer will be discussed.

Dynamic Visual Acuity

Ludvigh (1941) and his associates at the U. S. Naval School of

Aviation Medicine did much of the early work on dynamic visual acuity.

Ludvigh and Miller (1958) asked observers to discriminate the orientation of the small gap in Landolt C targets as they moved at velocities

ranging from 0 to 120 deg/sec. The exposure time was in all cases .4

sec and illumination level was 25 ft-c. Thuir data are reproduced in

Figure 39. The three curves represent three groupings of observers on

the basis of performance. All curves have been fit by the equation:

DVA = a + bx3 (13)

F in min of arc of the gap. The constant a represents static visual

acuity (that is, acuity for non-moving tarlots) and b is a slope constant characteristic of an observer's acuity loss withincreased speed,

and velocity is in deg/sec. Variability in static visual acuity among

observers is low (32%), possibly because all observers were screened

for 20/20 vision or better. However, despitce this "matching" for

static acuity, variability among dynamic acuity scores (b fastor) was

large, 224%. Fergenson and Suzansky (1973) directly measured the correlation between static and dynamic acuity for 24 observers, unscreened and uncorrected for static acuity and found it to be essentially zero.

.10
I !1-

I Target Lob

1 2 Target Lob o---*

2 Torget Air .-- o / S9-,

F- -

.4-

3-

2-

0 I

0 10 20 30 40 50 ýo io 80 90 100o110

Angular Velocity in Degrees per sec

Figure 39. Observed and computed threshold values of all subjects

grouped according to performance level. The circles, crosses, and

triangles are the objerved values, the continuous lines are graphs of

the equation Y = a+bx (from Ludvigh & Miller, 1958).

106

I'-
Interestingly, Ludvigh and Miller note that if some of their

poorer observers had been provided with low-vision aids that magnified

the targets, performance would have decreased, because not only the gap

in the C, but also the velocity of the target would have been magnified

by the device. Observers with poor dynamic acuity suffer much greater

loss from motion than they gain by an increase in the size of the gap (since OVA increases as the third power of velocity).

A follow-up study (Miller 1958) produced very similar data when

the observer moved (rotated) but the target remained stationary.

DynamTc-i-c•ity, however, was slightly worse when the observer was

rotated, rather than when the target moved. Miller also found that

although static visual acuity did not improve much above 5 to 10 ft-c

of illumination, dynamic visual acuity improvea up to 125 ft-c, which

was the highest illumination he used (see Figure 40).

In both of these studies, the observer was allowed to track the

target with his eyes, but Ludvigh and Miller thought that errors in tracking allowed the target's image to move across the retina and cause

visual smearing. Brown (1972a, 1972b, 1972c) was also concerned with

such errors as well as with visual loss caused by viewing the target

with parafoveal areas of the retina. Consequently, he measured dynamic acuity both with fixation at a stationary point and with tracking.

Figure 41 shows data from Brcwn (1972c). Four observers fixated on a

stationary spot and the target was presented at various eccentricities

and velocities. Like Ludvigh an5 Miller, Brown used Landolt C targets;

background luminance was 14 cd/m and contrast was .74 (see Appendix A,

SFormula 2). Standard deviations are about 1/., of the means. OVA was

roughly linear with velocity but this is probably because Brown's range

o,- velocities was much less than that of Ludvigh and Miller. Acuity

also decreases fairly linearly with retinal eccentricity.

Brown (1972a) measured OVA (and at the same time recorded eye

movements) while the observer pursued the target with his eyes. Brown

concluded that tracking errors were the main causes of loss in OVA. He

also noted that dynamic acuity improved with practice. Brown (1972b)

repeated these measurements while varying the contrast of the targets. As Miller found when varying illuminatinn, contrast affects DVA much

more than static acuity (see Figure 41, the break midway in the functions are practice effect artifacts of the experimental design). These

results agree with Ludvigh (1941) who found that a 20-fold increase in

the contrast of a Snellen chart improved acuity by only one line.

When the eyes pursue a moving target, the target's slippage across

the retina is considerably less than that of its background. Although

most OVA studies concentrate on the visibility of the pursued target,

Mackworth and Kaplan (1962) were interested in the observer's acuity

ror non-pursued background objects. They had observers pursue an

object moving horizontally at various speeds, from 0 to 120 deg/sec.

When the object reached the midpoint of the display, a test object

#lashed on for 99 msec, about 54' above the moving object. The test

object was a set of three bright bars on a dark background. The luminance of the bars was variable and acuity was measured as the minimum

resolhable stripe width. Figure 42 shows that luminance (and in this

107
I'

IS .84 fo

.20 fo

16

1.0 fc [ ~o 25.0 to

129.0 fc

I ; 6 -

2

0 20 40 60 80 100 120

ANGULAR VELOCITY IN DEGREES PER SECOND

Figure 40. The effect of increased angular velocity of rotation on

visual acuity at each of six levels of test chart illumination (from

Milier, 1958).

108
Fi

CONTRAST (%1

6 23

5

36

4 -

0 70 Hz

; 3

K>

2

0 20 40 60 80 100

TARGET ANGULAR VELOCITY (°/S)

FigLre 41. Mean DVA as a function of target angular velocity at 4

taget contrast levels (from Brown, 1972b).

109
S8 J

0 VERTICAL

r HORIZONTAL

6 S". [ 6O, 07I,nL

r i

S4 0.,28 mL l

I- 3

2

k 2

0 15 30 45 60 75 90 1 05 120

.. TARGET SPEED IN DEGREES PER SECOND

Figure 42. Minimum resolvable stripe width in minutes of visual angle

as a function of target speed in degrees of visual angle per second.

Thresholds for horizontal and vertical stripes were obtained at three

luminance levels. Each point is the average of eight determinations

on one subject (from Mackworth & Kaplan, 1962).

110
F case contrast) again affected dynamic acuity more than static acuity. The figure shows that vertical bars were degraded more than horizontal

ones. T,is is because "visual smearing" from object pursuit would be

primarily along horizontal axis. Since the smearing would be perpendicular to the bars in the vertical condition, it would wash-out the

effective contrast of the bars and reduce resolution. In the horizontal condition, smearing would tend more to enlongate the bars rather

than to reduce the pattern- contrast as much. For the same reason many

experimenters note that the orientation of the gap in a Landolt 0

target is important. Mackworth and Kaplan claim that t~e facilitating

effects of movement in the 60 ml condition (191 cd/m ) were due to

glare reduction.

"pparent" mo,,ment reduced DVO in much the sp'e manner as real

movement. Bi'eitmeyer, Love and Wepman (1974) alterinated two circular

: acuity targets, 1.125 deg in diameter and 1.125 deg apart. The targets

were discs with one edge slightly flattened. The subject had to judge

the presence or absence of the flattened edges. Each disc was presented for 15 msec and at a range of SOA's (stimulus onset asynchronies). SOA of 95 msec produced both the best apparent movement, and

the worst acuity -- a drop off from 45% to 25% correct discriminations.

Non-Acuity Measures

The preceding discussion has shown that movement reduces visual

acuity. As for flicker effects, acuity is not a complete measure of an observer's capabilities, and with other types of low visibility dis- plays, movement may improve performance.

As mentioned previously, researchers often use the contrast

threshold of targets as a measure of visibility. As an example,

Krauskopf (1962) measuring the contrast threshold of a thin (20 40')

long line, tried to improve its visibility by moving it back and forth.

For oscillations of 4 Hz, amplitudes of up to 12 min, (average velocities of up to 1.5 deg/sec) improved visibility (i.e., lowered contrast

threshold). For 32 Hz, however, as amplitude increased up to 12 min,

(average velocity up to 12.3 deg/sec) visibility decreased.

More complete information on visibility may be gained by testing

with a series of grating displays. Van Nes, Koenderink and Bouman

(1967) made such measurements at photopic levels with moving sinusoidal

gratings. The borders of the display (1.2 x 2.40) did not move, so the

display looked like a moving endless striped belt seen through a window. They specify motion in terms of the temporal modulation (in Hz)

at any point on the screen. The relationship between velocity and

temporal frequency for a moving grating is:

drift rate

Velocity (deg/sec) (14)

spatial freq.

where drift rate is in Hz and spatial frequency is in c/deg.

SI~i
Van Nes et al. (1967), used three temporal frequencies, zero, one,

and 10 Hz. For high spatial frequencies, the 10 Hz rate made gratings less visible than the zero or one Hz conditions, but at low spatial frequencies one Hz produced the greatest improvement in visibility.

Tolhurst, Sharpe and Hart (1973) made very similar, but mose extensive

hmeasurements with a 6.5 deg diameter display of 100 cd/m mean luminarnce. They also found that visibility was enhanced by movement if

spatial frequencies were low (Figure 43). Peak visibility occurred at

a temporal frequency of 5 Hz, regardless of the velocity, for spatiRl

frequencies of 8 Hz or less. Sharpe (1974), however, made very similar

measurements at a location 10 deg into the periphery (display diameter

was 3.5 deg), and found that optimal temporal frequency changed with

spatial frequency. Optimal temporal frequency for a 1.5 c/deg grating

was 5 Hz (3.3 deg/sec) but for a lower frequency grating (.8 c/deg), a :,Jgher enmporal ?requenty was better (10 Hz o, 125 deg/sec. For a

higher spatial frequency grating (5.5 c/deg), a lower temporal frequency was optimal (2 Hz or .36 deg/sec). Sharpe thought that the

periphery may be "tuned" to detect rapidly moving coarse detail.

Van Nes (1968) reinterpreted the data from van Nes et al. (1967)

in terms of visibility expressed as a ratio of the contrast threshold

of a moving grating relative to that of a stationary grating (Figure

44). All the data are from one observer, but a second observer showed

similar results except that his peak visibility gain was only about

1.7. These graphs show more clearly that the facilitating effects of

movement are restricted to slow speeds and low spatial frequencies.

Figure 45 shows, that the effects hold only for photopic luminance levels.

In general, whether movement improves or degrades visibility I depends on the spatial characteristics of the target and the type of

task the observer is required to perform. This may explain app;rent

descrepancies in the dynamic visual acuity literature. For instance,

Breitmeyer, et al. (1974) found that apparent motion reduced acuity in

a manner similar to real movemnt, yet they note in their paper that

. "2 Erikson and Colgate (1970) performed a seemingly similar experiment, in

, which the observer recognized letters in apparent motion, and found no

visibility loss. Breitmeyer et al, (1974) thought this apparent contradiction was reasonable since a pattern recognition task does not

necessarily require attention to iine detail (high frequency information) and may, in fact, sometimes be improved by eliminating such

infor,.:jtion (Harmon & Julesz, 1973).

More support for the interaction between task and spatial frequency content is provided by a visual masking experiment by Carpenter

and Ganz (1972). They showed that high frequencies were effective

maskers for an acuity task but lower frequencies were effective maskers

for a detection task.

Research in Applied Settings

Dynamic visual acuity has generated more interest among applied researchers than other visual phenomena because of its obvious impor- tance to military and civilian transportation problems. For example,

112
"i0

1300 -- "

100

I+L i I_ I A ,Id . . .. . .. pi I ,

0.3 1 3 10 30

-SPATIAL FRQUEMCY (C/DEG)

Figure 43. The sensitivity to gratings drifting in one direction as a

function of their spatial frequency. The results for four temporal

frequencies are shown: stationary (0); 2.5 c/sec (0); 6 c/sec. (A);

and 11.5 c/sec. (i). The continuous curve has been drawn by eye

through the open circles; the dashed line has been drawn by eye through

the data for 11.5 c/sec and then shifted up the sensitivity axis to fit

the data for 6 c/sec (from Tolhurst, Sharpe, & Hart, 1973).

113
3I

350td

-40

1.. .14cpd

02 34

"VELOCITY IN DEGREES PER SECOND

Figure 44. Visibility-improvement factors for four spatial frequencies

as a function of the grating velocities, at an average retinal illuminance of 850 td (from van Nes, 1968). 1144

11
3

AS. 21

L.0

i .. .. w .. . - - !- - " - n-' - -

8. 5 t

i Ii

I _ _ _ __ _ __ _ _

0 1 2 3 4 5 6 7 a

. Spatial frequency In cycles per degree

L ,,

'Il

Figure 45. Visibility-improvement factors for three average retinal

"illuminances as a function. of the spatial frequencies at a gratingvelocity of I deg/sec (dps) (from van Nes, 1968).

115
Goodson and Miller (1959) tested the ability of an observer in a lowflying aircraft to see fine details on the ground. They flew observers

past large single or double Landolt C targets. In order to compare

their data to laboratory data, the plane was equipped with a special

observation box that limited the exposure duration of the target to .4

sec. The aircraft flew at speeds that resulted in target speeds of 20,

69 and 90 deg/sec (Goodson & Miller, 1959) state that under combat

conditions maximum target speed would be 110 deg/sec). Actually, the

target speed was not constant during the exposure and these values are

more nearly the median speed that the observer was exposed to. Figure

46 shows their data compared with other laboratory data. The data are

similar although acuity was actually better in the air. They claim

that this is probably due to deceleration of the target near the end of

the exposure.

toGilson (1971) was also interested in dynamic acuity in the aviation setting. He gave observers the task of constantly centering a

drifting needle in an aircraft-localizer/glideslope indicator. He then

oscillated the entire display with an amplitude of 10 deg of visual

angle, at a speed of either 5 or 30 deg/sec. It is not suprising that

oscillation made the observer's task more difficult (Figure 47), but

interestingly he found, as had Miller (1958), that changes in luminance

affect performance more when dynamic acuity is required, than when

static acuity was required.

Throughout this section, the attempt has been made to clearly

demonstrate that dynamic an4 static acuity are only very weakly related, if at all. This has been furt:ier supported by Shinar, Mayer and

Treat (1975) by using a battery of visual tests designed by Henderson

and Burg (1974) to discriminate between good and bad automobile

drivers. Shinar et al. (1975) found that DVA was correlated with other

motion tests but not with non-motion related tests, including static

acuity. More importantly, DVA was the measure that best discriminated

between accident-prone drivers and a control group.

As was previously discussed, theoretically oriented studies have

found that movement may improve detection or recognition performance,

but, not acuity. This is also true of studies oriented towards application of data on visibility. Peterson and Dugas (1972) found that

movement improves the visibility of targets embedded in random or

"cluttered" scenes. They used a high resolution T.V. system with a 31

x 40 deg screen of 38 cd/mi mean luminance. The target size was 1/4

deg square and motion was an oscillation 43' in amplitude. The backgrounds used were: a dot matrix, an aerial photo of forest terrain and

a photograph of felt (random texture). Some of the results are presented in Figures 48, 49 and 50. The ordinate is the probability of

detecting the target. Figures 48 and 49 show data collected with the

dot matrix background, each at a different contrast. Increasing contrast from .3 to .5 (see Apoendix A) was equivalent to increasing speed

by one deg/sec. Figure 50 shows the results for the aerial photo

background. In Figure 51 these data are replotted in terms of detection fields, the area around any fixation in which the target must be

present to be detected. The larger the field, the greater the detectability of the target, and motion increases this field greatly. Figure

116

- I.
i

I1"

I Target Lab *--

10- 2 Target Lab *..-.-

9- 2 Target Air e-..-' , ,

S7-

6-,

50.

... A

I 20 3"

4 -

•j0 10 20 30 40 50 60 70 80 90 I001I l

Angular Velocity in Degrees per sec.

Figure 46. Means for different methods of testing visual acuity (from

Goodson & Miller, 1959).

117
r

I 

i

• 35-

ml

RED FILTER (050) o--o

BLUE FILTER (050) D-O

I 30 ND FILTER (050) - •" ~~~~~ND FILTER (016)NOFLE(1) ,"•

SMEAN 25-

INTEGRATED

ER ERROR

20.

(ARBITARY UNITS)

10-

• . 5"

0 5 10 15 20 25 30 I! MEAN RETINAL - IMAGE SPEED (DEG/SEC)

Figure 47. Average absolute velocity of the display image passing over

the retina. These values were computed from the differential velocity

of the eye and the display. (From Gilson, 1971).

118
Ii,

"I

100

F 680

Target sneed. dep!.sec

a.. 40

20

05 10 15 20 25

•" ~Timne (.sm•)

Figure 48. Targets detected within a given time period on the dot

pattern background (contrast = 0.3) (from Peterson & Duoas, 1972).

119
II

0 -

! i

I

jso

S31

j Tmrqeat speed. deR/sec.

i V

I I

~iR.20

_ _.I i I I I

S 5 10 is 20 25 30

Time (mc)

II

- ,Figure 49. Targets detected with a given time period on the dot pattern background (contrast = 0.5) (from Peterson & Dugas, 1972).

"I 120
30 -___

s0 07

400

40 20M 

FM.D/M

20

0 5 10 I5 20 25 30

Tuff (sac)

Figure 50. Targets detected within a given time period on the aerial

photo background (from Peterson & Dugas, 1972).

121
Ij~

140 --

• I

' AeriRl Dhntn 120 - t

• II

800

I T)nt. Q. .5 Felt I /

60 II

,/ D Dot. C = 0.3

C Ij

... 40

II

20

2 4 6 8 In L2 14

r reret speed (dqg*,"9

Figure 51. Variation of detection field with speed (from Peterson &

Dugas, 1972).

122
52 shows the detection .me for a stationary and moving target as a

function of distance of the target from fixation (dot matrix background; the observer did r~at scan the screen). Beyond six deg eccentricity, motion reduces det,:ction time considerably.

These results are simi',ar to those of Alexander (1970) for detect.

ing a target embedded in random noise. He investigated effects '~f

flicker (see previous chapter), but he also noted that movementdf

either the target or the noise improved performance.

This phenomenon has already found practical application ini a rad r

technique called "time-compression" (Scallan, Roscoe & Williges, 1971 .

SThey found that if several frames of radar information are stored .bri

video disks and then played back quickly, the target will appear to

move and be much easier to detect among noise and other sources of

radar clutter.

r• The reader now has a general idea of when movement will help an •. .!observer perform a visual task and when it will impair. But there are

still many more data that should be gathered. Goodson and Miller

(1959), in discussing their dynamic acuity results, note that Ludvigh

(unpublished) has considered the problem of relating acuity data collected in the field to more complex tasks in a way that wculd allow

prediction of performance on the complex task. But just as the rela- tion between dynamic and static acuity is weak, so might be the relation between dynamic acuity and complex recognition tasks. Especialli if as Breitmeyer et al. (1974) suggest, dynamic acuity demands use of

the high spatial frequency information in a pattern, while during a

complex recognition task an observer may rely on information at lower

frequencies.

A more promising approach would be to collect field data on the

visibility of moving sinusoidal gratings similar to those of van Nes

(1968) and of Tolhurst et al. (1971) and then analyze only data from the particular spatial frequencies which are important to the complex task under consideration. This latter approach is exemplified by

Harmon (1974), who selectively filtered various bands of spatial frequencies from portrait photographs and measured the ability of observers to recognize the subjects. By combining these techniqt:es, an

engineer would know what frequencies were being "filtered" by the

display motion, and now this would affect the observer's task.

123
II . ~110

F 90

70 Target speed

0 Static

X 13 deR!/ec

50

300

. 301 

1~2-

0~0

1 xi 

0 4 8 3.2 is 20

Angle from fixation point (deg)

Figure 52. Detection time as a function of distance from fixation

point (from Peterson & Dugas, 1972).

124
CHAPTER 14

Temporal Events Causing Distortion

Perceptual distortion is an ambiguous and difficult term. In this

chapter it refers simply to a perceptual difference on a non-intensity

dimension (e.g., length, size, color, etc.) between a steady-state

display and the same display when fleshed, flickered, or moved. Note

that flicker-induced brightness enhancement is not included in this

definition since it involves an intensity dimension (i.e., brightness).

Since there is a large body of literature on this particular topic, it

has been treated separately (see chapter entitled Brightness Enhancement).

In the case of movement, distortion often seems to be an indirect

consequence of errors in eye movement during pursuit tracking of the

moving object. The eye cannct always keep up with the object and the

observer's visual system does not seem able to appreciate the error

fully. As an example Mack and Herman (1972) found that when obervers

tracked a small target moving along a 200 track at 4.5 0 /sec, they

underestimated the length of the target's motion by 17% -- it only

appeared to move 16.50. At a faster speed (19.5 deg/sec) the error was

only 6%.

Similarily, Bridgeman, Mayer, and Glen (1976) had observers track

a "ramp" motion (i.e., the target's motion had both horizontal and

vertical components, the horizontal component greater than the vertical). Observers tended to track only the horizontal component.

"ignoring" the vertical. As a result, the vertical component was more

accurately perceived and the motion appeared to have a steeper slope

(i.e., had a greater vertical component) than it actually did.

Errors in eye-tracking also make the circular path of a target

look smaller than its actual path. Coren, Bradley, Hoenig, and Girgus

"(1975), and Bradley (1977) had observers track a target moving in a

circular path 200 in diameter. At slow velocities, the eye tracked the

target well and distortion was small (Figure 53); at high speeds, the

observer was unable to pursue it and so inferred a velocity derived

from the movement of the target's image on the retina. At intermediate

speeds, however, the eye could pursue, but could not keep up. The

path of pursuit of the eye is smaller than that of the target's path,

and so the target's path seems smaller in diameter at these speeds.

This same type of distortion makes square or triangular motion paths

appear to be "bowed" inward (Festinger & Eastman, 1974; see Figure 54).

Figure 54 also shows that the distortion present during fixation of a

stationary point is much less than during the pursuit condition, This

suggests that in some applied situations, distortion may be lessened by

instructing the operator to fixate on a stationary point in the display

and not to follow the target with his eyes.

Pursuit eye tracking causes a different type of distortion in displays that simulate smooth motion -- television, computer graphics, motion pictures, etc. To economize on film, or reduce computer overhead,

125
• , 20

18I

1-4

-16

tq.

-U TRAcCK

14

.--- U FIXATION

0.5 1.0 1.5 2.0 2.5 5.0

ROTATIONAL VELOCITY, C/SEC

Figure 53. The apparent diameter of the target, path under fixation and tracking conditions is plotted as a function of the speed of target rotating (from Coren, Bradley, Hoenig, & Girgus, 1975).

126
Figue of 5. he Prcetio athof taget ovig i a quae o

triagulr lies pah. ndicte Sold he hysial aths an tI

[row indicate___ th ieto fmto. Dahdlnsdsrb h

pecie pahs (Fo-et r& ata,17)

/ N 127
a "moving" object may appear in the same location for more than one

frame before changing location. Some motion picture projectors use

a multiple-blade shutter to accomplish this. By presenting the same

frame more than once, flicker rate can be increased without having to

increase the number of frames of film. Certain of these systems provide a flicker-free picture, but if the eye tracks a small moving object in the scene, the observer may see multiple images of that

object. Kintz and Witzel (1972) found that when a two-bladed projector

was used, observers saw two images of the moving object (the projector

shows each frame twice). Likewise, a three-bladed projector produced three images. The image is displaced abruptly from one position to the

next, but the eye is tracking smoothly, thus for any given position, each presentation of the object falls on a different part of the ret- , R ina, and several images are formed. Hempstead (1956) and Braunstein

(1966) found the same effect with computer generated displays, and

Riley (1977) noted a similar effect with LED displays in which the

elements are sequentially refreshed.

Another type of distortion may occur even when the observer fix-

"ates on a stationary reference point. Ansbacher (1944) found that a L !rotating arc of light (about 360 off the circular path along which it

moved) seemed to shorten to a fraction of its length when rotated at sub-fusional speeds. This phenomenon is generally referred to as the

Ansbacher effect. He found that under conditions in which the radius of the motion path was 14.3 deg, the shrinkage increased as the speed a

was increased from zero to 1.3 revolutions per second (r.p.s.). At I, this maximum speed, the arc seemed to shrink to one-fourth of its

physical size. Stanley (1966, 1968) using an almost identical display,

found the same result and in addition varied the length of the arc.

The effect is not as strong for short arcs (see Figure 55, from

Stanley, 1968). Also, the effect does not occur for black arcs on a

white background, (Marshall & Gordon, 1973) and if there is an effect,

the arcs appear to be slightly longer when rotating (Stanley, 1966).

,•.• _'.Gov-don, The effect 1973).is not seen under conditions of dark adaptation (Marshall &

SIn a series of ingenious experiments, Day (1973) provides strong

• evidence that the Ansbacher effect results from a type of masking.

Cotsidering the temporal 5timulation at any one point along the path of

the arc: when the leading edge passes, the sudden onset of light pro- duces a strong sensation which results in a short period of reduced

"sensitivity afterwards. (See chapter on Flash Sensitivity.) This

may explain why longer arcs suffer more shrinkage than shorter ones.

The longer ones provide the pioper temporal interval between the leading and trailing edge, so that the trailing edge is not seen. This

masking hypothesis may also explain why a black arc does not behave in

the same way. In practical terms, this theory can be translated into a

rule for predicting when 'he Ansbacher effect will occur; in order to

see the effect, both the leading and trailing edges of a moving target

must stimulate the same retinal area. Ansbacher (1944) himself provides

support for this rule. In one case he did not use an arc, but rotated

a tilted straight line (i.e., not tangent to the path) and thus each

part of the line swept over different parts of the retina. In this

case he did not see any shrinkage. In another experiment, he sequenti128
PhysicRl

K lenRth

7 nf arc

(cms)

12

6

-. I 5

9

4

V#

a:

S 3

L ; 2

,3

i,

I p

30 45 65

Speed of arc rotation (r.p.m.)

Figure 55. Mean ratio estimates of length plotted against speed of arc

rotation for each length of arc (from Stanley, 19G8).

'1 129
ally illuminated adjacent, non-overlapping sections of arc, in other 1 words, produced "apparent" movement of the arc and not real movement.

Again, in this case each part of the arc stimulated a different retinal

r area and shrinkage was observed. Throughout these observations, fixation had always been at the center of the circular path.

Day (1973) found that the radius of the path had to be at least

110 in order for the effect to be reliably seen and for shrinkage to be

appreciable. It seems that the effect is restricted to the periphery

of vision. Stanley (1970) also found that the effect was increased if

a stationdry random pattern was superimposed on the display.

The display designer would be interested in knowing if this distortiorn also applies to targets that are moving linearly rather than in

a circle. It seems that there is no reason that the effect would not

occur if a bright rectangle of 5 to 10 deg in length moved parallel to

its orientation at speeds of 40 to 60 deg/sec. In this case, eccentric

ity would change as it moved, and so might its apparent length as it

moved across the retina. Pollack (1953) observed shrinkage of a

linearly moving 10 spot of high velocity, but provides few details.

Tynan and Sekuler (1975a) observed that motion can produce a

i peculiar kind of contour completion effect. That study generated a

moving granting display of low spatial frequency (.75 deg/sec) on a CRT

and placed a long rectangular mask over the center, orienting it perpendicular to the bars. In the occluded area, a dim grating is seen,

moving at the same speed and in phase with the flanking gratings. When

the occluder is positioned parallel to the bars, the effect is not

seen. Also, if the grating's motion is stopped, the illusory grating Sd'isappears. Gratings of greater than 3 c/deg spatial frequency do not

produce strong illusory gratings. The effect is not due to scattered

light within the eye because the illusory grating can be produced by

flanking gratings on only twice their own contrast threshold, In fact, S. illusory gratings only .3* in height can produce illusory gratings

that span a 30 accluder. A similar effect occurs if the center of a

moving random dot display is occluded. In this case illusory dim

random dots fill the occluded area. Weisstein, Maguire, and Berbaum

"(1977) have repeated Tynan and Sekuler's observations and determined

that the illusory gratings can produce motion aftereffects much as real

gratings do.

The distortions produced by flashes and flickering are more puzzling and varied than those produced by motion. These distortions range

from mere changes in perceived luminance (similar, but not quite the

same as enhancement effects), to illusory contours, as well as colors

or drastic distortions in contours and colors that may be present 'in

the display.

In the chapter on Brightness Enhancement, the types of flickering targets that were discussed were fairly simple. If more complex

waveforms are used, however, peculiar phenomena may occur. For ex- - i ample, Anstis (1967) and Walker (1974) found that exposure to a "ramp"

flickering waveform produced a brightness aftereffect. If the ramp was

one of gradually increasing luminance (with an abrupt return to the

130
lowest luminance), a subsequently viewed steady light seemed to grow

gradually dimmer. The opposite aftereffect was produced by a ramp

waveform that produced gradually decreasing luminance (see Figure 56).

The 

should 

optimum 

be at 

frequency 

least two 

of 

log 

the 

units 

ramp 

(Anstis, 

is about 

1967). 

one Hz 

Walker 

and the 

used 

amplitude 

a mean

I

luminance of about 240 cd/m,.

Anstis reports that neither a point source nor a Ganzfeld target

is effective in producing the aftereffect, and retinal location and

color do not seem to matter. In addition, Walker found that the

increasing-luminance waveform produced a brighter display than the

decreasing-luminance waveform. There was no effect of waveform on CFF

or Talbot brightness, howevor.

For over 150 years, it has been known that observing a large homogenous flickering screen (usually 40o-600) produces a variety of illusory shapes and movement. Smythies (1957) describes some of these:

herring-bone patterns; checkers, spirals, whirlpools, concentric circles, families of parabolas, etc., and linear motion, oscillations, El; rotations, and all of them are often accompanied by vivid colors.

An example of two of these patterns is shown in Figure 57 from

Young, Cole, Gamble, and Rayner (1975). In each figure, only part of

the pattern is drawn, but these contours are perceived as surrounding

the centerpoint. In Figure 57, the center elements were hexagons or

dots of 5' to 12' in size; the larger grid elements in Figure 58 were

about 1.4 to 1.60 across. The optimal frequency for seeing the fine

pattern was pout 10 Hz and the threshold luminpnce for seeing them

about 10 cd/m 

. For the larger pattern the op imal frequency range was

10 to 30 Hz and threshold luminance was one cd/mv.

These findings agree with a series of studies done by Remole

(1974). He found that different types of illusory patterns were seen

on -i homogeneous screen at different luminances and temporal frequencies. At low photopic luminances, one sees stationary geometric patterns and color (Remole, 1973b, see Figure 59). The geometric patterns

were seen better with binocular than monocular vision. At even higher

luminances, movement was again seen -- swirling clusters of geometric

shapes, reversing birection of rotation every two to four seconds

(Remole, 1974). The lumiinance thresholds for this movement as a function of frequency were very similar to those for geometric figures, but

displaced one-fourth to one-half log units higher. Unlike some of the

other distortions discussed so far, there seems no way to prevent these

powerful effects.

Consider the less extreme situation of a smiller Oatterned display, which has been reviewed before for effects of flicker on sensitivity and visibility. Then, consider the effect on the perceived

spatial characteristics of the pattern; as studies of visibility,

grating patterns have been a popular tool for manipulating spatial

parameters precisely. In general, temporal modu'iation.of 'gratings produces an increase in the apparent spatial frequency of the patterns.

131
GRADUAL LUMINANCE INCRAEASE

(CLOCKWISE ROTATION)

DIM--ING AFTEREFFECT

"TrADY INTERMEDIATE LUMP'ANS1

.If

TIPAE

I- OBJECTIVE LUMINANCEi

GRADUAL LUMINANCE DECREASE

OUNTERCLOCKI4fSE ROTATION) . - MOMENTARY APPARENT BRIGHTNESS

-- AVERAGE APPARENT PRI!!TANIESS

SSTEADY INTER.IEDIATr LUMNIY'A•"'E S_ _ ....................

W "BIGHTENING AFTEREFFECT

TIME

r Figure 56. Diagrammatic illustration of objective luminance and apparent brightness as functions of time and rotation direction for a

.•, 'produces right-hand gradient disk. Clockwise rotation of a right-hand disk gradual increases in luminance arid sudden decreases across

time, arid counterclockwise rotation produces gradual decreases and

sudden increases in luminance. The sawtooth waveforms correspond only

approximately to the illumi'tance cnanges occurring at each point in the

retinal image of the rotating disk, since the luminance gradient in the

disk is stepwise rather than smooth. Momentary apparent brightness

L roughly follows the sawtooth waveform of objective luminance. Average

apparent brightness is greater for clockwise rotation. Exposure to

gradually increasing luminance (clockwise rotation) produces an aftereffect of apparent dimming in a subsequently presented stationary

uniform gray disk of intermediate luminance. Exposure to gradually

decreasing luminance (counterclockwise rotation) produces an aftereffet

of brightening (from Walker, 1974).

132
Fa

LI

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SFigure 58. Main contours of the grid pattern. The measurements are I;' , expressed in visual angle (minc * 1 S.E.M. , n = 5) (from Young, Cole,

Gamble, & Rayner, 1975).

• ' 134I
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10 2030 4
Presenting a grating as a short flash (in this case contrast is

modulated rather than luminance) produces an increase in the apparent

spatial frequency if the flash duration is in the range of 20 to 60

msec (Tynan & Sekuler, 1974; Kulikowski, 1975). Both found the effect

was optimal for low spatial frequencies (Figure 60, from Kulikowski).

Kulikowski found up to a 30% increase in spatfa,l frequency while Tynan Sand Sekuler found only 10%. This difference, as we shall sef, may be

due to the different mean luminances used: 5 and .2 cd/mr respectively.

Virsu and Nyman (1974) found a very similar effect when they

turned the contrast of a grating on and off at a rate of eight Hz (60

msecl "on" time, see Figure 61.) Again low spatial frequencies produce

the greatest effect, and optimal contrast was about .4. The effect was

greatest at photopic luminance levels.

L More complex types of temporal modulation may produce even greater

increases in the apparent spatial frequency of a grating. Reversing

the phase of a grating at a high rate (sometimes called counter-phase 

4

L flicker) may double the spatial frenuency (Kelly, 1966; Richards &

Felton, 1973; Kulikowski, 1975; Virsu, Nyman, & Lehjio, 1974). In this

case, the higher the temporal frequency (below CFF) the closer •he

pattern comes to doubling its apparent spatial frequency. Though this

type of flicker would be rare in an applied setting, Virsu, et al.

(1974) increased apparent frequencies by up to four times, using mult"-

phase flicker. Note that the spatial frequency increases caused by thc.

on - 'off" types of displays seem to be distinctly different lreo

those due to flicker accompanied by phase changes.

Findings by Erlebacher and Sekuler (1974) suggest that these

similar distortions may occur in non-repetitive targets. They looked

at the effect of

2 flash duration on the perceived length of a thin 2.60

line of 28 cd/mr luminance, presented against a dim background. As

duration was reduced from 20 to 30 msec, line length reduced by about

4%. Although small, the effect would probably be greater for figures "of greater area and higher luminances.

There is at least one case when flashing makes a target seem

longer than it is. Burns, Mandl, Pritchard, and Webb (1969) foundthat

if a dim, 13' diameter spot was flashed against a dim background at 

a

location 10 deg in the periphery for a duration of 10 to 50 msec, it

often appeared as a line of 5' to 30' in length, and was, indisti,-

guishable from.a real line. The orientation of these illusory !ines

was usually close to horizontal, but when two dots were flashed and

both looked like lines, they did not always havq the same orientation.

This means that the lines were not caused merely from visual smearing

of the dots due to eye movements. The effect is not critically dependent on location, luminance, or duration.

One case *has already been mentioned, in which an observer sees

color although no chromatic objects are present: occurring along with

the illusory figures seen when viewing a large flickering field. However, there are other color effects for which specific colors are

determined by specific temporal waveforms of achromatic objects. Most

136
130

i

110

ft

100 .

0" 0 20

Snatial Preauancy c/dag

Figure 60. Spatial frequency match at a contrast of 0.5 and at a

presentation time of 50 msec (from Kulikowski, 1975).

137
T I F -

I/.

110

120

.-. f~~O%~1001

11 L ~ 0cclsdg

______ _____110

10 0.5 cycle deg f

-10 10 4 cycles deg -1

M 120 ~

(V ~1201

S 110 - C- 110 " iI

0.25 cycle de-[ 100 100 A

__________-______I I I I

0 1 0 2ycl32

0 0.5 2 8 32 3 22 0 0.5 2 

Temporal 6equency (Hz)

Figure 61. Apparent spatial frequency as a function of temporal frequency, spatial frequency, and adaptation level. Contrast 0.82; adap- tation level for the comparison grating was a constant 5 td. (from

Virsu & Nyman, 3974).

V+ 138
of this work has been done with a spinning disk type of stimulus called

"Benham's top" (see Figure 62, from Sheppard, 1968). Three circular

bands are seen when this disk is spun and each one has a different

color. For a review of this work, see Cohen and Gordon (1949). These

reviewers point out, however, that these subjective or illusory colors

may also be produced with just flickering lights. In fact, Festinger,

Allyn, and White (1971) have in a way "cracked the code" of subjective colors, and by using various complex temporal waveforms can produce

almost any color.

Sheppard (1968) reports that rather simple temporal waveforms can

produce subjective colors in black and white photographs or on black

and white television. In fact, before color television became common, "colored" television commercials were broadcast in Europe by fiickering

the image. The colors occar at temporal frequencies between 5 and 10

Hz and at a pulse-to-cycle fraction of one-eighth or one-fourth.

r, Campenhausen (1973) found that the visual system is very sensitive

to changes in the temporal relationships that produce colors in displays like Benham's top. A change in phase of less than one msec

between the flicker in two parts of the top produces a noticeable

change in color. If these effects are applicable to a large population

of observers, it might be possible for a display to transmit information about subtle changes in the temporal relationships among the

elements of a complex system by making use of these color effects. See

also Carter (1980) for a review of how color improves search performance.

In addition to producing colors in achromatic objects, temporal

modulation can distort the color of chromatic ones. Flash duration has

two effects on color. Kinney (1965) found that colors desaturate as flash duration is reduced over a range of 400 to 50 msec. The author

Sused both five and one degree diameter targets of 3.183 cd/m 2; desaturation was greater for the larger target. Luria (1967) and Kaiser

(1968) demonstrated that short flashes of low luminance (less than 43

td and 300 ns in the latter case) produced artificial tritanopia or

"blue blindness". One consequence of this blindness is a loss uf the

ability to differentiate colors in the blue-green part of the spectrum.

Flicker also produces dramatic changes in perceived color. Ball

and Bartley (1966), and van der Horst and Muis (1969) found that colors

at the low end of the spectrum looked like colors normally associated

with shorter wavelengths; reds, for instance, looked more like orange

or yellow. Very short wavelengths, however, appeared more like colors

associated with longer wavelengths (see Figure 63). In addition, Ball

arid Bartley (1966) noticed considerable desaturation of short wavelength colors. These effects occurred only at luminances above 1600

cd/m , and Nelson and Bartley (1961) found that a pulse-to-cycle fraction of about .25 was optimal for producing both the hue shifts and

desaturation effects. These effects coincide closely with brightness

enhancement (van der Horst & Muis, 1969).

L 139
W' UW' - ZZ2TL ... z7I.,. - -.

r

L.

I

I

I.

Figure 62. Benham's top (from Skppard, 1968).

140

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.[X I -15I uI ! I ! ~

PLICUR F-U 100

Figure 63. The hue shift es a function of flicker frequency for several wavelength!. Retinal illuminance 4800 td (from van der Horst and

Muis, 1969).

141
GLOSSARY F

Absolute motion threshold: the minimum velocity of an object at which

an observer can tell that the object is moving.

Adaptation level: 'the observer's sensitivity to light, determined by

the mean luminance of the display and by how long the observer has

been exposed to this mean luminance. It takes only five to ten Sminutes to adapt to photopic luirinance, but 20 to 40 minutes to

adapt to scotopic luminance. This term is often used interchangeFably with "mean luminance", but in these cases it is assumed that

Sthe observer has adapted to the mean luminance level.

• !Ansbacher effect: the apparent shrinkage of a bright, rotating arc.

Apparent motion: the illusion of motion between two sequentially

presented, spatially separated objects.

Aubert-Fleischl phenomenon: a moving object appears faster when

tracked with the eyes than when the observer fixates on some

' stationary object in the display.

Autokinetic effect: a small light viewed in an otherwise dark room will appear to move in small random steps, presumably as a consequence of eye movements.

Benham's disk (or top): a disk with a black and white pattern on it

that appears colored when rotated (see Figure 62).

Bloch's law: Below some critical duration, the percept is controlled

by energy (the product of duration and luminance). Consequently,

luminance and duration can be interchanged without changing the

percept. Usually this law refers to the detection of a flashed

target, but may refer to several other types of visual function r" : (i.e., acuity, color naming, etc),

Broca-Sulzer effect: With increase in the duration of a flash, brightness increases up to a maximum and then declines with further

increase. The "enhancement" of brightness at a particular duration is the Broca-Sulzer effect.

BrUcke-Bartley effect: The brightness of an intermittent (flickering) light is enhanced over that of a steady light of the same mean

luminance when the intermittency is on the order of 8 - 13 Hz.

CFF (Critical flicker frequency): the frequency of a flickering light

above which the light appears to glow steadily.

Contrast; for a patterned display, the ratio between the bright and

dark parts of the pattern. Several formulas for contrast are

given in Appendix B.

142
Difference limen or threshold: the minimum perceptible change in a

stimulus.

Extended source: a luminous object that is larger than ahout 20' of

arc.

Ferry-Porter law: The critical flicker frequency (CFF) increases with

the log of the luminance of the flickering object.

Ganzfeld: from the German meaning "whole field"; a homogeneous field

m that fills the entire visual field of the observer. Standing in

vary dense fog would produce a Ganzfeld.

Granit-Harper law: The criltical flicker frequency (CFF) incireases with

the log of the area of the flickering object.

Illuminance: the effectiveness of a source of light such as a light

bulb in stimulating the human eye. The standard measure is the

lumen.

Induced motion: the illusory motion produced in a stationary object by the mot-on of other ,objects around it, or the modification of the

perceived motion of a moving object by those around it.

Korti's Laws: a set of simple -equations that relate various determinents of apparent movement and predict, somewhat imperfectly,

the conditions under which apparent movement ought to be seen most

strongly.

Landolt ring or "C": A common stimulus for measuring acuity, it is a

high contrast circle with a small gap in one of four locations -- up, down, left, or right. The observer's task is to indicate the

position of the gap. The tester uses rings with various size gaps

to measure the observer's acuity.

Linearvection: the illusion of self-movement when moving contours

stimulate an observer's peripheral field of view.

Luminance: the ability of an extended light source such as reflective

paper to st"iruiate the human eye. The most common measure is

candelas per square meter (cd/m 2 ).

Mach bands: the illusory enhancement of contrast near the horder

between two areas of different luminance.

MAE (Motion aftereffect): the illusion of motion of a stationary

object after prolonged observation of moving contours. The illusory motion is in a direction opposite that of the previously

seen motion.

Mesopic luminance: the luminance range between photoplc and scotopic.

Photopic luminance: luminances above about 10 cd/m2 or 100 td.

143
i7 1

Point source: as a rule of thumb (although there is no universal

agreement), a source smaller than about 20 minutes of arc.

Scotopic luminance: luminances below about 10 cd/m 2 or 5 X 102 td.

Suprathreshold: above the minimum signal strength needed to detect,

this term usually denotes that the object or signal is easily

"detected.

Talbot's law: At frequencies above flicker fusion, brightness is

proportional to time-average luminance. Usually it assumed that

the brightness of the "Talbot level" and that of a steady light of

the same time-average luminance are equal.

Troland (td): a unit of retinal illuminarce that takes into account

the area of the pupil through which the stimulus is viewed. On

the assumption that the light passing through the pupil is proportional to its area, the product of luminance and pupillary area

describes retinal illuminance. One troland is produced when a

luminance of I cd/m2 is viewed through a pupil of I mm2 (see

Appendix A).

Visual angle: the arc tangent of the target size divided by the distance of the observer from the target. This provides a standard

unit for the size of the image of an object on the retina.

Visual masking: the decrement in the visibility of a target due to the

presence of another target in c0ose spatial and/or temporal proximlty to it.

Visual persistence: the extension of the visual effects of a flash

beyond the offset of the flash itself.

Weber's law: used to describe the case when difference thieshold along

some stimulus dien& on is proportional to the strength of the

signal (i.e., a constant, where Al is the difference

* threshold of I, the signal level).

J

144
APPENDIX A

K The troland (td) is the unit of illuminance on the retina. It is

often preferred to measures of luminance for experiments that examine

the effects of changes in stimulus luminance because it eliminates

variation in the effective stimulus which come from changes in pupil

size. The formula for computing trolands is

Trolands = (luminance in cd/m2 ) X (area of pupil in mm2 )

where area = 1/4.n. D2 (D denotes the diameter of the pupil).

In most experiments which use the troland unit, the pupil diameter

is controlled by placing a small aperture very close to the observer's

eye. This "artificial" pupil is always smaller than that of the natural

pupil so that effective pupil size remains constant across changes in

light level, or the momentary physiological state of the observer.

It is difficult to compare two sets of data when the luminance in

one case is in troland units and in the other case is in units of

stimulus luminance, because in the latter case the size of thc pupil,

in this case the natural pupil, is unknown. Yet the data collected in

units of luminance may be more valuable to the applied scientist because artificial pupils are unlikely to occur in the applied setting.

One way around the problem of comparing data given in units of retinal

illuminance to those given in luminance units is to take advantage of

tables which give the average expected pupil diameters for observers of

different ages at various luminance levels.

"A

145 F

I: .

• - I

*i i

1, 145,
APPENDIX B

Contrast is the ratio of luminance in the brightest and darkest

parts of the visual field, but this ratio is calculated in several

different ways depending oil the nature of the stimulus. For displays

consisting of one or a group of separate objects, such as letters,

spots, figures, etc. , the most commcn 'Formula for contrast is:

Lt bi

contrast - Lb1

where Lti target luminance and Lb is background luminance.

If the pattern is repetitive, such as sinusoidal or square-wave

bars, the contrast of the pattern is represented by:

2 Lmax Lmin

contrast Lma + L (2)

where L is the maximum luminance in the pattern and L i s the

mi ni mum M¶~mi nance. This ratio is sometimes multiplied tV100 and

called the "percent modulation" of the pattern. Notice that this ratio

varies only between zero and one, while the ratio calculated by equation (1) can assume any value.

*1 146
APPENDIX C

seveOur primary data base has been Robert Sekuler's index file of

several thousand articles. Also, Commander Robert Kennedy of the Naval

Biodynamics Laboratory (now of the Canyon Research Group) allowed us

the use of his article files. In addition, we searched through

Ergonomics Abstracts from 1969 to the present, Psychological Review and

Psychologi Bulletin from 1968 to the present, and all of Human

Factors. We also searched two more data bases through Northwestern

University's computer-assisted information service. The first was

Psychological Abstracts data base (1967-present), and the search

yielded 679 printouts at a cost of $74.40. The other was the U. S.

National Technical Information Service data base (1964-present), which

I' yielded 67 printouts at a cost of $74.60. We estimate that the proportion of these articles that actually were incorporated into this review

was less than ten percent. Many of the articles that were used in the

final version of this review were not found in the search phase of this

project, but were referenced by the articles we initially started to

read.

Our main difficulty was obtaining articles of an applied orientation. Many had to be ordered through NTIS on microfilm, and others were

difficult to obtain because they were University or corporation inhouse reports. Luckily, Commander Robert Kennedy's files provided us

with some of this material.

147

.... . . ~~- . . ---- ..: ; - T
J

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167
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168
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II

:I .L

,17

I- _ _ _ _ _:
AUTHOR INDEX

Aarons, 33 Chapanis, 85-87

Ailslieger, 29 Charpentier, 78

Alexander, 99, 123 Cheatham, 85

Allan, 19 Cohen, J., 139

139 Cohen, R. L., 19 

Anglin, 90 Cole, 10, 131, 133, 134

Ansbacher, 128 Colgate, 112

Anstis, 32, 130, 131 Collins, 80, 92

Arend, 92 Corballis, 31

Aubert, 18 Coren, 125, 126

Corwin, 93, 95

Baker, 96 Cream, 7, 76, 80 - Ball, 12, 13, 93, 139 Cristal, 38

Banks, 32 Croft, 29

Baron, 97

Bartlett, 81, 82, 85 Day, 31, 32, 128, 130

Bartley, 85, 97, 98, 139 deLange, 69-71

Basler, 45 Cichgans, 27, 52, 54

Bauer, 52 DIck, 29

Baumgardt, 57 Diener, 27

Benson, 45 Dugas, 116, 119-122, 1?4

Berbaum, 130 Duncker, 38

Bevan, 57

Biederman-Thorson, 45 Eason, 85, 86

Bird, 71, 72 Eastman, 125, 127

Black, 97 Efron, 78, 80

Bonnet, 18-20, 31 Ellingstad, 28

Boring, 33 Emerson, 31

Borkenhagen, 47 Eriksen, 80

Bouman, 65, 111 Erikson, 112

Bourassa, 97, 98 Erlebacher, 136

Bracewell, 70 Ettinger, 75

Braddick, 34, 36 12 Bradley, 125, 126 Fankhauser, 47

Brandt, 27, 52, 54, 55 Favreau, 31

Braunstein, 128 Fehrer, 80, 81 SBreitmeyer, 111, 112, 123 Felton, 136

"Bridgeman, 125 Fergenson, 105

. Brooke, 75, 76 Festinger, 125, 127, 139

Brosgole, 38 Foley, 75

Brown, 71 Ford, 4

Brown, B., 105, 107, 109 Forsyth, 71, 85-87

Brown, J. F., 19, 27, 49

Brown, J. L., 97, 99 Gamble, 131, i33, 134

"Brown, R. H., 23, 24, 28 Ganz, 112

Burns, 116, 136 Gebhard, 8, 86

Byham, 86 Gerathewohl, 89, 97

Gerhard, 28, 29

Campenhausen, 139 Gibson, 10

Carpenter, 38, 112 Gillingham, 13

171
Gilson, 116, 118 Kaflin, 31

Gtnsburg, N., 67, 77 Kahneman, 34, 96

Giorgi, 76 Kaiser, 139

Girgus, 125, 126 Kane, 32

Glad, 93 Kaplan, 107, 110, 111

Glen, 125 Keck, 32

Goldstein, A. L., 25 Kelly, 67-71, 73-77, 99, 136

Goldstein, D. A., 7 Kennedy, 23

Goodson, 115, 117, 123 Kimball, 28 Goodwin, 7 Kinchla, 19

Gordon, D. A., 43, 139 Kinney, 139

Gordon, S., 128 Kintz, 128

Gottsdanker, 24, 25 Kirkwood, 99, 102-104

Graham, 18-20, 33 Kitchel, 5, 7, 8

Gregory, 32 Kitterle, 95

Grindley, 31 Klein, 49

Grossberg, 80, 81 Koenderink, 111

Gr.sser, 93, 94 Koenig, 52, 54

Guedry, 45 Kolers, 33-36, 38

Krauskopf, 111

Haber, 78-80 Krebs, 10

Haines, 57, 58, 81, 84 Kubovy, 96

Hamerman, 47, 48 Kugelmass, 75

Hamwill, 77 Kulikowski, 136, 137

Harmon, 112, 123

Hart, 112, 113 Lamb, 7

Haselrud, 13, 62 Landis, 75, 77

Hecht, 76 Lange, 45

Heimstra, 28 Lavender, 32

Held, 52 Layton, 13

Hempstead, 128 LeGrand, 85

Henderson, 116 Lehjio, 136

Henderson, D. C.. 20, 22 Leibowitz, 18-20, 22, 43, 45,

Herman, 125 47

Herrick, 56, 57, 62, 64-66 Levinson, 71 [ " Hershenson, 78, 79 Lichtenstein, 4

Hillman, 57 Link, 43, 45, 46

Hoenig, 125, 126 Locke, 31

Holland. 30-32 Loomis, 39

, Holmgren, 39 Love, 111

Hylkema, 76 Ludvigh, 105-107, 123

• • Lunkenheimer, 93

Ikeda, 13, 62, 63 Luria, 139

I.sabelile, 43

Mack, 125

. Jacobs, 10 MacKay, 32

i .Jenney, 5, 7, 8 Mackworth, 107, 110, 111

J.-Jrison, 1.2 MacLeod, 81, 82, 85

Johansson, 25, 55 Magnussen, 93

Johnson, 18-20, 43, 47 Maguire, 130

Johnston, 10, 11, 13-15 Mandl, 136

Jordan, 30 Mansfield, 90-92

SJulesz, 34, 112 Marks, 92, 93

Marshall, 128

172
Mashour, 23 Scott, 30-32 - -

Masland, 31 Sekuler, 12, 30-32, 39, 41,17, 49-

Mates, 19 51, 53, 99-101, 130, 136

Mayer, 116, 125 Sen, 71

McColgin, 21, 22, 43-45 Sgro, 34

McNaughton, 13 Sharpe, 112, 113

MCWhirt, 32 Sheppard, 139, 140

Miller, 105-108, 116, 117 Shinar, 116

Misiak, 77 Shlaer, 76 Miorin, 29 Singer, 57, 62

Mowbray, 8, 71, 72, 86 Smith, S. L., 7

Muis, 93, 139, 141 Smythies, 131

Soules, 97

Nachmias, 99 Sperling, 36-38

Nakayama, 39 Spigel, 33

Nelson, 97, 139 Standing, 78, 80

Neuhaus, 34-3b Stanley, 128-130

Norman, 96 Stevens, 23, 30, 49

Nyman, 136, 138 Strelow, 31

Suzansky, 105

SOsaka, 90, 92 Takeuchi, 13, 62

Palella, 32 Taylor, 10, 12, 13, 57, 59-61, 97

SPantie, 30-32 Teichner, 10

Peterson, 116, 119-122, 124 Thompson, 25, 27

Pickett, 12 Thorson, 45

Pierenne, 76 Tolhurst, 85, 112, 113, 123

Pollack, 130 Torres, 20, 43, 45

Poppel, 57 Treat, 116

Pouthas, 31 Tyler, 20, 43, 45

Powell, 30, 32, 39, 40 Tynan, 39, 41, 47, 49-51, 53,

Pritchard, 136 99-101, 130, 136

Raab, 80, 81, 90 Uetsuki, 62

Rabel,, 93, 94

RacilliT, 23 Valerie, 4, 43, 45, 46

SR-tliff, 39 van de Grind, 93, 94

.Rayner, 131, 133, 134 van den Brink, 65

Remole, 95, 131, 135 van der Horst, 93, 139, 141

.Richards, 136 van tes, 111, 112, 114, 115, 123

Riley, 128 Verrip, 76

Rinalducci, 92 Virsu, 136, 138

Ri e 5 Von Bruecke, 49

SRueoehr ,25, o Rogers, 75 Walker, J. T., 23, 27, 130-132

SRoscoe, 123 Walker, P., 39, 40

Rosenbaum, 28 Wasserman, 93

-Roufs, 56, 57 Waygood, 76'

i•Runeson, 4, 25, ZE Weale, 77

Rudolph, 32 Webb, 136

Webster, 13, 62

Scallafi, 123 Weiner, 28

Schmerler, 5, 24, 25 Weisstein, 130

Schmidt, 47 Welde, 7, 76, 80

173
Wendt, 23

Wepman, 111

Wertheimer, 33

Westheimer, 97

White, C. T., 4, 85, 86

White, C. W., 139

Wilkinson, White, J.,W., 92 31

Wist, 27

Witzel, 128

Wolman, 34

Yessenow, 23

Young, 131, 133, 134

Zihl, 57

S174
SUBJECT INDEX

absolute motion threshold, 18 constant distance hypothesis,

acceleration, 4, 20, 24, 25 18, 20

acuity, 13, 16, 24, 97 contour completion, 130

acuity for non-pursued objects, contrast, 27, 97

107 contrast and DVA, 107

acuity and duration, 96 contrast and MAE, 32

acuity visual distortion, 8 contrast and visibility, 111

aerial reconnaisance, 116 contrast enhancement, 95

age effects, 13, 14, 76 contrast sensitivity, 99

airborn target acquisition, 55 critical duration, 56, 62, 65,

anisotrophy, 85 96

Ansbacher effect, 9, 128

Ansbacher effect, definition, darkness enhancement, 93

128 deceleration, 25

apparent motion, 5, 30, 33, 36, design criteria, 5, 7, 8, 19,

111 22-24, 28, 29, 32, 33, 36,

apparent motion, definition, 33 45, 49, 55, 56, 62, 65, 71,

apparent motion in periphery, 75, 76, 80, 81, 85, 86, 90,

45 92, 99, 105, 107, 111, 112,

apparent velocity, 25, 27, 28 116, 123, 125, 130, 136

attention, 8 detection, 97

difference threshold for

background luminance, 81 velocity, 23, 45

Beta motion, 33 direct scaling, 23, 90

Benham's top, 139, 140 direction uncertainty, 12

Bloch's law, 56, 80, 90, 96 directly sensed motion, 18

Bloch's law and eccentricity, displacement, 22

57 display design, 67, 70, 89

brightness aftereffect, 130 distance-time reciprocity, 20

brightness enhancement, 8, 85, duration discrimination, 78

90, 92, 93, 97 duration uncertainty, 11, 12

brightness in MAE, 32 dynamic visual acuity (DVA), 8,

Broca-Sulzer effect, 90, 92 105, 107, 112, 116, 123

Brdcke-Bartley effect, 90, 93, DVA and driving, 116

97

eccentricity, 32, 43

centrifugal . centripetal eccentricity and flicker, 57

motio , 32 economy of film, 125

CFF, 7, 67, 71, 85, 131 enhanced visibility by motion,

CFF and size, 75 116

change indicator, 16 enhancement, 123

CIE, 13 elevator effect, 55

coding medium, 7 exposure duration, 4, 19, 28,.

collision, 5 81, 92

color and CFF, 76 eye movements, 8

complex patterns, 99

computer generated display, 36, Ferry-Porter law, 75

39, 128 fixation, 4, 16

constant displacement hypothesis, fixation and induced motion, 38

20 flashdetection, 56

175
flash duration, 7, 81 lower speed threshold, 18

flash sensitivity, 7, 85 luminance, 27

flash train, 62, 65 luminance effects, 20, 116

flicker, 7-9, 16, 57, 67, 71

flicker and acuity, 97 mach bands, 39, 95

flicker and apparent line MAE duration, 30, 31

length, 136 magnitude estimation, 23, 30, 90

flicker and eccentricity, 75 masking, 128

flicker and luminance, 75 Military Standard, 4, 16, 17,

flicker and perceived spatial 67

frequency, 136 motion aftereffects (MAE), 5,

flicker and periphery, 76 30, 32, 130

flicker and saturation, 57 motion contrast, 39, 42

flicker and size, 75 motion parallax, 24

flicker and spatial frequency, motion prediction, 28

76 flicker and television, 76 nonsense figures, 99

flicker and visibility, 99

flicker discrimination, 86 object recognition, 123

* flicker sensitivity, 7, 67 oscillation, 43, 111

frame of reference, 

5

frequency doubling, 136 perceived duration, 78

, functional field, 13 perceived flicker rate, 86

perceived numerosity, 85, 86 

4

Ganzfeld, 67, 131 perceived rate, 8, 85

Granit-Harper law, 70, 75 perceived spatial frequency, 99,

gratings, 27, 85, 97, 99, 111, 136

112, 123, 130, 131, 136 perceived speed and eccentricity,

49

heads up displays, 99 perceived speed in periphery, 49

hysteresis, 25 perceived velocity, 

4

perceptual persistence, 80

illisory colors, 131 perimetry, 13

illusory contours, 131 peripheral field, 7, 16, 20, 22,

individual differences, 13, 47, 43, 65, 81, 112

55, 77, 105, 107 phenomenal slowing, 19

induced colors, 9, 131, 136, Phi motion, 33

139 pilots, 7, 116

induced flicker, 95 position uncertainty, 11, 12

induced form, 131 practice effects, 125

. induced motion, 38 psychometric function, 10

induced motion, definition, 38 pursuit eye mavements, 125 A .inferred motion, 4, 18, 20

intermittent motion, 29 quantizing display, 

5 i IS•, 33 . ,3radar, 7, 29

kinesthesis, 52 random dots, 34, 36

Korte's law, 33, 34, 36 reaction time, 12, 29, 47, 80,

81, 85, 89

linearvection, 7, 16, 52 real aný illusory motion, 32

letter recognition, 112 reference lines, 

4

line length, 19 reference marks, 19, 20, 25, 55,

long-term, MAE, 31 76

low spatial frequencies, 112

176
refraction and peripheral velocity discrimination, 23, 24

motion, 47 45

retinal smearing, 105 vertical vs. horizontal, 111,

rod and frame motion, 38 125

vestibular, 52

selection and training, 125 vibration, 71, 128

self-motion, 52 visibility enhancement by simulaLion, 55 motion, 112

sinusoidal modulation, 70 visual field, 13

size, 19, 20 visual phantoms, 

9

size and velocity, 27 visual tracking, 107

size of velocity, 27

size uncertainty, 11 Weber-Fechner law, 70

SOA, 34 Weber's law, 24

spatial discrimination, 96

spatial distortion, 128, 130, 131

spatial factors in MAE, 31

spatial frequencies, 27, 123

spatial frequencies acuity, 

8

spatial structure, 25

spatial summation, 57

spatio-temporal interactions,

112

speed of MAE, 31

spped threshold, 18

speed uncertainty, 12

spiral aftereffect, 30

stimulus uncertainty, 10, 22

stroboscopic illumination, 29

stroboscopic motion, 32

suprathreshold, 23

suprathreshold flicker, 78

sustained vs. transient

responses, 85

tactical information display,

99

Talbot brightness, 1Z1

television, 116

temporal frequency, definition,

temporal impulse response, 62

temporal modulation, 

8

temporal uncertainty, 11, 12

time sampled display, 25, 123

L time sampled motion, 29

tracking, 4, 23, 28, 29, 125

Troxler effect, 57, 62

S:1 vection, 112

:.i velocity, 23

velocity adaptation, 25

velocity and MAE, 30

177
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NIJMlEH uF ADURESSLEbS 22R

TO)TAL NuMBER OF COPIES 406

182 -U.S. GOVERMHENT PRINTINS OFFICE: 1981--361-662/2804
[!SUPPLEMENTARY

0

/,(

INFORMATI

i [ I

\ •
. ITNCTASSIFTRfl SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered)

REPORT DOCUMENTATION PAGE READ INST&rCTIONS

BEFORE COMPLETING FORM

1. REPORT ,lUMBER |2. GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

Technical Report 540

4. TITLE (and Subtitle) S. TYPE OF REPORT & PERIOD COVERED

SOURCEBOOK OF TEMPORAL FACTORS AFFECTING Final

INFORMATION TRANSFER FROM VISUAL DISPLAYS

6. PERFORMING ORO. REPORT NUMBER

7. AUTHOR(&) S. CONTRACT OR GRANT NUMBER(&)

S u R. Sekuler & P. D. Tynan

SR. S. Kennedy N61756-76-M-5961

S. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT, PROJECT, TASK

Psychology Department, Northwestern University, AREA & WORK UNIT NUMBERS

Evanston, Illinois; Canyon Research Group,

Westlake Village, CA

It. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

US Army Research Institute for the Behavioral June 1981

and Social Sciences, 5001 Eisenhower Avenue, 13. NUMOER OF PAGES

Alexandria, VA 22333 177

IS. MONITORING AGENCY NAME & ADDRESS(I dflllerent from Controlling Office) 15, SECURITY CLASS. (of Ofl* report)

Unclassified

ta, DECLASSI FIC'ATION/IDWNGRAOING SCHEDULE

IS. DISTRIBUTION STATEMENT (oa tlh. Report)

Approved for public release; distribution unlimited.

17. DISTRIBUTION STATEMENT (of the abstract emtered In Block 20, It dilferent frtm Report)

IS. SUPPLEMENTARY NOTES

This report was funded as a collaborative effort by the U.S. Army

Research Institute with the U.S. Naval Pacific Missile Test Center, Point

Mugu, California and U.S. Naval Biodynamics Laboratory, New Orleans,

Louisiana.

19. KEY WORDS (Continue on rever** side it necessary and identity by block number)

Temporal factcrs Vision Illusions

Human Engineering Design Criteria Vection Dynamic Visual

Visual Displays Motion Perception Acuity

Military Standards Flicker Display Design

Contract Sensitivity Brightness Enhancement 20. AI'TRACT (Cm soir mum &M N nemoa mad tdetlify by block member)

This report collects in one document the important research literature

on temporal factors in vision. Over 350 scientific articles are cited

herein and this represents approximately 10 percent of the data base which

was consulted. The literature searched was comprised of the following: 1)

several thousand articles (under the general rubric temporal factors and

information processing) from existing reprint files of the authors and others; 2) Ergonomics Abstract, Psychological Bulletins, Psychological

DD 13 173 EMDTORoMOF INOVSS i OOLETE UNCLASSIFIED

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6. PERFORMING ORG. REPORT NUMBER

7. AUTHOR(a) ' CONTRACT OR GRANT NUMBER(&)

S. PERtFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT, PROJECT, TASK

AREA & WORK UNIT NUMBERS

,,. CONTROLLING OFFICE NAME AND ADDRESS I1. REPORT DATE

13. NUMBER OF PAGES

14. MONITORING AGENCY NAME & AOORESS(hI different from Controlling Gillce) 15. SECURITY CLASS. (of this report)

ISr, OECL ASSI FIC'ATI ON/ DOWN'GRADIN G SCHEDULE

IS. DISTRIBUTION STATEMENT (of thle Report) j

17. DISTRIBUTION STATEMENT (of the abatract entered In Block 20, iI different from Report)

14. SUPPLEMENTARY NOTES

IS. KEY WORDS (Continue on revaere stds it necessary and Identify by block number)

30. A3ST19AC? (Cin6aie - ,eeres h noeematy widei•itit by block number)

DD EDITION OF I NOGV 6R IS 0teOLET URM1473 

SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered)
.UNCLASSIFIEDSECURITY CLASSIFICATION OF THIS PACK(Whmu, Dai SAWte64c1

(Item 19 Continued)

Spatio Temporal Interactions

(Item 20 Continued)

Reviews and Human Factors for the last 12 years; 3) a listing from two

automated look-up systems (Psychological Abstracts 1967-present and National Technical Information System 1964-present). An integrative review

of the literature is provided and three chapters are included which deal

with application of these findings to display design. The subject matter

is perception of temporal events--specifically motion perception (real and

apparent) and flicker/flash sensitivity. A small chapter covers some

temporally based phenomena which distort or degrade perception. Features

of these phenomena may be observed in visual displays. Only studies which

report findings which are robust enough to be expected to be important

outside the laboratory are included. Where sufficient aata were available,

equations are provided to the engineer for the calculation of design

criteria (e.g., peripheral motion threshold, contrast thresholds, contrast

thresholds and age, etc.). Where gaps exist in our scientific knowledge,

recommendations are provided for applied research. General guidelines are

offured for incorporating design criteria into Military Standard 1472 for

perceptions due to temporal events.

UNCLASSIFIED

SECURITY CLASSIFICATION OF THIS PAGErWptn Dote Entered)
SUPPEMET'ARY

INFORM-ATON
;ECURITY CLASSIFICATION OF THtS PA3E (When Date Entered) .

REPORT-DOCUMENTATION PAGE READ INSTRUCTIONS " BEFORE COMPLETING FORM

I. REPORT NUMBER 12. GOVT ACCESSION No 3. RECIPIENT'S CATALOG NUMBER

Technical Report 540

4. TITLE (ad Subtitle) S. TYPE OF REPORT &k PERIOD COVERED

SOURCEBOOK OF TEMPORAL FACTORS AFFECTING Final

INFORMATION TRANSFER FROM VISUAL DISPLAYS

6. PERFORMING ORG. REPORT NUMBER

7. AUTHOR(&) 8. CON rRACT OR GRANT NUMBER(&)

SR. Sekuler & P. D. Tynan I~ N61756-76-M-5961

R. S. Kennedy

S. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT, PROJECT, TASK

Psychology Department, Northwestern University, AREA& WORK UNIT NUMBEfS

Evanston, Illinois; Canyon Research Group, 20162722A777

Westlake Village, CA

11. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

US Army Research Institute for the Behavioral June 1981 and Social Sciences, 5001 Eisenhower Avenue, 13. NUMBEROF PAGES

Alexandria, VA 22333 177

14. MONITORING AGENCY NAME & ADDRESS(If dlffetret from Controlling Office) 15. SECURITY CLASS. (of this report)

Unclassified

15m. DECL A551 FIC ATION/ DOWN GRADING

SCHEDULE

II. DISTRIBUTION STATEMENT (of this Retpott)

Approved for public release; distribution unlimited.

17. DISTRIBUTION STATEMENT (of the abstract intered in 31ock 20, iif dIfront from Report)

18. SUPPLEMEMTARY NOTES

This report was funded as a collaborative effort by the U.S. Army

Reseerch Institute with the U.S. Naval Pacific Missile Test Center, Point

Mugu, California and U.S. Naval Biodynamics Laboratory, New Orleans,

Louisiana.

19. KEY WOROS (Continue on roveres aids if necessry and Identify by block number)

Temporal factors Vision Illusions

Human Engineering Design Criteria Vection Dynamic Visual

Visual Displays Motion Perception Acuity

Military Standards Flicker Display Design

Contract Sensitivity Brightness Enhancement

I&. AEM1"N ACT Camuthae - m m e se neeseey and idefhiif by block number)

This report collects in one document the important research literature

on temporal factors in vision. Over 350 scientific articles are cited

herein and this represents approximately 10 perceat of the data base which

was consulted. The literature searched was comprised of the following: 1)

several thousand articles (under the general rubric temporal factors and

information processing) from existing reprint files of the authors and

o-hers; 2) Ergonomics Abstract, Psychological Bulletins, Psychological

FR, 1473 EDITION OF I NOV S5 IS OUSOLETE UNCLASSIFIED

SECURITY CLASSIFICATION OF THIS PAGE (^en Dte Entered)
SECURITY CLAS.IPICAAIOM OF THIS PA@U09(Wt Dia blomta

SECURITY CLASSIFICATION OF THIS PAGE('IWhen Dota EntetedJ
UNCLASSIFIED

SECURITY CLASSIPICATION OF THIS PAOiQVImm, Data Arelei)

(Item 19 Continued)

Spatio Temporal Interactions

(Item 20 Continued)

Reviews and Human Factors for the last 12 years; 3) a listing from two

automated look-up systems (Psychological Abstracts 1967-present and National Technical Information System 1964-present). An integrative review

of the literature is provided and three chapters are included which deal with application of these findings to display design. The subject matter

is perception of temporal events--specifically motion perception (real and

apparent) and flicker/flash sensitivity. A small chapter covers some

temporally based phenomena which distort or degrade perception. Features

of these phenomena may be observed in visual displays. Only studies which

report findings which are robust enough to be expected to be important

outside the laboratory are included. Where sufficient data were available,

equations are provided to the engineer for the calculation of design

criteria (e.g., peripheral motion threshold, contrast thresholds, contrast

thresholds and age, etc.). Where gaps exist in our scientific knowledge,

recommendations are provided for applied research. General guidelines are

offered for incorporating design criteria into Military Standard 1472 for

perceptions due to temporal events.

UNCLASSIFIED

SECURITY CLASSIFICATION OF THIS PAGE('Whon Date Entered)
SECURITY CLASSIFICATION OF TN'S PAGE (ften Date Rntered)

REPORT DOCUMENTATION PAGE "READ INSTRUCTIONS

BEFORE COMPLETING FORM

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7. "TITLE (and 3.bf,111.) S. TYPE OF REPORT & PERIOD COVERED

6. PERFORMING ORG. REPORT NLIMBER

7. AUTHOR(&) '. CONTRACT OR GRANT NUMBER(&)

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT, PROJECT. TASK AREA a WORK UNIT NUMBERS

I I. CONTROLLING OFFICE NAME AND ADORESS 12. REPORT DATE

13. NUMBER OF PAGES

14. MONITORING AGENCY NAME A AOORESS(l1 dilff•men from Coniroling Office) IS. SECURITY CLASS. (of this report)

IS&. DECLASSIFICATION/DOWNGRADING

SCH EDU LE

IS. DISTRIBUTION STATEMENT (of this Repori)

17. DISTRIBUTION STATEMENT (of the ebstract entered in Block 20, It different from Report)

IS. SUPPLEMENTARY NOTES

19. KEY WORDS (Cmotinue on reverse side It necessary and Identify by block number)

20L ANSTRAC?' m tverese o If neeesemid ideelty by block numbwb)

DO Fm 103 corOm or I Nov 6S Is OBSOLETE

SECUlrTY CLASMiFICATION OF THIS PAGE (When Date knterect)