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FAA-AM-79-12 ~®OO~ILfE"HB 

VISUAL SEARCH PERFORMANCE DURING SIMULATED RADAR 

WITH AND WITHOUT A SWEEPLINE 

.,£~LfJ1 NA L 

. . :; . 

Richard I. 1backray 

R. Mark Touctstone 

FAA Civil Aeromedical Institute 

P.O. Box 25082 

Oklahoma City, Oklahoma 73125 

MARCH 1979 

Document is available to the public through the 

National Technical Information Service, 

Springfield,Virginia 22161 

Prepared for 

U.S. DEPARTMENT OF-TRANSPORTATION 

Federal Aviation Administration 

Office of Aviation Medicine 

Washington, D.C. 20591 

. -... ·- . . •:; 
NOTICE 

This document is disseminated under the sponsorship of the 

Department of Transportation in the interest of information 

exchange. The United States Government assumes no liability 

for its content or use thereof. 

I 
Technic:::~l ~eport Documentation Page 

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

FAA-AM-79-12 

4. Titl~ and Subtitle 5. Report Date 

VISUAL SEARCH PERFORMANCE DURING SIMULATED RADAR MARCH 1979 

OBSERVATION WITH AND WITHOUT A SWEEPLINE 6. Pe~forming Organization Code 

a. Performing Organization Report No. 

7. Authorf s) 

RICHARD I. THACKRAY AND R. MARK TOUCHSTONE 

9. Performing Organizotion Name and Address 10. Work Unit No. (TRAIS) 

FAA Civil Aeromedical Institute 

P.O. Box 25082 11. Contract or Grant No. 

Oklahoma City, Oklahoma 73125 

13. Type of Report and Period Covered 

12. Sponsoring Agency Name and Addre!.S. 

Office of Aviation Medicine 

Federal Aviation Administration 

800 Independence Avenue, s.w. 14. Sponsoring Agency Code 

Washington, D.C. 20591 

15. Supplto-mentary Notes 

Work was performed under Tasks AM-C-78/79-PSY-68. 

16. Abstract 

A study was conducted to determine whether or not the presence or absence of a radar 

sweep line influences attentional processes and, hence, the speed with which critical 

stimuli can be detected. The visual display was designed to approximate an advanced, 

highly automated a1r traffic control radar display containing computer-generated 

alphanumeric symbols. Twenty-eight men and women, paid volunteers with no previous 

air traffic controller experience, were tested over a 2-hour session with half of 

the subjects assigned to the sweep condition and half to the no-sweep condition. 

Sixteen targets appeared on the sc-reen at all times, with 10 signals (a designed 

change in the alphanumerics) randomly presented during each 1/2-hour of the test 

session. Mean detection latencies, long detection times, and missed signals all 

incre~:ed significantly over the task session. Although the no-sweep appeared to be 

generally superior to the sweep condition in all measures of detection efficiency, 

none of the differences was significant. Concomitantly recorded measures of 

saccadic eye movements revealed a pattern of change in mean fixation duration which 

parallelzd the patterns of change in performance during the task session. However, 

as with performance, mean fixation durations for the sweep and no-sweep conditions 

did not differ, nor were individual differences in scanning activity related to 

performance. Possible reasons for the lack of relationship between scanning 

activity and performance are discussed. 

17. Key Words 18. Distribution Stotement 

Air Traffic Control Vigilance Document is available to the public 

Attention Visual Search through the National Technical Information 

Automation Service, 

Eye Movements Springfield, Virginia 22161 

Performance 

19. Sec:urity Clouif. (of this report) 20. Security C/assif, (of this page) 21 .. No. of Pages 22. Price 

Unclassified Unclassified 15 

Form DOT F 1700.7 (8-72l Reproduction of !ompleted poge authorized 

\ 
VISUAL SEARCH PERFORMANCE DURING SIMULATED RADAR OBSERVATION 

WITH AND WITHOUT A SWEEPLINE 

I. Introduction. 

Some of the current air traffic control (ATC) radar di>plays employ a 

visible rotating sweepline while others do not. Laboratory studies of eye 

movements during radar search have typically found that the eyes tend to move 

in a circular fashion when a sweepline is present and follow an irregular 

pattern when a sweep is absent (2,3,15,16). Gerathewohl (3) has expressed the 

belief that this tendency of the eyes to follow the circular motion of a 

sweepline may be responsible for complaints of fatigue, headache, drowsiness, 

and other somatic symptoms expressed by radar operators. On the other hand, a 

visible sweepline may have certain beneficial effects, since some controllers 

feel the sweepline provides a type of organization to the scanning process 

that is lacking in radar displays without a sweep (1). 

There have apparently been no studies reported in which monitoring 

efficiency and/or indices of subjective fatigue are compared under radar 

viewing conditions with and without a sweep. The primary purpose of the 

present study, then, was to make such comparisons. The task employed was 

designed to simulate a highly automated air traffic control system in which 

the observer passively monitored a display containing alphanumeric symbols for 

infrequent but "critical" changes. 

In addition to this primary purpose, several other aspects to the study 

were included either for exploratory purposes or for the purpose of extending 

the findings of our previous studies of complex monitoring, The first of these 

dealt with an examination of possible relations between frequency of eye 

movement fixations and performance. Of particular interest was the detailec 

examination of extreme detection latencies (maximum and minimum values) for 

any evidence of concomitant changes in mean fixation duration. The results of 

several previous studies of complex monitoring (4,12,13) suggest that maximum 

latencies appear to reflect lapses of attention or failures to maintain 

scanning, while minimum latencies provide an estimate of the individual's 

maximal state of alertness at any given period during the course of a 

monitoring session. We hoped that a measure of scanning activity would reveal 

whether long detection times occurred in spite of frequent scanning, or 

whether they were the result of an interruption in scanning. The second aspect 

consisted of an evaluation of the effects of increased task difficulty on 

monitoring performance. Two of our previous investigations (12,13) used the 

same basic display as that employed in the present study. However, in these 

earlier studies, subjects were required simply to detect and respond to a 

readily identifiable stimulus change (a "999" appearing in the altitude 

portion of an alphanumeric data block). The present study sought to determine· 

1 
the effects of increased informatio~ processing (the requirement to detect 

any change in altitude above or below designated upper and lower limits) 

both on performance ievels and on che pattern of performance decrement. 

II. Method. 

Subjects. Twenty-eight university ~tudents, 12 men and 16 women, served 

as subjects (Ss). Half of the Ss were randomly assigned to the sweep and 

half to the no-sweep condition. Subjects ranged in age from 18 to 29 years. 

None had any prior experience with the task used nor did any have training in 

air traffic control. All were righthanded. 

Design and Task Appar1tus. All task programing and recording of 

responses were accomplished using a Digital Equipment Corporation (DEC) 

PDP-11/40 computer. The ccmputer was interfaced with a VT-11 (DEC) 17-inch 

(43 em) cathode-ray tube (CRT), which served as the S's display. The CRT was 

located in a console resembling an air traffic control radar unit. The 

stimuli (target,;) consisted of small rectangular "blips" representing the 

locations of given aircraft. Adjacent to each target was an alphanumeric 

data block. Data blocks comprised two rows of symbols: the top row, 

consisting of two letters and three numerals, identified the aircraft, while 

the bottom row of six numerals indicated its altitude and speed. The first 

three of these numerals gave altitu-de in hundreds of feet and the last three 

gave groundspeed. 

For the task condition in which a simulated radar sweepline was employed, 

the sweep made one complete clockwise revolution every 6 seconds. A target 

was updated as to location and any change in its data block moments after the 

sweepline passed the target's prior location. Targets normally moved in a 

linear fashion unless a course change was necessary to avoid target overlaps. 

All aspects of the no-sweep condition, including the clockwise sequence in 

which targets were updated, were identical to those of the sweep condition, 

since the no-sweep condition was obtained by simply setting the intensity of 

the sweepline to zero. The critical stimulus or signal to which the S was 

instructed to respond consisted of a change in a target's displayed altitude 

to a value greater than 550 or less than 150. The values of the increases or 

decreases in altitude were randomly determined; except that the changed 

altitude value could not be greater than 599 or less than 100. Ten such 

critical stim~li appeared in each 30-minute period; five occurred in the first 

15 minutes and five in the second. The S's response to a critical stimulus 

consisted of rr~ssing a button held in the right hand and then holding a light 

pen over the critical target. The light pen caused the altitude portion of 

the data block to revert to its previous value. If the S failed to detect a 

critical stimulus within 1 minute, the data block automatically reverted to 

its previous value. Marker channels on a Beckman Dynograph signaled the 

onset of a critical stimulus and the occurrence of the required button press. 

All performance data were recorded by the computer for subsequent processing. 

2 
The same target display file was used for all Ss and was initially 

constructed from a computer program which assigned an altitude, groundspeed, 

identification, entry point, and exit point to each of the targets. All 

assignments were randomly determined except for the following restrictions: 

(i) altitudes had to fall within the "normal" range of 150 to 550 (in hundreds 

of feet), (ii) groundspeeds had to fall within the range of 400 to 550 knots, 

and (iii) the entry and exit points of a given target could not be separated 

by less than 300 along the circumference of the simulated radar screen. In 

addition, time of critical stimulus occurrence and the target in which it 

occurred were randomly determined with the restriction that two tar$e~s could 

not contain critical stimuli at the same time. 

Physiological Recordings and Instrumentation. Beckman m1n1ature 

biopotential electrodes were attached directly above and below the right eye 

and at the outer canthi of both eyes. Leads from the vertical and horizontal 

pairs of electrodes were connected to two separate channels of the Dynograph 

and recorded with a 3.0-second time constant. These channels served as the 

two primary electro-oculograph (EOG) channels and, because of the relatively 

long time constant, recorded both following and saccadic movements. In order 

to extract only the faster saccadic movements fo~ computer processing, the 

output of the primary horizontal channel was recorded on a third channel by 

differentiating the EOG with a time constant of 0.03 seconds. (Only hori-· 

zontal movements were computer processed because of eyeblink artifacts in the 

vertical recordings.) The resulting positive and negative pulses were led to 

two Schmidt triggers set for positive and negative inputs respectively, an OR 

gate, and hence to one of the digital inputs of the computer. These input 

pulses were also displayed for monitoring purposes on a fourth channel of the 

Dynograph. 

The computer and other recording apparatus were located in an adjacent 

room from which the S was visible through a one-way mirror. Indirect lighting 

was used in the S's ;oom, and the level of illumination at the display was 

21.5 meter-candl;s. This level approximates that used in operational air 

traffic control environments. 

Eye Movement Calibration. The gain of the primary horizontal channel on 

the Dynograph was initially adjusted to yield a 1-mm peak-to-peak deflection 

to a 50-~V, 1-Hz input signal from a Grass Square Wave Calibrator. The gain 

controls of the Schmidt triggers were tben adjusted to just fire at the peak 

of each positive and negativ~ excursion of the ~~libration signal. Yolloving 

this, each S's horizontal as well as vertical eye movements were calibrated 

using an optical table with chinrest support. Subjects were irtstructed to 

fixate points at 90° and 270° on the circumference of a 22-cm circle which 

subtended a visual angle of 20°. A similar procedure •N'as followed for vertical 

eye mcvements, except that points at 180° and 3600 were used. The gain 

controls of the primary horizontal and vertical channels were adjusted to yield 

peak-to-peak deflections of 20 rom as the eyeg were deflected to the extremes 

of the circle. Thus, 1 mm of pen deflection equnled 1° of eye movement. Any 

3 
horizontal saccadic movement equal to or greater than this value caused an 

output from one of the Schmidt triggers. 

Procedure. On arrival the S was taken to the experim~~tal room, 

orientation instructions were given, the ~ vas instrumented for physiological 

recording, and eye movements were calibrated. Then a 9-point subjective 

rating scale was administered dealing with present feelings of attentiveness, 

fatigue, tension, irritation, and boredom. 

The S was seated in a straight-backed chair directly facing the console. 

The circular display area of the screen subtended a visual angle of 

approximately 20° at the S's viewing distance. The minimum separation of 

alphanumeric targets at this distance was approximately 2.4°. Although a 

rigidly fixed head restraint would have been desirable in order to eliminate 

head movements, this was not considered feasible in view of the length of the 

t~sk session. Instead, each S was instructed to sit straight in the ehair 

with his/her head directly facing the screen at all times. While this 

procedure is not optimal, since small head movements produce apparent eye 

movements indistinguishable from true eye movements, it was expected that 

error resulting from head movements would be randomly distributed across 

conditions and within Ss. Periodic observations revealed that virtually all 

Ss complied with instr~ctions to keep gross head movements to a minimum. 

The task instructions emphasized the necessity of pressing the button 

immediately upon detection of a critical stimulus. The S was told that a 

critical stimulus (any altitude value greater than 550 or less than 150) could 

occur in any target at any time, regardless of the current altitude values of 

the targets. It was explained that occasional large changes in altitude would 

not normally occur in en actual radar system, but that this departure from 

normal conditions wa~ necessary to insure that all targets would be given 

equal priority in scanning. Following the taped instructions, the S was given 

a 4-minute practice period containing six critical stimuli. -

After the 2-hour task session, the S completed a second form of the 

subjective rating scale. This form was identical to the first except that the 

S was asked to rate each item, plus one additional item dealing with task 

monotony, on the basis of how the s felt near the end of the test period just 

completed. 

Measurement of the Performance and Physiological Data. Performance data 

were computer processed and the following measures were obtained on each S 

for each 30-minute period (all latency measures refer to the time from 

critical stimulus onset to the button press): 

(i) 

(ii) 

(iii) 

(iv) 

Mean response latency to critical stimuli correctly identified. 

Single longest latency to a correctly identified critical stimulus. 

Single shortest latency to a correctly identified critical stimulus. 

Number of critical stimuli misseu. 

4 
For eye movements, the computer identified each correct response (button 

press) and then determined mean fixation duration from the intersaccadic 

interval data contained in the 30-second interval immediately preceding this 

response. (Mean fixation duration can also be considered an index of fixation 

frequency. Consequently, although the data were analyzed only in terms of 

rnean fixation durations, subsequent discussions may refer to mean fixation 

duration and frequency of fixations interchangeably.) If a critical stimulus 

was missed, the 30-second interva.l prior to the time the stimulus timed out 

was analyzed. Average values derived from the above 30-second intervals were 

also obtained for each 30-minute period. To eliminate various forms of electronic and/or physiological noise from the data, all apparent fixation 

durations of less than 100 ms were rejected by the analysis program. 

III. Results. 

Performance Data. Figure 1 shows mean detection latencies across 

30-minute periods for all critical stimuli, as well as mean maximum and 

mini~um latencies, for both the sweep and no-sweep conditions. Analyses of 

40~ 

36 

32 

28 

(/) 24 0 

z 

0 20 

(.) 

w 16 

(/) 

12 

8 

4 

0 

--o o----- ............ --

MAXIMUM LATENCIES 

--- _.........- ••--• Wlth.Sweep ................ 0 ....... 

o--o Without Sweep 

MEAN LATENCIES -==@> 

• • --o-- . ---- o-------o-----

MINIMUM LATENCIES 

o--------e--- 8 ----Q 

2 3 4 

30- MINUTE PERIODS 

Figure 1. Mean, maximum, and minimum detection latencies 

for the two display conditions. 

variance applied to these three sets of data revealed significant main effects 

for the four 30-minute periods for mean latencies, F(3, 78) "-' 7. 38, I'. < . 01; 

maximum latencies, F(3,78) = 4.07, I'.< .OS; and minimum latencies, F(3,78) = 

S.OO, I'.< .OS. Although the data presented in Figure 1 suggest slightly 

faster detection latencies when no rotating sweep is employed, analyses of 

variance revealed no significant main effects (I'.> .10) for the sweep vs. 

5 
no-sweep conditions for .any of the th1·ee latency measures and no significant 

interactions (£ > .10). 

With regard to missed scimuli, four critical stimuli were missed during 

the first half-hour, two during the second, and eight each during the third 

and fourth half-hours. Because of the relatively low frequency of occurrence 

of missed stimuli in each half-hour, these stimuli missed by Ss in each of 

the two experimental groups were summed over the four 30-minute periods and 

a chi-square test was conducted. A comparison of the number of Ss in each 

group missing no stimuli with those missing one or more yielded a 

nonsignificant chi-square of 2.33, df = 1, 1'. > .05. 

Since a secondary purpose of the present study was to examine the effects 

of increased task difficulty on performance, the data of Figure 1 were 

combined and are shown in Figure 2. Also shown in this figure are the data 

40 

36 

32 

28 

(/) 

24 0 

z 

0 20 

(.) 

w 16 

(/) 

14 

8 

4 

0 

o--o Present Study 

6---6 Thackray et al. ( 1977) 

• • Thackray et al. (1978) 

.......... / 

o------o 

MAXIMUM LATENCIES 

o--- ~- - ~ ...... _____ 0, 

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

--- __ 6 

-----

MEAN LATENCIES _o-- 0------o-----

__ o 

-- '------------e---- 0 --------i 

------ MINIMuM LATENCIES ______ ! ______ ! A----------~---------- ----------

2 

30- MINUTE 

3 

PERIODS 

4 

Figure 2. Comparison of the combined-group latency data 

of three studies. 

from our two earlier studies (12,13) for comparable experimental conditions. 

It is obvious that the trends across studies are virtually identical. In all 

three studies performance remains relatively uniform or even improves during 

the first hour, but becomes worse during the second. (The similarity of trends 

is not the result of some idiosyncrasy in the arrangement of targets, interstimulus intervals, etc. Although the same target display file was used in 

the two previous studies, a completely new file was created for this study.) 

The principal difference between the findings of the present study and those 

6 
of the two earlier ones is the greater magnitude of the obtained detection 

latencies. Average values across all four time periods in this study were 

30.4, 12.1, and 2.4 seconds for maximum, mean, and minimum latencies respectively, while the combined data of the two previous studies yielded values of 

18.8, 7.6, and 2.0 seconds for these same three measures. Thus, the requirement of the present study to detect altitude values exceeding upper and lower 

limits rather than the simple identification of a 999 increased detection 

latencies, but apparently had little or no effect on the patterns of 

performance change. 

Subjective Data. Separate t tests applied to the rating scale data 

revealed no differences (E > .05) between the sweep and no-sweep groups at 

either the beginning or end of the experiment. All measures except those 

derived from the tension-relaxation scale changed significantly (E < .01) from 

the first to the second measurement period. Statements on the scales corresponding to cne mean ratings obtained at the completion of the task period 

suggested that the Ss were only slightly bored, were mildly annoyed, felt more 

tired than usual, were reasonably relaxed, were rather inattentive, and felt 

the task to be very monotonous. Actual obtained mean values are not presented, 

since they would add nothing to the verbal descriptions just given. 

Eye Movement Data. Mean fixation durations for both groups 

Figure 3. Analyses of variance conducted on these data revealed 

710 

700 

690 

en 

680 

Cl 670 z 

0 

u 660 

w 

en 650 

...l 

...l 640 

:E 

630 

620 

610 

0 

• • With Sweep 

o--o Without Sweep 

o----o Combined Groups 

~ .... 

,........ ,<f .... ,., 

''o" / 

\/ 

2 3 

30- MINUTE PERIODS 

are shown in 

a significant 

4 

Figure 3. Mean fixation durations for horizontal eye movements. 

7 
Figure 4. Sample recordings of eye movements during task performance with sweep (top record) and without sweep (middle 

record) compared to eye movements when instructed to 

simply follow the sweep (bottom record). Each record 

represents 30 seconds. 

8 
main effecc for 30-minute periods (F(3/78) = 5.14, £ < .01), but no 

difference between the sweep and no-sweep groups (p > .10) and no significant 

interaction(£> .10). Consequently, the data of both groups ~'~e combined 

and are shown in this same figure. It is readily apparent that the pattern of 

change in fixation durations resembles the performance patterns (especially 

maximum latencies) shown in Figure 2. A Newman-Keuls test applied to the 

combined data of Figure 3 revealed no differences between the first, third, 

and fourth 30-minute periods but all three differed significantly from the 

second (£ < .OS). 

Scanning Patterns. Figure 4 compares the pattern of horizontal and 

vertical eye movements of a S instructed to sin~ly follow the rotating sweep, 

with the eye movement patter~s of two randomly selected Ss (one with and one 

without sweep) while performing the task. Each segment ~epresents 30 seconds. 

Neither of the segments taken during task performance shows any evidence of 

the cyclic pattern present in the bottom pattern. A more precise comparison 

was made by examining the recordings of each S's eye movement patterns and a 

judgment made as to whether the S had been exposed to the sweep or no-sweep 

condition. Both experimenters made separate, blind judgments. Chi-square 

tests of the resulting frequencies were nonsignificant (£ > .05) for both sets 

of judgments. 

Relationship of Eye Movements to Performance. Each ~·s mean detection 

latency for each separate 30-minute period was compared with his/her mean 

fixation duration. No significant relationships between mean fixation duration and mean detection latency were obtained. The correlations for periods 

1 through 4 were .22, -.09, -.03, and -.06 respectively(£> .05). Further 

analyses were conducted on each ~·s extreme detection latencies. It will be 

recalled that all eye movement data were obtained from the 30-second interval 

that preceded each detection response (button press). Thus, for each separate 

30-minute period, mean fixation durations in the 30-second intervals 

associated with maximum detection latencies were compared with mean fixation 

durations in the 30-second intervals associated with minimum latencies. 

Separate~ tests revealed none of the comparisons to be significant (£ > .OS). 

Average fixation durations associated with maximum and minimum latencies for 

the four 30-minute periods were 694 and 685, 640 and 644, 658 and 648, and 

689 and 720 ms for periods 1 through 4, respectively. It is evident that there 

is no consistent pattern of differences in these data. 

A final analysis consisted of comparing average fixation durations 

associated with missed critical stimuli with fixation durations associated 

with minimum detection latencies. The procedure was similar to that just 

desc,ibed for maximum and minimum detection latencies. Because of the small 

number of missed stimuli, however, separate comparisons were not made for each 

30-minute period, but only for the session as a whole. Although the mean of 

fixation durations associated with missed stimuli was greater than the mean of 

fixation durations associated with comparable minimum latencies (693 and 650 

ms), the obtained~ of 0.98, df = 14 was nonsignificant (£ > .05). 

9 
IV. Discussion. 

As noted previously, studies of eye movement patterns during radar 

observation have shown that the eyes generally tend to follow a rotating 

sweepline by means of a series of closely spaced fixations (3,15,16). These 

early studies, however, were attempts to simulate systems in which the 

primary task was frequently the simple detection of a new "pip" on the 

screen. Given such a task, a faint radar return might easily fade from view 

if the operator were not constantly attending the sweep. Thus, a search 

pattern in which the eyes are closely coupled to the rotating sweep would 

serve to optimize detection of weak signals. Under such task conditions, a 

circular search pattern might ~ell produce the types of physiological 

symptoms described by Gerathewohl (3). 

Contemporary ATC radar systems, however, typically employ a 

computer-generated graphic display containing a variety of alphanumeric and 

other symbols. In such systems, the task is not only to note the appearance 

of a new target, but to detect and make appropriate decisions with regard to 

any significant change in the alphanumeric information displayed. Given a 

relatively large number of targets to monitor, a great deal of information 

must be processed. Thus, although targets in the present study were updated 

in a clockwise fashion moments after the sweepline passed, it was the 

impression of both experimenters (wh;:, served as pilot Ss) that it was 

virtually impossible to process information rapidly enough by using a search 

pattern in which the eyes attempted to follow the sweep. (It will be recalled 

that the sweep made one revolution in 6 seconds.) Apparently, most, if not 

all, Ss in the sweep group experienced the same difficulty, since no 

differences were found between this group and the group that monitored without 

a sweepline in eye movement patterns, fixation durations, or detection 

latencies. Nor were there any differences between groups in perceived 

effort, fatigue, or attentiveness. Had the ,task been simply to acknowledge 

the appearance of a new target on the display, or had the rotation speed of 

the simulated radar sweepline been considerably slower, quite different 

results might have been obtained. However, the requirement to recognize 

departures from designated altitude limits appeared to approximate a 

realistic monitoring requirement, and the speed of sweep rotation was within 

the range of contemporary ATC radars. Thus, the results suggest that the 

presence of a sweepline neither adds to nor detracts from efficiency when the 

primary task consists of monitoring a complex display for the appearance of 

occasional, critical alphanumeric changes. 

with regard to the overall changes common to both groaps, the patterns of 

change in mean, maximum, and minimum detection latencies were quite comparable 

to those obtained in two previous studies using a similar radar simulation 

(12,13). The principal difference was the longer detection latencies found 

in the present study, presumably because of the increased difficulty in 

recognizing the critical stimulus changes. The pattern emerging from all 

three studies is that of relatively uniform performance during the first hour 

10 
followed by a general decline during the second. Interestingly enough, the 

horizontal eye movement data revealed a pattern of fixation durations that 

appeared to parallel the patterns that have been obtained for performance. 

D~tailed comparisotiS of mean fixation duration in the present study with 

varim:s measures of performance efficiency, however, failed to yield any 

evidence of a significant covariation. 

What evidence exists that might bear on the expected degree of relationship between frequency of eye movements and detection efficiency in a task of 

this type? Unfortunately, comparisons must be made with other types of 

performance tasks, since studies directly analogous to the present one 

apparently have not been conducted. 

With regard to the general pattern of eye movements during prolonged 

performance, the typical findir.g appears to be a decline in the frequency of 

fixations. This has been reported during simulated driving (10), piloting a 

helicopter (11), and performance of a simple vigilance task (8). This decline 

is apparently a manifestation of fatigue (11) and parallels a decrease in 

performance efficiency (8). 

Studies attempting to relate inciividual differences in the frequency of 

visual fixations to performance have generally reported some evidence of a 

positive relationship between frequent eye movements and superior performance. 

The findings that would seemingly be most directly applicable to the present 

study are those obtained from simple"vigilance ~asks. Schroeder and Holland 

(8) found high correlations between frequency of fixations and detection 

performance, with higher frequencies of eye movement related to higher detection rates. Similar findings were reported by Mackworth, Kaplan, and Metlay 

(5). However, in both studies, these relationships were obtained using a task 

condition in which Ss continuously monitored two or more dials. It is not 

very surprising that Ss whose eyes shifted more frequently between dials 

detected more signals~ Nevertheless, other studies involving some form of 

visual search have also reported that frequent eye movements may be related to 

superior performance, although the evidence is far from conclusive. Snyder 

(9) found that Ss in a simulated air-to-ground search tas~ who had lower mean 

fixation times detected more targets. However, the number of Ss was too small 

to warrant detailed statistical analysis. Thomas and Lansdown-(14) reported 

that, out of five radiologists searching roentgenograms, the single 

radiologist who detected the most lesions had the shortest mean fixation 

durations. Schoonard, Gould, and Miller (7), on the other hand, found that 

good inspectors of integrated circuit c~ips were more rapid in locating defects 

than were poor insrectors, but mean fixation durations did not differ among 

inspectors. 

Most of the evidence to date scggests some degree of relationship between 

frequency of eye movements and performance efficiency in visual search or 

monitoring tasks. The parallel trends obtained for eye movement and performance in the present study seem to support these previous findings. However, 

11 
if tbe parallel trends do, in fact, imply a relationship, why was there no 

evicL .. ·=e of any correlation between individual eye movement data and 

performance? 

Lack of reliability or validity of the method of recording scanning 

activity does not appear to be a satisfactory explanation. While only horizontal eye movements were recorded, it seems rea3onable to assume that this 

measure would be proportional to total eye movement activity, s;_nce any 

scanning pattern employed in searching the display would necessarily require 

both horizontal and vertical mov<Jments. (It was impossible to accurately 

correlate horizontal with verti~al eye movements in our recordings because of 

the contamination caused by blinks in.the vertical data. Stern and Bynum 

(11), however, have reported that horizontal and vertical saccades covary for 

most Ss in a visual search task.) Reliability of the horizontal mean fixation 

durations proved to be quite high. An estimate of reliability based on an 

analysis of variance (17) of the data across 30-minute periods yielded a value 

of .94. Also, correlations obtained at the beginning and end of the session 

between hand-scored horizontal eye movements and the pulses resulting from the 

differentiated EOG were high (.90 and .82, £ < .01), indicating that the data 

processed by the computer were reliable measures of horizontal eye movement 

activity. Finally, fixation durations in the present study fall within the 

range (estimated from their data) of mean horizontal fixation durations (641 

to 943 ms) obtained by Stern and Bynum (11) for helicopter pilots during 

flight. They also fall within the range (430 to 1,815 ms) of total eye movement fixation durations reported by Gerathewohl (3) for radar tasks of 

varyi~g difficulty. 

The lack of any difference in the present study between mean fixation 

durations in the intervals preceding maximum detection latencies or missed 

stimuli and in intervals preceding minimum detection latencies suggests the 

hypothesis that critical targets were, at times, fixated witho·,t being "seen" 

as critical events, and that the number of these fixations witi,cut recognition 

varied in some~ perhaPs>stochastiC, manner within the session. Studies of 

simple vigilance peiform~w.-e have found that signals are frequently missed 

even when photographic measures-of eye fixation points indicate that the S 

was fixating the stimulus event at the time the critical signal occurred -

(5,8). This has also been reported to occur in more complex tasks such as 

searching roentgenograms (6, p. 363) or air-to-ground surveillance (9). 

Virtually nothing is known concerning the frequEncy of occurrence of this 

phenomenon or the factors that may influence or .~ontribute to it (6, pp. 362-

364). 

If the above hypothesis is correct, fr~quency of scanning (or mean 

fixation duration) may be a poor correlate of detection latency under task 

conditions similar to those employed in the present study. Whether the use 

of a more easily detected critical stimulus (such as the 999 employed 

previously) or the use of Ss with extensive radar experience (such as 

journeyman contro~cers) might have changed the relationship between scanning 

12 
and performance cannot be answered with complete certainty from this 

experiment. The use of trained controllers might have resulted in a higher 

correlation between scanning activity and detection latency, since, as noted 

earlier, there is suggestive evidence that pilots who scan more rapidly 

detect more ground targets (9), and similar findings have been reported for 

radiologists searching roentgenograms (14). However, it should be 

emphasized that both of these investigations used professionals (test pilots 

or radiologists), and both report that Ss would occasionally fail to see 

targets that they had actually scanned.- There is no reason to believe that 

air traffic controllers would be any different in this respect. We hope to 

conduct future studies using equipment to continuously record actual eye 

fixation points during monitoring performance to compare the extent of 

covariation between scanning and performance for different levels of critical 

stimulus difficulty. Such studies would provide definitive information on 

how frequently critical targets are fixated without recognition and on the 

factors (e.g., experience, age, fatigue) that may contribute to the 

occurrence of this phenomenon. 

13 
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14 
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15