‘Taste 1. Order of stimulus presentation for all subjects. Five readings were obtained for each spiral size at
each distance. The spiral sizes were presented in the order indicated for each distance.

Boperimental Day 1 Boperimental Day & Eoperimental Day $
Distance Spiral Size Distance Spiral Size Distance Spiral Size
Budject (Feet) (Inches) (Feet) (Inches) (Feet) (Inches)
Group A
KW 4 48,16 8 8,16,4 16 16,4,8
Le 16 8,16,4 8 16,8,4 4 84,16
RF 4 16,8,4 16 16,8,4 8 48,16
Iw 8 16,4,8 4 16,4,8 16 84,16
DW 16 4,168 4 4,16,8 8 4,16,8
RW 8 84,16 16 48,16 4 8,16,4
cs 4 48,16 8 8,16,4 16 16,4,8
BS 16 8,16,4 8 16,8,4 4 84,16
WF 4 16,8,4 16 16,8,4 8 48,16
cM 8 16,4,8 4 16,4,8 16 84,16
GL 16 4,16,8 4 4,168 8 4,16,8
WP 8 84,16 16 4,8,16 4 8,16,4
Group B
JT 4 2,4,8,16 8 16,8,4,2 16 2,4,8,16
RM 16 16,8,4,2 8 2,4,8,16 4 16,8,4,2
Group C
SG 36 16,14,12 42 12,14,16 48 16,14,12
CB 36 16,12,14 48 14,12,16 42 16,12,14
DS 42 14,12,16 48 16,12,14 36 14,12,16
MK 42 12,16,14 36 14,16,12 48 12,16,14
ON 48 14,16,12 42 12,16,14 36 14,16,12
JT 48 12,14,16 36 12,14,16 42 16,14,12
5 15

SPIRAL DIAMETER

o— ic"
13 13 8°
—-s 4"

" a " *
9

©

o
a

DURATION OF SPIRAL AFTEREFFECT
(Seconds)
x
~

3

2
i
ow

Ne2
1 1 1 1 L 1 1 n 4 1 1 L L n 1 L 4
2 4 6 8 10 i2 I4@ 6 18 20 2 4 6 8 10 2 4 6 8 2%
VISUAL ANGLE (Degrees)
Ficure 1. Mean duration of the spiral aftereffect as a function of visual angle for Group A (N=12; visual alley

dark) and Group B (N=2; visual alley illuminated). Two subjects in Group A reported no aftereffect for
the 16-inch spiral at 4 feet (18°56’ visual angle).

Ewperiment B. Another group (N=2) served jlluminated. The mean SAE duration values for
as a replication and further check of the results each of the size-distance settings are plotted as a
noted above. These subjects viewed the same function of visual angle in Figure 1, and of min-
three spirals plus a new spiral (2-inch diameter) ares/sec in Figure 2. Confirmation of the results
at each of the three distances and with the alley from the larger group of subjects was obtained.



L

2.

REFERENCES

Costetxo, C. G.: Further Observations on the Spiral
After-Effect. Percept. Mot. Skills, 11:324, 1960.

Fozarp, J. L., Fucus, M. Pater, M., and Smira
A. M.: Effect of Combinations of Six Presentation
Conditions on the Duration of the Spiral Aftereffect.
U.S. Govt. R & D Report, AD-623-976 (Dec.), 1965.

3. Freup, S. L.: Duration of Spiral Aftereffect as a

Function of Retinal Size, Retinal Place, and Hemi-
retinal Transfer. Percept. Mot. Skills, 18 :47-58, 1964.

. GRanit, R.: On Inhibition in the After-Effect of

Seen Movement. Brit. J. Psychol., 19 :147-157, 1928.

. Hortanp, H. C.: Some Determinants of Seen After

Movements in the Archimedes Spiral. Acta Psychol.,
(Amsterdam), 14 :215-222, 1958.

Hortanp, H. ©.: The Spiral After-Effect. Interna-
tional series of monographs in experimental psy-

10.

chology. Vol. 2, New York: Pergamon Press, 128 pp.,
1965.

McKenzigz, R. E. and Harrman, B. O.: The Effects
of Size, Speed, and Inspection Time on the Duration
of the Spiral Aftereffect. USAF SAM Report, No.
62-18, Brooks AFB, Texas, 1961.

Pickersem1, M. J. and Jeeves, M. A.: After-Effect
of Movement Produced by a Rotating Spiral.
Nature, 182 :1820, 1958,

. Scorr, T. R.: Summary of Principal Findings on a

Visual Aftereffect of Motion and Brain Pathology.
Newsletter for Research in Psychol., (Aug.) 38-46,
1962.

Scorr, T. R, and Notanp, J. H.: Some Stimulus
Dimensions of Rotating Spirals. Psychol. Rev.
72 :344-357, 1965.

21066


located near the front of the box and was viewed
binocularly from a seated position at one end
of a 48-foot visual alley. The subject’s head
was positioned with a chin rest so that his line
of sight to the center of the spiral was parallel
to the floor. By shielding the light source and
leaving the alley in darkness, few cues were
available regarding the distance of the spiral
from the observer. Duration of the aftereffect
was measured with timing equipment activated
by depression of a microswitch located at the
subject’s position.

Ewperiment A. Twelve male subjects were
used in the first part of the study which involved
a repeated sessions design with each subject re-
ceiving every condition in random order. The
subject attended a 1-hour session on each of 4
successive days. Spirals with diameters of 4,
8, and 16 inches were each used at distances of
4, 8, and 16 feet. These nine conditions produced.
a total of five different visual angles: 1°19’,
2°94’, 4946’, 9°39", and 18°56’.

Each subject was given a demonstration of
the operation of the spiral. While watching the
spiral, the subject was instructed to maintain
visual fixation on the center of the stimulus and
immediately after cessation of the rotatory mo-
tion, as the aftereffect commenced, to depress a
microswitch and to keep it depressed until the
aftereffect was no longer perceived. The subject
was then given a group of three practice trials.
The spiral was rotated for 15 seconds during
each of the trials and there was a 2-minute break
between trials,

Immediately following these familiarization
trials, each subject received a series of five prac-
tice trials for each of three conditions (the 4-inch
spiral at 4 feet, the 8-inch one at 8 feet, and the
16-inch spiral at 16 feet). The duration of the
stimulus and length of time between practice
trials was the same as that allowed during the
previous three familiarization trials. The sub-
jects were also given 5-minute rests between
conditions.

On each of the 3 following days (Experimental
Days) the instructions were repeated and the
experimental trials were conducted with the
alley in darkness. No further practice was given.
Five judgments were obtained for each size-
distance condition. By the end of the third
Experimental Day the subject had viewed the
three spiral sizes at each of the three distances.

The order of presentation of the various spiral
size-distance combinations was counterbalanced
among subjects (Table 1).

Ewperiment B. A reliability check and exten-
sion of the test conditions were accomplished by
using two new subjects, illuminating the visual
alley, and introducing an additional spiral
(2-inch diameter displayed at 4, 8, and 16 feet).
The order of presentation for these two subjects
also appears in Table 1.

Experiment C. A different group of six sub-
jects viewed spirals with diameters of 12, 14, and
16 inches at distances of 36, 42, and 48 feet. The
visual angles produced by these combinations
were 1°12’, 1°29’, 1°35’, 1°51’, and 2°7’, and
were within the range of the three smallest visual
angles used in the preceding experiments. The
distances were selected in an attempt to obviate
possible accommodation-convergence effects which
might have affected the data collected in Experi-
ments A and B. The order of presentation
appears in Table 1.

Hl. Results

Lzperiment A, An average of the five dura-
tion scores was obtained for each subject for
each of the nine spiral size-distance settings.
These scores were then averaged to yield a mean
duration score for the group for each of the
nine conditions and were then plotted as a func-
tion of visual angle (Fig. 1).

Mean scores for each subject were submitted
to an analysis of variance. The effect of visual
angle was found to be significant (.05 level).

The angles were then tested for trends. Al-
though linear and quadratic tests were significant,
the quadratic component accounted for 50% of
the variance, whereas the linear component ac-
counted for only 25%. Thus, the relationship
between visual angle and the duration of the
SAE would appear to be curvilinear with the
critical break occurring approximately between
2°-4° of visual angle.

Using the formula from Scott and Noland’
for determining speed of normal rotation, mo-
tion values in minares/sec were calculated for
each of the size-distance settings, and SAE
duration was then plotted as a function of the
speed of eliciting motion (Fig. 2). Contrary
to the report of Scott and Noland”, the duration
of the aftereffect appears to increase only up to
a point between 30-60 minares/sec and then
declines.


(Seconas)

DURATION OF SPIRAL AFTEREFFECT

ivan
[

SPIRAL DIAMETER
is
oo 8"

o
aro

20 40 60 80 100 120 140 160 180 200 220 240 260
SPEED OF ELICITING

20 40 60 € 100 120 140 160 180 200 220 240 260

MOTION — (Minores /Sec)

Ficure 2, Mean duration of the spiral aftereffect as a function of the speed of eliciting motion for Group A
(N=12) and Group B (N=2),

Eupertment C. Mean group SAE duration
values were calculated for each of the nine size-
distance settings and plotted as functions of
visual angle and of minarcs/sec (Fig. 8). The
duration values increased in a relatively linear
fashion as functions of increasing visual angle
and increasing speed of eliciting motion. It is
to be noted that only one of the visual angle
conditions (2°7’) used in this phase of the study
approached the range (between 2°24’ and 4°46’)
within which a decrease in aftereffect occurred

in Experiment A, and that the maximum speed
of eliciting motion for these spirals was less than
30 minarcs/sec.

IV. Discussion

The results obtained here are in close agree-
ment with the data and conclusions presented
by Granit* in his study of the waterfall illusion,
ie., that the optimum duration of the aftereffect
occurs between 2°-4° of visual angle. The pres-
ent data also support the reports of Pickersgill

VISUAL ANGLE

5 13

ir]

i

Ble 12

wi

2

eu " SPIRAL DIAMETER

z3 oe 16"

£510 10 1"

Fs o_o

OS a—a 12"

6 9 9

2

Ss = =

- 8 N=6 8 N=6

s

a 1 in n n n n rn n 1 petitirirtirerti rir tiriitiviid
20° 40° P1920 1°40" 2°20" 2°40" 3° 5 10 15 20 25 30

SPEED OF ELICITING MOTION (Minarcs/Sec)

Ficure 3, Mean duration of the spiral aftereffect as a function of the visual angle and the speed of eliciting
motion for Group C.

4


AM 68-10

THE SPIRAL AFTEREFFECT:
Influence of Stimulus Size and Viewing Distance on
the Duration of Illusory Motion

William E. Collins, Ph.D.
David J. Schroeder, M.S.

Released by Approved by

Zge€ 1d Shit Site.

J. Roperr Drutz, M.D.
Cuter, Crvmp AEROMEDICAL
INsTITUTE

P. V. Sxecer, M.D.
Feperat Am Surceon

May 1968

Department of Transportation

FEDERAL AVIATION ADMINISTRATION
Office of Aviation Medicine



and Jeeves® and of Fozard et al.?. That Holland*
found no significant effects (his mean data were
not reported) may be due to the fact that the
two angles which he employed (4° and 6°) were
too close to the optimum visual angle to show
statistical significance. A similar explanation
might account for the results of McKenzie and
Hartman’ who obtained a shorter SAE duration
(4.91 sec) at 2°8’ of visual angle than at 4°14’
and 6°22’ (where the mean duration was 5.51
sec in both cases) ; in any event, the SAE dura-
tion did not increase for the largest visual angle
even though the speed of eliciting motion prob-
ably averaged less than 100 minarcs/sec.

Only one study appears to show a clear di-
vergence from the results of this investigation.
Although his main interest was in retinal place
and hemiretinal transfer effects, Freud’ also re-
ported an increase in SAE duration from 2°
through 8° of visual angle regardless of whether
predominantly cone or rod areas were stimulated.
Since Freud had subjects view the spiral with
only one eye whereas most studies involve bi-
nocular stimulation, it was thought that such a
difference might account for the lack of agree-
ment. A brief examination of this possibility
(using the two subjects who participated in
Experiment B) resulted in approximately identi-
cal data for uniocular and binocular conditions.

Scott and Noland’ sought to resolve some
conflicts in findings among studies in which
spiral size, type of spiral, distance from the ob-
server, and motor speed were different, by taking
into account the usually neglected variable of
speed of eliciting motion. The authors point

out that a frequently ignored factor in SAE
studies is that the speed of motion of the stimulus
pattern across the retina varies with the viewing
distance. Based upon calculations from data
reported by Granit‘, Scott®, and Freud’, Scott
and Noland concluded that SAE duration in-
creased up to 182 minarcs/sec (then declined)
and that the aftereffect appeared highly predict-
able on the basis of information about the speed
of eliciting motion. Data from the present study,
however, show increasing SAE durations only
up to 30-60 minarcs/sec, and a further analysis
(calculations of the speed of eliciting stimuli)
of the work of Fozard et al.? shows a wide range
in minarcs/sec during which peak SAE dura-
tions occur depending upon other manipulated
variables (Table 2). In both of the latter studies
the visual angle appears to have a critical effect
on peak duration. It seems clear, then, that both
the visual angle and the speed of eliciting motion
are variables of significance in determining the
duration of the spiral aftereffect.

Vv. Summary

This study examined some effects of stimulus
size and distance on the persistence of one type
of illusory motion, viz., the spiral aftereffect.
Duration of SAE was investigated with stimuli
of 2, 4, 8, 12, 14, and 16 inches in diameter.
The distance between the observers and the ro-
tating spirals was varied to produce visual angles
between 1°12’ and 18°56’ of arc. Data indicate
that the duration of illusory motion reaches peak
values between approximately 2°-4° of visual
angle.

Taste 2, Speed of eliciting motion (minares/sec) calculated from the data of Fozard et al.* and the durations
of the spiral aftereffect (in seconds) which they obtained under various test conditions.

25 rpm 100 rpm 250 rpm
Spiral Spirat Visual Minares SAE Minarcs SAB Minares SAE
Turns Rize Angle Per Seo Duration Per Seo Duration Per Seo Duration
av 2°23" 19.9 18.5 7.8 15.6 198.9 16.8
15 8" 4°46" 39.8 18.9 159.1 18.6 397.8 14.9
16” 9°23’ 79.6 143 818.2 13.7 795.6 12.0
a” 2°28" 12.0 19.7 479 17.8 119.7 19.8
25 8” 4°46" 22.5, 18.9 90.1 19.7 225.2 20.1
16” 9°23" 45.1 16.1 180.2 13.2 450.5 24.7
4” 2°23" 8.6 20.5 34.2 19.7 85.6 19.3
3.5 8" 4°46" Ww 23.2 68.5 23.8 171.3 21.4
16” 9°23" 34.3 20.4 187.1 19.7 342.7 16.3



Qualified requesters may obtain Aviation Medical Reports from Defense Documentation

Center.

The general public may purchase from Clearinghouse for Federal Scientific and
Technical Information, U.S. Dept. of Commerce, Springfield, Va. 22151.


The Spiral Aftereffect: Influence of Stimulus Size and

Viewing Distance on the Duration of Illusory Motion

I, Introduction

Visual illusions represent a significant problem
area in aviation research. Although many such
illusions are the result of misleading cues, others
appear as an aftereffect of real motion. Included
among the latter is the spiral aftereffect—the
apparent reversed motion of a spiral after it
ceases spinning.

Previous studies dealing with the relationship
between the duration of the spiral aftereffect
(SAE) and the visual angle subtended by the
spiral have led to contradictory results. Holland®
indicated no significant difference in the duration
of the aftereffect with a change in visual angle
reported elsewhere’ to be from 4° to 6°. Similar
findings were noted by McKenzie and Hartman’
with visual angle variations of 2°8’, 4°14’, and
6°22’.

Contrary to the results summarized above,
Pickersgill and Jeeves* found » nonlinear rela-
tionship between spiral size and the duration of
the SAE. An increase from a 3-inch spiral
(2°52’) to a 6-inch spiral (5°44’) produced a
significant increase in the duration of the after-
effect; however, an increase from a 6-inch spiral
to a 12-inch one (11°26’) resulted in a shorter
duration. Costello' employed a visual angle of
6°30’ for two distance conditions (an 814-inch
spiral at 6 feet and a 3-inch spiral at 214 feet)
and reported significantly shorter effects for the
3-inch spiral. Freud* demonstrated statistically
significant size effects on the SAE when the
spirals subtended visual angles of 2°, 4°, and
8°, The relationship appeared to be linear; as
the spiral size increased there was a correspond-
ing increase in the duration of the aftereffect.
Fozard, Fuchs, Palmer, and Smith? obtained
SAE duration measures with 4-inch, 8-inch, and
16-inch spirals at a distance of 8 feet (2°23’,
4°46’, and 9°28’, respective visual angles). They
reported a slight increase in duration of the
aftereffect from the 4-inch to the 8-inch spiral,

but a notable decrease when the 16-inch stimulus
was used.

In a study concerning the duration of the
“swater-fall illusion,” a related phenomenon,
Granit* found that an increase in the size of the
drum produced a significant increase in the
duration of the aftereffect, with an optimum
duration occurring between 2°-4° of visual angle.
The increase in duration could only be produced
by a larger sized drum, not by moving the drum
closer to the subject.

Scott and Noland have reevaluated conclu-
sions of the Freud? and Granit* studies from
the viewpoint that changes in the viewing dis-
tance produce changes in the speed of stimula-
tion. Thus, by comparing data from Scott®,
Freud? and Granit‘, they report that the after-
effect increases for stimulating speeds between
30-132 minutes of are per second (minarcs/sec)
and that results reported as due to viewing dis-
tance may be accounted for by stimulus speed.

In previous studies of SAE, ‘authors altered
visual angles either by using spirals of different
size or by changing the viewing distance. Pick-
ersgill and Jeeves’, McKenzie and Hartman’,
and Fozard et al. used spirals of various sizes,
while Freud? and Holland® changed the viewing
distance. In the present study both spiral size
and distance from the observer were system-
atically varied.

II. Method

Apparatus. The spirals were three-throw
arithmetic spirals, produced by photographic
means, with diameters of 2, 4, 8, 12, 14, and
16 inches. The spirals were attached to a shaft
driven by a variable speed motor with speed of
rotation set at 80 rpm. A timing system started
the spiral rotating, determined the stimulus
duration, and stopped the rotation.

The spiral and motor were enclosed in a box.
The spiral was illuminated by two 8-watt bulbs