; AD-A 190 641

DOT/FAA/AM-881 An Evaluation of the Effects of

.. e: Offfice of Aviation Medicine High Visual Taskload on the

Washington, D.C. 20591 Separate Behaviors Involved in

"Complex Monitoring Perfo'mance

6

Richard I. Thackray

R. MarL. Touchstone

Civil Aeromedical Institute

Federal Aviation Administration

Oklahoma City . OK 73125

January 1988 DTIC

,ELECTE

§,MAR 2 41988

This document is available to the public

through the National Technical Information

* •Service, Springfield, Virginia 22161

° 0

US Deportment of Transportation

Federal Aviation Administration

88 3 22 094 _ _,, ° . .... .. ...... , , ... ..... , .-..... . ., ... . ,. .•.• ,,.• , .. . ., , . ,. 
L , . N.. .. A ,- ,-, No . I R. t' C-oag No.

FAA-AM-88-1

JAN EVALUATIO, OF THE EFFECTS OF HIGH VISUAL January 1988 TASKLOAD ON THE SEPARATE BEHAVIORS INVOLVED -P. P O r...ga tion Code

IN COMPLEX MONITORING PERFORMANCE

.. ... ... . . ......... Performing Orgoanm oton Report No.

7. Author's)

Richard I. Thackray and R. Mark Touchstone

9. P..,orm,ng Orgomization Name and Address 10. Work Unit No. (TRAIS) FAA Civil Aeromedical Instietute

. P. 0. BOX 25082 ii. Contract ot Giant No.

Oklahoma City, Oklahoma 73125

13. Type of Repori and Period Covered

0e Av aT I on Mediclno

Federal Aviation Admlnlstratlon

800 Independence Avenue, S.W. 74. 'ponsoring Agency Code

Washington D.C. 20591

15 5iippl ne'rta ry Notes

This work was performed under tasks AM-C-813-PSY-104 and AM-D-87-PSY-104.

-16. Abatuct

2 Operational monitoring situation@. In contrast to typical laboratory vigilance tasks, generally

Involve more than Just stimulus detection and recognition. They frequently Involve complex

multidimensional discrlmitiotlons, Interpretations of significance, decisions as to appropriate

action. Implementalon of actions, and evaluntion of consequences. A simulated air traffic control

(ATC) task was developed to study the effects of prolonged monitoring on a number of such behaviors

embedded In the context of the task. All subjects performed the took ,inder relatively high vlou..l

taskicod conditions for a single 120-mmn seselon,_)Two types of critical events requiring different

levels of Information processing for detection were employed. One type of event consisted of a

readily detectible loss of altitude Information In on alphoraumeric data block; a second type of

event Involved the detection of two aircraft at the same altitude on the same flight path, This

latter event required continuous, successive comparleons of data blocks in order 4o be detected.

Following detection, a decision was made as to whether or not the situation might redult In a

, potential conflict (collision). blemsuree derived from the Implementation of each type of deoleon

I"enabled acquisition of data on •short-term memory, decision tl,e and decision errors, procedural

errors, and speed of motor movement. The results revealed that time to detect aircraft at the some

4I altitude Increased significantly over the monitoring period as did omission errors for this type of

event. Detection time for the more readily detectible alphanumeric changes Involving loss of

altitude Information showed no evidence a! Impairment, nor was any Impairment found for any of the

other task behaviors that were measured. The findings ore discussed with reference to previous

studies suggesting that complex monitoring primarily affects attentlonal ,rocessle and that the rate

of decline In attention appears to be related to the degree of Information processing required for

event detection.

17., Ki Words 18. Ditributio, Staot.ment

Air traffic control, attention, Document Is available to the pubil.;

automation, monitoring, performance, through the National Technical

vigilance ¶ . I Information Service,

Springfield, Virginia 22161

19. Security CIUSS4. (of this r~oI20. Seci.~rty Classif. (of this pages) 21. N;. of Pages 22. ,c

Unclassified Unclassified 13

"Form DOT F 1700.7 8e-72) Reprodu'.ion of completed page authorized

.-. ~ ~ C~

% 
NOT ICE

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 contents or use thereof.

Access5ion For

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kAU 111,P"1''
AN EVALUATION nF THE EFFECTS OF HIGH VISLIAL TASKLOAD ON THE

SEPARATE BEHAV$ORS INVOLVED IN COMPLEX MONITORING PERFORMANCE

1. Introduction

It Is Increasingly recognized that modern operational vigilance tasks,

such as those related to air traffic control, nuclear control room

operation, security-surveillance systems etc., Involve more than simply

"detecting and responding to Infrequent critical events. They frequently

Involve complex multld~menslonal discriminations in which stimulus

detection or Identification may be followed by Interpretation of

significance, decisions as to appropriate action, Implementation of

actions, and evaluation of consequences (Craig 1984, Mackle 1984). Yet,

traditional vigilance studies, for the most part, seldom look at behaviors

other than those directly related to stimulus detection. This would

appe3r to be true not only for laboratory studies using simple vigilance

tasks, but for studies of complex monitoring performance as well (see

Davis and Parasuraman 1982, Parasuraman 1988 for recent reviews).

:n an effort to examine the effects of prolonged monitoring on behaviors

other than Just stlmulue detection, we have developed a laboratory

simulation of an air traffic control (ATC) task that Incorporates many of

the aspects of real-life monitoring situations. As It Is currently

configured, the task simulates an Intermediate level of ATC automation In

which the computer acts as an aid to the controller In resolving aircraft

conflict situations. Although monitoring for Infrequent event detection

constitutes the principal task requirement, the task was developed to

enable acquisition of data on short-term memory, decision making.

procedural errors, and speed of motor movement.

Our Intltlal study with this task examined the relationship of both visual

taskload and target difficulty to detection performance (Thackray and

Touchstone 1985). Subjects monitored either 8 or 16 alphanumeric targets

In order to detect critical events requiring different levels of

Information processing for detection. One type of event consisted of a

readily discernible change in the contents of an alphanumeric data block;

a second type of critical event Involved the detection of two aircraft at

"the same altitude on the same flight path. This latter event required

continucs, successive comparisons of data blocks In order to detect Its

"occurren;e. While the more readily detectible events showed no evidence

of performance decline at either level of visual taskload, the more

difficult to detect altitude events showed evidence of impairment that was

signiflcant:y related to taskload; the number of ouch events not detected

Increased significantly under the higher, but not under the lower,

taskload condition. Fatigue, resulting from the effort required to

continuously scan and process Information from a large number of targets,

was offered as a possible explanation for this Impairment. This

explanation was supported by the finding of a significant decline In

critical flicker frequency (CFF) that occurred under the 16-target, but

not the 8-target condition.

Because elements of the task Just described were still being developed at

the time the above study was conducted, only data relating to detection

efficiency (time and errors) were analyzed In that study. The present

A~4

R, % R

.* R ' -
study represents an extension of this earlier one and was conducted to

determine whether the apparent fatigue resulting from prolonged monitoring

under high taskload conditions affects only attentional processes or

whether other behaviors relevant to complex monitoring show Impairment as

well. Effective allocation of function In Increasingly automated systems

requires Information on how prolonged monitoring may affect all

performance aspects of such tasks, not just those related to attention.

The present study also sought to provide further Information on the visual

behavior of subjects during times when critical events are missed.

Findings obtained In several of our previous studies suggest that critical

events (e.g., altitude changes) are either missed (Thackray and Touchstone

1985) or are responded to with excessively long detection times (Thackray

and Touchstone 1980) In spite of the fact that subjects appear to be

scanning the display throughout the session. In the current study,

videotaped recordings of eye movement activity and facial orientation were

obtained In order to assess visual behavior of subjects during those times

when missed events occurred.

2. Methods

2.1 Sjublo . Forty-eight men and women, all paid university students,

volunteered to participate in the study. Subjects ranged In age from 18

to 29 years, had 20/20 uncorrected vision, were nonsmokers, and had no

*• prior experience with the task used or previous ATC training. None were

currently taking any prescription medication on a regular basis.

2.2 Appjj atu I.Aa T .QsJDsgn. The basic experimental equipment consisted

of a Ulgital Equipment Corporation (DEC) VS11 19-in (49-cm) graphics

display, keyboard, and Joystick, all of which were Interfaced with a VAX

11/730 computer (DEC). The computer was used both to generate Input to

the display and to process subject responses. The VS11 was Incorporated

Into a console designed to closely resemble an ATC radar unit. Two

diagonal, nonlintersecting flight paths were located on the display, along

"which aircraft targets could move In either direction. A given PIrcraft's

location was displayed as a small "blip" on the flight path, and an

adjacent alphanumeric data block Identified the aircraft and gave Its

altitude and groundspeed. Aircraft were updated In position and any

change In alphanumerics every 6 sec. Figure 1 shows a typical target

pattern as displayed to the subject, with the total console-display

* configuration shown In Figure 2.

The subject's task was to continually monitor the display for one of two

types of change in the alphanumeric data blocks. The duration of each

type of change (referred to as a critical event) was 90 sec; If a subject

failed to detect a critical event within this gO-sec period, the data

* block containing the change reverted to its previous state.

'The first type of critical event was readily detectable and consisted of

three X's in place of the three altitude numbers In a given data block.

Subjects were told that this replacement of an altitude value signified

that a transponder malfunction had occurred resulting In a loss of

altitude information. Upon detection of such an event, subjects were told

to press a designated button on the console, move a joystick-controlled

VI I l i D I l
LVI

FIGURE 1. A TYPICAL TtRGET CONFIGURATION AS DISPLAYED TO THE SUBJECT.

-

r- .......--.... o m

FIGURE 2. THE SIMULATED ATC WORK STATION. ONLY THE CONSOLE ON THE LEFT

WAS USED IN THIS STUDY.

3

I M
cursor over the data block containing the critical event, and to press

another button on the joystick control unit. This last response

"Ocorrected" the malfunction by replacing the threa X's with the previous

altitude value. The second type of crltlcl event was more difficult to

detect, since It was not Immediateiy apparent. This event was the it occurrence of two aircraft at the same altitude on the name flight path.

As soon as such an event was noted, subjects pressed a second console

button. It was next determined whether the tvo iýlrcraft were moving

towards each other, away from each other, or In the same direction. On

the basis of this determination, subjects then oresyed elther a "Conflict"

button (indicating that the aircraft were movlno tws' dz- iech other) or a

"No Conflict" button (indicating that the alrcr&it i,%re eciher moving away

from each other or were moving In the same diractson). In order to

prevent overlapping data blocks, all aircraft In this study were assigned

a constant speed of 450 knots. Thus, only targets moving towards each

other would constitute a potential conflict situation. Followlng a

"conflict" decision, the cursor was positioned over one of the two

conflicting aircraft, and the joystick control button was pressed. This

caused the computer to assign a new altitude value to one of the two

conflicting aircraft and display this value, along with the aircraft's

Identification In a box at the lower left of the screen. Subjects then

verified that tne computer-assigned altitude did not result In a conflict

with some other aircraft on the flight path. If no new conflict was

created, a keyboard entry was made that assigned the new altitude value to

one of the two previously conflicting aircraft. (Although subjects were

led to believe that a computer-assigned altitude might occasslonally

result In a conflict with some other aircraft, In actuality this never

occurred.)

Whenover a "no conflict" response was made, no further action ensued, since no change In altitude was required. Subjects were told that the

altitude of one of the two nonconflicting aircraft would eventually change

to some other value (this change always occurred 60 sec after the no

conflict response was made) and that they had to remember that they had

responded to this particular pair of aircraft. If they failed to remember

and responded a second time, a memory error was recorded.

The number of targets on each flight path was kept equal at aHi times; as

one left the screen, another appeared. Nine critical events occurred In

each 30-mmn period, with no more than one event present at any given time.

Of these nine events, three were XXX's, three were conflicting altitude

changes, and three were nonconflicting cringes. These events were

arranged In a quasI-random order with th, restriction that each of the

three types of events had to occur at least once In both the first and

second 15 min of each 30-mmn period. Subjects were given no Information

regarding the frequency of events or their order of occurrence. The times

between events (Interstlmulus Intervals) ranged from 126 to 302 sac with a

mean of 200 sec.

2.3 Ylde RnJird.Ing Methodology,

A miniature Sony CCD TV camera was mounted In the lower left corner of the

console at an approximate 45 degree angle to the subject's face. The

output of this camera was combined, by means of a special effects

generator, with the output of a second camera located to the rear of the

4

IN. . . R%
subject that was used to record the contents of the simulated radar

display. The combined outputs of both cameras were displayed on a video

S.• monitor. A small indicator light, not visible to the subject, was located

above the console and was momentarily Illuminated each time a critical

event occurred. Continuous videotapo recordIngs enabled subsequent

playback and analysis of the subject's visual behavior times when

critical events were not detected.

2.4 Procedure

On arrival, subjects were played a taps recording that stated that this

experiment was Dart of a series of studles designed to investigate the

role of the controller in Increasingly automated ATC systems. They werp

told that the task wa3 designed to simulate an Intermediate level of ATC

automation In which computer aids are used to assist the controller. They

were then given task Instructions and separate practice In responding to

each kind of critical event.

In order to add a greater element of realism to the task, a tape recording

of background noises recorded In actual air traffic control radar rooms

was played continuously during the 2-hour task session. Sound love: of

this noise at the subject's head location was 82 dBA. It was not expected

that this would have any effect on performance, since an earlier study

using a previous version of this monitoring taok failed to find any

significant performance effects of this noise at a considerably higher (80

dBA) level (Thackray 1G82), At the completion of the 2-hour task period,

subjects were given a thorough debriefing concerning the purposes of the

experiment.

3. Results

3. 1 Ta~r.get DetectLL~ gnLM&A IJm ErL.rar .L Om IJasLIon,1

As described eartler, tubjecte monitored the display for the occurrence of

either one of two types of events. The first type of event, signifying an

altitude malfuction, consisted of an XXX that replaced the three-digit

altitude value In an alphanumeric data block; the second type of event,

constituting a potential conflict or no conflict situation, could only be

detected through continuouw comparisons of each target's altitude with the

altitude values of all other targets cn a given flight path.

Figure 3 shows mean detection times across 30-mln periods for both types

of event. Separate repeated measures analyses of variance (ANOVAS)

applied to these data revealed no significant change across the 2-hour

session in detection time for altitude malfuctlon events (F(3/141)-1.68,

p:.05), but a significant Increase in time to detect possible conflict/no

conflict situations (F(3/141)-15.47, p4.001).

With regard to errors of omission, the more readily detectable malfuction

events were never missed by any of the subjects. For aircraft at the same

altitude, however, 71% of all subjects missed at least one of these

occurrences during the two-hour session. Since the actual proportion of

events missed relatije to events presented was rather small, It was

decided to compare omission rate during the first and second hours of task

.....ii.w.-
performance rather than during separate 30-min periods. Combinlng across

subjects and events revealed that 21 of the conflict/no conflict events

were missed during the first hour and 77 during the second, yielding miss

rates of 4% and 13% respectively. A Wilcoxon comparison of the first and

seccnd hours revealed the Increase In miss rate to be significant (p4.05). 30

28-

"26-

CONFLICT NO CONFLICT EVENTS

22

20

0

z i8

o

U 16 w 14

12

S0 •--0 -- - ~ ...-.. ~ -0

ALTITUDE MALFUNCTION EVENTS

I 23 4

30 MINUTE PERIODS

FIGURE 3. MEAN DETECTION TIMES ACROSS 30-MIN PERIODS FOR BOTH LEVELS OF

EVENT DIFFICULTY.

3.2 Decision .•an Decision Errors,

Following a subject's response to the detection of two aircraft at the

same aititude, a decision was made as to whether the situation represented

a potential conflict or a no conflict situation. The time from detection

response to decision response was obtained for each altitude event for

each subject with means displayed In Table 1. Also shown In Table 1 are

data for a second measure of decision time. This measure consisted of the

TABLE 1. MEAN TIMES (IN SEC) FOR SEVERAL MEASURES OF DECISION

BEHAVIOR DURING THE TWO-HOUR SESSION.

Thirty-minute Periods

Measure 1 2 3 4

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

Conf/No Conf 6.29 5.77 6.12 6.11

Decision Time

Time to Accept 4.36 3.79 3.95 3.67

Alt Resolution ----------------------------------------------------------
time between a subject's Interrogation of the computer for its suggested

resolution to a conflict and acceptance of this resolution. Separate ANOVAs p•. formad on the two sets of data shown In Table I revealed no

evidence of any Increase ot decrease In conflict/no conflict decilon time

across the 2-hour session (F(3/141<1.00) nor any evidence of a significant

change In acceptance time for computer-generated altitude resolutions (F(3/141-2.29, p>.05).

Decision errors were recorded whenever a conflict decision was made to a

no conflict situation or a no conflict decision to a conflict situation.

If the incorrect decision was then followed by a sequence of behaviors

appropriate to the decision made, this would suggest an Incorrect

Interpretation of the altitude event; If the Incorrect decision was

followed by a sequence of behaviors that would have been appropriate to

the opposite deciolon, one could Infer that the subject had made a

careless error, In not pressing the button Intended. Only 3 errors of the

latter type were documented, suggesting that carelessness was not a

significant factor In Incorrect decisions. With respect to the former

type of error, 14 were made during the first hour and 8 during the second,

yielding error rates of 2% and 1% respectively. The Wilcoxon comparison

of first and second hours was nonsignificant (p>.05).

3.3 Motor Movement Time.

In order to obtain an Indication of possible change In the speed of motor

activity with time on the task, measures were obtained that reflected the

time taken by subjects to move the joystIck-controlled cursor from the

bottom of the screen and locate It over the data block containing a

critical event. Two similar, but separate measures of such behavior were

obtained; those associated with correcting malfunction events and those

associated with resolving altitude conflicts. Mean times for each measure

are shown In Table 2. Separate ANOVAs yielded no evidence of a

significant change In time to complete either of these two movement

sequernces during the 2-hour session (F(3/141) <1.00 In both cases).

TABLE 2. MEAN CURSOR MOVEMENT TIMES (IN SEC) ASSOCIATED WITH

RESOLVING MALFUNCTION EVENTS AND ALTITUDE CONFLICTS.

Thirty-minute Periods

Measure 1 2 3 4

Movement Times 6.86 7.22 e.47 6.45

for Malfunction

Events

Movement Times 8.60 6.59 7.25 6.86

for Altitude

Conflict Events

----- -- -- -- -- --- -- -- -- --- -- -- -- --- -- -- --
3.4 Milli.lry. LError&,

Wheneve' a no conflict decision response was made to two aircraft at the

same altitude on the same flight path, the altitudes of these two aircraft

remalned the same for a 60-sec period following the decision response.

During this time period, If a subject failed to remember having previously

responded to these two aircraft and made a second detection and decision

response, a memory error was recorded. The frequency with which such

errors occurred was found to be quite small. During the first hour of the

session, 4% of the no conflict situations were responded to twice, while

during the second hour, the error rate declined to 3%. A Wilcoxon test

revealed this decrease to be nonsignificant (p>.05).

3.5 ErocduCrl E.rrors.

As described previously, detection responses to both malfunction and

altitude conflict events were always followed by a sequence of behavior*

that served to resolve the particular event. Whenever any element of

these behavioral sequences was performed out of order, was omitted, or an

Incorrect element added to the sequence, a procedural error was recorded.

Such errors, like the memory errors above, occurred quite Infrequently,

with an error rate of only 2% during the first hour and 4% during the

second. A Wilcoxon test performed on these data revealed the Increase In

errors from the first to the second hour to be nonsignificant (p).08).

3.8 YLnag AQLlyalm gL QmuJi.gL Errors,

Videotaped recordings of each subject's visual behavior during the session

were examined, specifically with regard to visual activity during times

when altitude events were not detected. Thus, for each missed conflict/no

conflict event, visual activity was examined over tne 90-sec period that

the event was present on the screen. Because of problems with the video

recorder, and because the subject's seating position at times prevented a

complete analysis of facial orientation and visual activity over the

entire 90-sec period, not all missed events could be analyzed. Of the 98

events missed by the subjects, there were 40 events for which visual

activity data was available during all of the 90-sec scoring period. As

Indicated earlier, the Intent of this analysis was not to provide precise

Information on fixation times, fixation points, or scanning patterns, but rather simply to gain Information on general visual activity during times

when subjects failed to detect aircraft targets at the same altitude.

From preliminary viewing of the tapes, It was determined that any portion

of the scoring period could be categorized In one of three ways: (1) Eyes

open, head oriented toward screen, continuous scanning; (2) Eyes closed;

(3) Eyes diverted from screen.

The above categories, while admittedly rather qualitative, served the

purpose for which they were intended. This was to ascertain the extent to

which the Increase In frequency of missed events that occurred during

monitoring could be attributed to subjects falling to detect these events

simply because their eyes were either closed or diverted away from the

display. Analyses of the tapes revealed that 97% of the scorable missed

events occurred during periods In which subjects had their eyes open and

were actively scanning the display. One event was missed because a

subject's eyes were diverted from the display, but no missed events could

i v 8
be attributed to a subject's eyes being closed during the time the event

was present.

4. Discussion

Detection times for the alphanumeric change used to Indicate an altitude

:. , malfunction rhowed no evidence of any Increase over the 2-hour session.

Mean detection time averaged 9.2 sac, and these events were never missed

by subjects. The time required to detect aircraft at the same altitude,

however, Increased significantly over the session, from an average of 19.8

sac during the first half hour to 28.8 sac during the final half-hour

period. In addition to the Increase In detection time, the frequency with

which such events completely escaped detection by subjects also Increased

significantly. Four percent were missed during the first hour and 13%

during the second. Taken together, these findings are consistent with

those obtained previously using this task under comparable taskload

conditions (Thackray and Touchstone 1985).

S3 Although the ability to detect aircraft at the same altitude showed clear

evidence of impairment over the 2-hour session, the processes contributing

to this impairment are not Immediately apparent. Clearly, the ability to

dotect such events Involves more than just attention; memory and scanning

woula also appear to be Important components. Yet with regard to the role

of memory as a contributor to this cecline, It should be noted that none

of the oier functions or subtask elements Involving memory that were

measured In the present study showed any evidence of decline during

monitoring. Thus, neither failures to remember having responded to a

particular no conflict altitude event nor failures to remember correct

procedural sequences increased In frequency during the session. In like

.. manner, although onl/ a gross assessment of scanning activity was possible from the videotaped recordings of visual activity, there were no obvious

Indications that scanning was not taking place during times when

behavioral evidence (missed events) might suggest Inattentiveness.

Further, the fact that detection times for the readily perceivable

malfunction events showed no change across the session would also suggest

that decreased scanning activity per so would not appear to be responsible

for the decline In abl;Ity to detect aircraft at the same altitude. One

S Is left to conclude, then, that the decrement associated with these events

would appear to be specific to attention. A similar conclusion was also

reached by Johnston 21 fLL. (1966) In an earlier study of complex

monitoring. Performance decrement under high taskload conditions was

found to result primarily from an Increase In lapses of attention, the

m agnitude of which did not appear to be uniquely affected by differences

In memory requirements of the task conditions employed.

Memory was not the only aspect of performance that failed to (:hangs during

monitoring. There was also no evidence of change In me -ures of decision

time, decision errors, or motor movtment time. These findings are

difficult to evaluate because, as noted earlier, stulies of complex

monitoring seldom report on behaviors apart from those directly related to

stimulus detection. However, a few comparisons can be made. In an early

* •,study by Adams 2t 1l. (1961), an air traffic surveillance task was used

to study the effect of prolonged monitoring on decision making, In

addition to the usual measures of target detection. Half of the subjects

•.4

V
made only a simDie detection response to an alphanumeric Symbol change

whils the remaining half were required, FoliowIng detection, to make a

four-choice evaluation Indicating the nature and location of the change

that had occurred. Over a 3-hour monitoring session, performance decl!ned

In the simple detection condition, but showed no evidence of decline in

the condition In which decisions were required. These findings 3uggest

that the decision requirements, rather than adding to performance

decrement, appeared to have prevented It.

With regard to motor movement time. a subsequent study by Adams AL Li.

(1962) again used an air traffic surveillance task to examine the effect

of tine consecutive daily monitoring sessions, each 3 hours long. on

detection time and on the movement time required to complete the detection

response. This latter measure consisted of the time bettween the initial

detection response and response to a second button on a panel 16 Inches

away. Although movement time did slow significantly within each session,

the actual magnitude of this slowing was remarkably small, amounting to

approximately 80 msec.

The findings of the present study that performance decline under high

(16-target) task'oad conditions was confined to attentional behavir, and

within that realm only to the more difilcult task of detecting two

aircraft at the same altitudo, would appear to support conclusions reached

by Davis and Parasuraman (1982) that Information processing demands placed

on the observer may be one of the more significant determinants of

performance decline in monitoring tasks. In order to examine this

possibility within the context of our previous research, a post hoc

comparison was made of the present findings with those of two of our

earlier studies. All studies were equivalent In terms of the number of

alphanumeric targets employed, critical event rates, and task durations.

The principal difference between studies was In the type of critical

events used. In the earliest of these studies (Thackray it al. 1979).

the critical event consisted of the replacement of an aircraft's normal

altitude value with the number 0999." This critical stimulus, much like

the malfunction events of the present study, was a readily apparent

stimulus change requiring minimal Information processing for Its

detection. In a subsequent study (Thackray 1982), critical stimuli

consisted of a chanoe In an aircraft's displayed altitude to a value that

Oither exceeded an upper limit or was below a lower one. Like the "999m

used In the earlier study, such changes could also be detected without

reference or comparison to any other information displayed on the screen.

Information processing requirements In the later study, however, would

seem to be greater since altltuds changes became signals not because. they

assumed some fixed numerical value, but because they were detected as

having a value that exceeded previously specified upper or lower limits.

Mean detection times obtained In these two previous studies, aiong with

data for the conflict/no conflict altitude events of the prese:t study are

uhown in Figure 4. Examination of this figure suggests that an Increase

!n the level of Information processing required for critical event

detection not only Increases average detection time, but appears also to

Influence the aecrement function. An ANOVA performed on the data of the

three studies supported these Impressions by revealing a significant

effect for processing level (F(2/101)-120.21, pt.001) and a sign!flcant

level by p~lrlods Interaction (F(8/303)-4.85, p4.001). Since the analyses

10

S • • ,- * ,*4• * . ~,?•.. .- • , ,'-.o .,,• ~ r *,, . - ' - -* - -.- • . -• • '. - :•. . . ..
for periods, it Is not surprising that It was also significant In this

analysis as well •F(3/303)-13.35, P<.O01).

3o0

28

26-

24 CONFLICT/NO CONFLICT EVENTS

22- 

(PRESENT STUDY)

Z 18

":A 0

S16

- 14 ....... 0

12 -

to 0 HI-LO ALTITUDE LIMIT EVENTS

(THACKRAY 1982)

8

6 ALTITUDE REPLACEMENT EVENTS

4 (THACKRAY. BAILEY, 5 TOUCHSTONE 1979) 0•r __ __ I ___

I2 3 4

30 MINUTE PERIODS

FIGURE 4. COMPARISON OF DETECTION TIMES FOR ALTITUDE EVENTS DIFFERING

IN INFORMATION PROCESSING REQUIREMENTS.

In our previous study comparing monitoring performance under 8- and

16-target conditions (Thackray and Touchstone 1985), It was hypothesized

that the requirement to pa.sslvely monitor large numbers of targets over a

prolonged period of time demands conslderablq effor., and that the greater

decrement In porformance found under the higher taskload condition was a

reflection of the fatigue resulting from this effort. The results of tho

present study suggest that such fatigue effects are confined primarily to

attentlonal processes; of the other behaviors that were measured (decision

making, short-term memory, ability to correctly carry out procedural

sequences, motor movement), none showed any increase In Impairment over

the 2-hour session. Further, the present study, In agreement with our

earlier one (Thackray and Touchstone 1985). found that It was not

detection of events that are readily apparent to the observer that showed

evidence of decline under hig!, taskload conditions. Rather, It was

detection of those events that require considerable Information processing

In order to oe "seen" by the observer that were most adversely affected bu

prolonged monitoring under these conditions. Data presented In Figure 4

suggest that Information processing demands required for targoet detectiun

may Interact with visual taskload to Influunce the rate of attentfona!

decline undei conditions Involving extensive scanning of multlpt, tir-.ftý

Because this Interpretation Is based on a post hoc Co 0 IT ), I• , ,

findings of several different studies, additional rp.rqnch t(o v-.

i1'

. j . . *1 * -
g 

of targets differing

systematically In processing requirements and presented under different

levels of visual taskload Is required before more definitive statements

can be made. Hopefully, such research will enable us to specify more

"precisely the kinds of stimulus events that would benefit most from

computer-aided detection, especially with the higher ratios of aircraft to

controllers that are anticipated under the more automated ATC systems

being contemplated (Swedish 1983).

I=

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116 I.. I' 'C ' M ýL

:,tF
References

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