THE EFFECT OF INCREASED MONITORING LOAD ON VIGILANCE Julv 1977 

PERFORMANCE USING A SIMULATED RADAR DISPLAY 6. Performing Organization Code 

8. Performing Organization Report No. 

7. Author's) Richard I. Thackray 1 J. Powell Bailey, and 

R. Mark Touchstone 

9. Performing Organization Nome and Address 10. Work Unit No. (TRAIS) 

FAA Civil Aeromedical Institute 

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

Oklahoma City I Oklahoma 73125 

13. Type of Report and Period Covered 

12. Sponsoring Agency Name and Address 

Office of Aviation Medicine 

Federal Aviation Administration 

8oo Independence Avenue, s.w. 14. Sponsoring Agency Code 

Washington, D.C. 20591 FAA 15. Supplementary Notes 

Work was performed under Task AM-C-77-PSY-61. 

16. Abstract 

The present study examined the extent to which level of target density 

influences the ability to sustain attention to a complex monitoring task 

requiring only a detection response to simple stimulus change. The visual 

display was designed to approximate a futuristic, highly automated air 

traffic control radar display containing computer-generated alphanumeric 

symbols. Forty-eight male subjects 1 equally divided into three groups, were 

exposed to density levels of 4, 8, or 16 targets. Ten critical stimuli 

(signals) were randomly presented during each half-hour of the 2-hour 

session. Detection latency to the critical stimuli in the 16-target condition 

was significantly greater than latency to the 4- and 8-target conditions. 

There was no evidence of performance decrement in the two lower density 

conditions. The 16-target condition showed a significant progressive increase 

in mean detection latency, which was primarily the result of an increase in 

long latencies. The hypothesized decline in attention associated with this 

condition appeared to be independent of any major change in arousal level. 

17. Key Words 18. Distribution Statement 

Air Traffic Control Vigilance Document is available to the public 

Arousal through the National Technical Informati

Attention Service, Springfield, Virginia 22161 

Automation· 

Performance 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

Unclassified Unclassified 14 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 
I. Introduction. 

Air traffic control systems are becoming increasingly automated. 

Assuming that this trend will continue as projected, the future radar 

controller may well find his primary role will be that of a system monitor 

rather than an active participant in traffic control. Under such conditions, 

controller intervention would be required only when certain types of 

malfunctions occur or system limits are exceeded that cannot be automatically 

detected and remedied by the computer (4). Since a malfunction or 

deviation from system limits not detected by the computer could be a quite 

infrequent occurrence, the controller would be required to maintain a high 

degree of sustained attention to a task in which he intervened only 

occasionally. Leaving aside the problems of boredom and job dissatisfaction 

that might result (12), the question of whether a controller can maintain the 

required level of sustained attention under these task conditions is a 

significant one. 

Laboratory studies of prolonged performance on vigilance tasks typically 

employ near-threshold, short-duration signals and commonly reveal a 

progressive increase in errors, primarily of omission. The increase is 

generally quite rapid initially and tends to level off (approach asymptote) 

within the first 30 minutes of work. While this phenomenon, which has 

become known as the "decrement function," is ba·sed almost entirely on 

results obtained by using simple tasks, it is often assumed to apply to any 

vigilance task regardless of complexity. This assumption, however, has been 

subject to intermittent criticism over the past 10 to 15 years. Elliott (8), 

who was one of the first of the critics, noted that decrement of this form 

was never found in studies of closely simulated asdic or radar search. He 

states that while a slow, continuous degradation of performance may 

occasionally occur during 2-hour watches, more commonly no such "fatigue" 

effects are observed. Other critics have pointed out that modern operational 

monitoring tasks involving suprathreshold long-duration signals, extensive 

scanning of multiple stimuli, and complex decisional processes are so 

different from the simple vigilance task that to assume the existence of a 

comparable decrement function is unwarranted on the basis of our present 

knowledge (18, 22). 

Much of the criticism directed toward assuming the existence of a 

decrement function in complex operational monitoring tasks stems from the 

series of studies by Adams and his coworkers at the University of 

1 
to either 6 or 36 symbols; the required response was a button press wheneve

a particular symbol occurred. Critical stimuli were rarely missed in either 

of the two density conditions. While detection latency was significantly 

longer in the condition of greater visual load, performance decrement was th

same in both conditions. The increase in latency, over a 3-hour period, 

amounted to approximately 1 second. The above authors interpret their 

results as supporting the general conclusion that any decrement found in 

operational monitoring tasks would not be of practical significance for mos

systems. 

Another recent series of studies presents evidence that vigilance 

decrements do occur in complex monitoring tasks and that the magnitude of 

decrement may well be of practical importance (13). All of these studies 

employed a cathode-ray-tube (CRT) display in which Ss observed an 8x8 matrix 

of computer-generated alphanumeric symbols. The required response was a 

button press each time a symbol was randomly added to or removed from the 

display. In one of the studies, four stimulus densities--4, 8, 16, 

and 32--were compared under two levels of signal frequency (36 and 90 per 

hour). Although all conditions were accompanied by increases in mean 

detection latency over the 100-minute session, by far the most pronounced 

increase occurred under the maximum-density, low-signal-frequency condition

in which mean latency increased from 8 to 22 seconds. An analysis of this 

particular condition revealed that the increase in mean latency was primaril

the result of a progressive increase, over successive time segments, in the 

duration of maximum or longest latencies. Minimum latencies showed no chang

during the test session. This pattern of change, which is really a reflecti

of increasingly skewed latency distributions, is a pattern that has been 

found to occur during prolonged performance in a variety of monotonous tasks 

(5, 10, 17, 24, 25). Although other explanations are possible, the most 

likely explanation for this pattern is that it is a reflection of increasing

frequent lapses or fluctuations of attention (3, 6, 13). 

As noted earlier, whether radar controllers can sustain adequate levels 

of attention to advanced display systems that may require very infrequent 

intervention is an important question. The studies just presented suggest 

tha~ the question has not yet been adequately resolved. 

The increase in mean latency reported by Howell, Johnston, and Goldstein 

(13) for their high-density, low-signal-freauency condition was substantiall

larger than the 1-second increase reported by Adams, Stenson, and Humes (2)

More striking, however, was the increase (from 24 to 46 seconds) in long 

detection latencies found by Howell et al. (13); if this increase reflects 

lapses of attention, it could have real operational significance. 

2 
In addition to performance, a number of physiological and subjective 

measures were also included in this study. These measures were some of the 

same ones employed in a previous study dealing with performance correlates 

reported boredom and monotony (23) and were included for exploratory 

purposes. We hoped they might serve to suggest possible reasons for any 

performance difference found among the various target density groups. 

II. Method. 

Subjects. Forty-eight paid male university students were randomly 

assigned to three groups of equal size. The groups differed only in the 

number of targets--4, 8, or 16--to which they were exposed. Half the Ss 

in each group were tested in the morning and half in the afternoon. None h

any prior experience with the task used. 

Apparatus and Task Design. All task programing and recording of respon

were accomplished by using a Digital Equipment Corporation (DEC) PDP ll/40 

computer. The computer was interfaced with a VT-11 (DEC) 17-inch CRT, whic

served as the S's display. The CRT was located in a console designed to 

resemble an air-traffic control radar unit. The stimuli (targets) consiste

of small rectangular "blips" representing the locations of given aircraft. 

Adjacent to each target, and connected to it by means of a short diagonal 

line, 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 altitude in 

hundreds of feet and the last three gave groundspeed. For a given target, 

the alphanumerics identifying the aircraft, its altitude and groundspeed, a

well as its entry and exit points were randomly determined except for the 

following restrictions: (i) altitudes had to fall within the range of 180 

600 (in hundreds of feet); (ii) groundspeeds had to fall within the range o

400 to 580 (in knots); (iii) the entry and exit points for a given target 

could not be separated by less than 30°; and (iv) all targets on the displa

had to have at least 2,000 feet of vertical separation. 

At the beginning of an experimental session, the screen contained eithe

4, 8, or 16 targets, depending upon the condition to which the S was 

assigned. A simulated radar sweepline, the scale radius of which equaled 6

miles, made one complete clockwise revolution every 6 seconds. A target wa

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 

3 
pgppy

signal to which the ~ was instructed to respond consisted of 999 appearing i

the altitude block. Ten such critical stimuli appeared in each half-hour 

period; five occurred in the first 15 minutes and five in the second. The 

mean intersignal interval was 3 minutes. Time of critical stimulus occurren

and the target in which it occurred were randomly determined with the 

restriction that two targets could not contain critical stimuli at the same 

time. The ~ response to a critical stimulus consisted of pressing a butto

located on the console 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 ~ failed to detect a critical stimulus within 1 minu

the data block automatically reverted to its previous value. 

The computer and other recording apparatus were located in an adjacent 

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

lighting was used in the ~ room, and the level of illumination at the 

Figure 1. The simulated air traffic control console with 

a typical stimulus pattern. Only the lower left 

button was used. 

4 
, y , p, y

Except for eye movements, the transducers and electrode placements used for 

recording have been fully covered in a previous study (23) and will not be 

described here. The eye electrodes were Beckman miniature biopotential 

electrodes and were attached directly above and below the right eye. The 

outputs were AC-coupled to the Dynograph by using a 10-second time constant. 

In addition to recording the physiological measures, the Dynograph recorded 

on one channel the output of a photocell recessed in the console shelf, 

which was used to monitor the S's hand placement. Marker channels on the 

recorder also signaled the onset of a critical stimulus and the occurrence of 

the required button press. Outputs from a cardiotachometer and a pulse 

integrator for body movement were led to the digital inputs of the computer 

for subsequent analysis. 

Procedure. On arrival the ~ was taken to the testing room, given 

orientation instructions, and then instrumented for physiological recording. 

Following this a nine-point subjective rating scale was administered that 

dealt with present feelings of attentiveness, fatigue, strain, boredom, and 

irritation. 

The task instructions emphasized the necessity of pressing the button 

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

critical stimulus represented some form of malfunction-not detected by the 

computerized radar system. Following the taped instructions, the 2 was given 

a 3-minute practice period containing three critical stimuli. After the 

practice period three blood pressure recordings were obtained and the 2-hour 

test period was initiated. The ~ watch was taken from him before the test 

period began. 

After the 2-hour test period, three measures of blood pressure were again 

taken and the ~ 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 obtained for each 

30-minute time period: 

(i) Mean response latencies to critical stimuli correctly identified. 

(ii) Number of button presses without a critical stimulus. 

5 
ppgpvy (5) w

the mean and standard deviation of heart rate for each successive 5-minute 

period. These were then averaged to give values for the four 30-minute 

periods. A separate computer program summed the number of pulses from the 

body movement integrator for each 30-minute period. Conductance levels for 

each half-hour period were averages based on measures obtained at the 

beginning and end of the 30-minute periods. The systolic and diastolic blo

pressure recordings taken at the beginning and end of the task period were 

measured according to the procedure described by Thackray, Bailey, and 

Touchstone (23). Vertical eye movement-eyeblink recordings were visually 

scanned for evidence of any eye closures during the intervals following eac

critical stimulus. 

III. Results. 

Mean detection latencies for the three target density conditions are 

shown in Figure 2. Analysis of variance applied to these data yielded a 

significant (£ < .05 throughout) main effect for density, F(2/45) = 37.56, 

but a nonsignificant main effect for the half-hour periods, F(3/135) = 2.26

2 3 4 

30- MINUTE PERIODS 

Figure 2. Mean detection latencies for the three target 

density conditions. 

6 
difference between these latter two density conditions. Tests on the s

effects of the interaction indicated that only the 16-target condition 

increased significantly in mean detection latency over the half-hour pe

(F3/135) = 6.55. 

Since the 16-target condition was the only one to show evidence of 

significant decrement, it was selected for detailed examination. Each 

individual's longest and shortest latencies in each of the half-hour tim

periods were obtained and the means computed. These values are shown in 

Figure 3. Trend analyses revealed a significant linear component for l

latencies, F(l/45) = 4.64, but not for shortest latencies, F(l/45) = 3.

This finding suggests that the increase in mean response latency for th

16-target condition shown in Figure 2 was apparently the result of a 

progressive increase in long response latencies rather than a general 

increase in all latencies. 

Errors of omission, commission, and procedure were virtually nonexi

over all conditions. Two button presses to critical stimuli were made 

30 

28 

26 

24 

22 

20 

(/) 18 

0 

z 16 

0 

u 14 

w 

(/) 12 

10 

8 

6 

4 Minimum Latencies 

2 /). 6 6 6 

0 

2 3 4 

30- MINUTE PERIODS 

Figure 3. Mean minimum and maximum detection latencies 

for the 16-target condition. 

7 
gyy 

evidence of apparent eye closures during the periods between each critical 

stimulus occurrence and the detection response (button press).* Two Ss in t

4-target, four in the 8-target, and six in the 16-target condition sh~ed 

evidence of eye closures during these periods. The geheral pattern was tha

of a single eye closure lasting an average of 2.4 seconds. Of the 34 

detection latencies accompanied by eye closures, 27 were found to exceed the 

mean latency to critical stimuli occurring within the same half-hour measure

ment period. Of these 27, 9 were long enough to have been identified by the 

computer program as "maximum latencies"; 5 of these 9 occurred under the 

16-target condition. Not surprisingly, eye closures tended to increase duri

the task session. Eleven percent of all stimulus periods containing closure

occurred during the first half-hour period, 23 percent during the second, 37 

percent during the third, and 28 percent during the fourth. 

Analyses of variance conducted on the physiological data revealed 

significant changes across periods (or, in the case of blood pressure, 

between pretest and posttest measures) for all variables except diastolic 

blood pressure. Heart rate variability and body movement increased over the 

measurement periods, while heart rate, conductance level, and systolic blood 

pressure decreased. There were no significant differences between condition

and no significant interaction effects. Since no between-group or interacti

effects were significant, the obtained physiological data are not presented

Analyses of variance were also applied to the subjective rating scale 

variables. Significant differences between measurement periods were obtaine

for boredom, irritation, fatigue, strain, and attentiveness. Attentiveness 

decreased, while all the rest increased. As with the physiological data, 

there were no significant differences between density conditions and no 

significant interactions. A separate one-way analysis of variance of the 

monotony data likewise revealed no significant difference between condition

Statements on the scales corresponding to the mean ratings for the above 

variables at the completion of the task period suggested that the Ss were 

only slightly bored, were mildly annoyed, felt more tired than usual, 

*It was relatively easy to visually discern the differences between the 

recordings of eyeblinks and vertical eye movements. An eye closure was 

assumed if a pen deflection occurred that was equal to or greater than the 

average blink amplitude and lasted for 1 second or longer. There was no way 

of determining, however, whether the eyes were completely or only partly 

closed during these times. Consequently, all references to eye closures 

should be taken to mean that there was evidence that the eyes were at least 

partly closed. 

8 
IV. Discussion. 

Although mean detection latency of the eight-target condition (3.7 

seconds) exceeded that of the four-target condition (2.5 seconds), the 

difference was not found to be significant. Nor was there any evidence o

performance decrement over the time period examined. 

As noted earlier, several studies of complex monitoring have employed 

long-duration signals with similar density levels and, although they diff

in a number of respects from the present study, it is of interest to comp

findings. Both Thackray et al. (23) and Adams et al. (2) used filmstrip 

simulation of contemporary radar displays with target densities of eight 

six respectively. Mean detection latency was 2.9 seconds in the former 

study and 2.4 seconds (estimated) in the latter. A direct comparison of 

findings of the study reported by Howell et al. (13) is more difficult, 

however, since their monitoring task incorporated an additional short term 

memory requirement. However, if one uses their low frequency (36 signals

hour) "add" condition, which most nearly corresponds to the conditions of 

the above studies, mean detection latencies for stimulus densities of fou

and eight were 3.9 and 4.9 seconds (estimated) respectively. Only Adams 

et al. (2) obtained any evidence of performance decrement at these density 

levels, and the increase in latency obtained in their study only amounted 

approximately 1 second over a 3-hour period. 

Combining the abov~ data across studies gives an overall mean detectio

latency of 3.4 seconds (SD = 0.96 second) for densities of four to eight 

stimuli. Thus, from a practical standpoint, it would appear that detectio

of simple stimulus change, while monitoring four to eight targets, is qui

rapid, is relatively uniform over a variety of task conditions, and shows 

virtually no evidence of the performance decrement associated with simple 

vigilance tasks. 

With a stimulus density of 16 targets, however, the present study 

revealed a progressive, significant increase in mean detection latency acr

the 2-hour period. This increase was found to be primarily the result of 

increase in long detection latencies, which, as indicated earlier, appear 

reflect lapses of attention. 

Why evidence of declining attention in both the present study and the 

previously described study of Howell et al. (13) should be confined to 

high stimulus density conditions is an interesting question. One 

possibility involves the change in ratio -of signals to observing responses 

density is increased. Howell et al. (13) found that the decrement associa

9 

----------------------------------------------------------------
Pickett (14), a high proportion of signals to observing responses results 

a high rate of reinforcement, and little or no decrement is obtained. If 

proportion is low, relatively fAw observing responses are reinforced and th

S becomes increasingly inattentive. In the context of a complex monitoring 

task, this could imply either (i) that scanning behavior would be reduced 

under conditions in which there is a low proportion of signals to observin

responses or (ii) that it would be maintained, but Ss might fail during 

scanning to "see" the critical changes or signals. 

Presumably, this theory could also be used to explain the results of th

present study. However, it does not appear to provide any explanation for 

the results obtained by Adams et al. (2). It will be recalled that these 

authors employed target densities of 6 and 36 stimuli while keeping the sig

rate constant at 12 per hour. On the basis of the above theory, one would 

expect a sizeable decrement under the greater density condition, yet the 

minimal decrement obtained was the same under both conditions. No ready 

explanation is apparent for the difference in findings of their study 

relative to the findings of the present study and those of Howell et al. (

Data obtained from the eye electrode recordings suggest that eye closu

tended to occur with greater frequency in the 16-target condition and to 

increase in number during the course of the session. Further, long detect

latencies appeared to be more frequently associated with eye closures in th

16-target condition than in either of the lower density conditions. Failu

to respond to the critical stimuli because the S's eyes were closed, howev

was probably not a principal reason for the long detection times. (It sho

be recalled that the average duration of eye closure during a critical 

stimulus-response period was only 2.4 seconds, while average maximum detec

latencies for the 16-target condition ranged from approximately 17 to 23 

seconds.) Rather the eye closure data suggest a reduction in alertness th

was probably greater in the 16-target condition than in the other conditio

While Jerison and Pickett's (14) theory may provide the most satisfacto

explanation for the apparent decline in alertness associated with the grea

density condition, one puzzling aspect remains. With the possible exceptio

of .frequency of eye closures, which is suggestive at best, the 16-target 

condition was found not to differ from the lower density conditions on any 

the other physiological and subjective measures of alertness, arousal, or 

attention that were employed. Heart rate variability, for example, has be

found to vary inversely with behavioral measures of attention in numerous 

studies (7, 9, 15, 16, 19, 21, 23, 25). Yet this measure increased equally 

for all conditions. If alertness and attention did decline more in the 

greater density condition, one can only conclude that these physiological 

subjective measures were too insensitive to detect the difference. 

10 
(habituate) and show more pronounced oscillations are phasic and, finally

tonic activation reactions involved. Since these reactions involve differ

levels of the central nervous system, their manifestations would differ n

only in degree, but in kind. Extending Haider's (11) formulations, the m

factors known to affect vigilance performance (e.g., signal frequency, 

intersignal interval, signal duration, task duration, rest pauses, knowled

of results, personality variables, fatigue) would presumably affect the 

course or rate of decline in selective attention and, hence, the extent an

type of involvement of the activating system. 

It is proposed that the decrement associated with the 16-target condi

involved a decline in attention that was apparently independent of any ma

change in arousal, although some changes in the EEG might have been noted 

this measure been employed (20). Further research is needed to better 

understand the nature of the variables that contribute to and are associat

with performance decrement in cor:lplex monitoring tasks. 

v. Conclusions. 

The results of the present study are consistent with those of other 

studies of complex monitoring in which low target densities (four to eigh

targets) and critical signal rates of 12 to 36 per hour have been employed

Latency of detection of simple stimulus change was found to be quite unifo

across the studies reviewed (Mn = 3.4 seconds; SD = 0.96 second) with litt

or no evidence of performance decrement. This finding is rather striking 

when one considers that the various studies differed on a number of 

potentially relevant variables; e.g., ratio of signals to stimuli or targe

type of display employed, length of prior~ training, task duration. It 

would suggest that scanning a typical radar display for the purpose of 

detecting relatively infrequent but readily observable and identifiable 

critical changes can be performed quite rapidly and efficiently when the 

visual load falls within this density range. 

There is less agreement among studies with regard to higher target 

densities. While a general increase in detection latency is to be expecte

with increases in target density, evidence of performance decrement 

associated with higher densities has not been obtained universally. The 

present study found a significant increase in mean detection latency acros

the 2-hour period that appeared to be due primarily to an increase in long 

detection latencies. Long latencies increased from a mean of 17 seconds i

the first half-hour to 23 seconds in the last. It was hypothesized that t

increase reflected a progressive increase in lapses of attention during th

session. However, as noted, not all previous studies have found a similar 

decline in attention associated with high target densities. Consequently, 

11 
employed in this study, may find it difficult to maintain the necessary high 

level of sustained attention required for rapid, consistent detection of 

target changes. Additional research to confirm or extend the present findin

is needed before further speculations or recommendations would be warranted

12 
Division, L. G. Hanscom Field, Bedford, Massachusetts, 1963. 

2. Adams, J. A., H. H. Stenson, and J. M. Humes: Monitoring of Comple

Visual Displays--II. Effects of Visual Load and Response Complexit

on Human Vigilance, HUMAN FACTORS, 3:213-221, 1961. 

3. Barmack, 

Efforts: 

J. E.: Boredom and Other Factors in the Physiology of Men

An Exploratory Study, ARCHIVES OF PSYCHOLOGY, 218:5-83, 1

4. Barnes, L. B., and D. L. Dickson: Optimizing the Role of the Air 

Traffic Controller, JOURNAL OF ATC, January-February, 1973. 

5. Bertelson, P., and R. Joffe: Blackings in Prolonged Serial Respond

ERGONOMICS, 6:109-116, 1963. 

6. Broadbent, D. E.: Decision and Stress, London, Academic Press, Inc

1971, pp. 56-57. 

7. Coles, M.G. H.: Cardiac and Respiratory Activity During Visual 

Search, JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 96:371-379, 1972. 

8. Elliott, E.: Perception and Alertness, ERGONOMICS, 3:357-364, 1960

9. Ettema, J. H., and R. L. Zielhuis: Physiological Parameters of Men

Load, ERGONOMICS, 14:137-144, 1971. 

10. Faulkner, T. W.: Variability of Performance in a Vigilance Task, 

JOURNAL OF APPLIED PSYCHOLOGY, 46:325-328, 1962. 

11. Haider, M.: Neuropsychology of Attention, Expectation, and Vigilan

l£ D. L. Mostofsky (Ed.), Attention: Contemporary Theory and Analy

Appleton-Century-Crofts, New York, 1970, pp. 419-432. 

12. Hopkin, V. D.: Human Factors in ATC. In: Swedish Society of 

Aeronautics and Astronautics, International Symposium on Air Traffic 

Control, Stockholm, Sweden, 12-14 March 1969, Proceedings (Stockholm

Swedish Society of Aeronautics and Astronautics), 329-339 and 341-34

1970. 

13. Howell, W. c., W. A. Johnston, and I. L. Goldstein: Complex Monito

and Its Relation to the Classical Problem of Vigilance, ORGANIZATION

BEHAVIOR AND HUMAN PERFORMANCE, 1:129-150, 1966. 

13 
15. Kagan, J., and B. L. Rosman: Cardiac and Respiratory Correlates of 

Attention and an Analytic Attitude, JOURNAL OF EXPERIMENTAL AND CHILD 

PSYCHOLOGY, 1:50-63, 1964. 

16. Kalsbeek, J. W. H.: Measurement of Mental Work Load and of Acceptable 

Load: Possible Applications in Industry, INTERNATIONAL JOURNAL OF 

PRODUCTION RESEARCH, 7:33-45, 1968. 

17. Kennedy, J. L., and R. c. Travis: Prediction and Control of Alertness: 

II. Continuous Tracking, JOURNAL OF COMPARATIVE AND PHYSIOLOGICAL 

PSYCHOLOGY, 41:203-210, 1948. 

18. Kibler, A. W.: The Relevance of Vigilance Research to Aerospace 

Monitoring Tasks, HUMAN FACTORS, 7:93-99, 1965. 

19. Mulder, G., and W. R. E. H. Mulder-Hajonides van der Meulen: Mental 

Load and the Measurement of Heart Rate Variability, ERGONOMICS, 

16:69-83, 1973. 

20. O'Hanlon, J. F., and J. T. Beatty: Concurrence of Electroencephalographic and Performance Changes During a Simulated Radar Watch and Some 

Implications for the Arousal Theory of Vigilance. In R. R. Mackie (Ed.), 

Vigilance: Theory, Operational Performance, and Physiological Correlates

Plenum Publishing Company, New York, 1977, pp. 189-201. 

21. Porges, s. W., W. A. Arnold, and E. J. Forbes: Heart Rate Variability: 

An Index of Attentional Responsivity in Human Newborns, DEVELOPMENTAL 

PSYCHOLOGY, 8:85-92, 1973. 

22. Smith, R. L., and L. F. Lucaccini: Vigilance Research: Its Application 

to Industrial Problems, HUMAN FACTORS, 11:149-156, 1969. 

23. Thackray, R. I., J. P. Bailey, and R. M. Touchstone: Physiological, 

Subjective, and Performance Correlates of Reported Boredom and Monotony 

While Performing a Simulated Radar Control Task. In R. R. Mackie (Ed.), 

Vigilance: Theory, Operational Performance, and Physiological Correlates

Plenum Publishing Company, New York, 1977, pp. 203-215. 

24: Thackray, R.I., K. N. Jones, and R. M. Touchstone: Self-Estimates of 

Distractibility as Related to Performance Decrement on a Task Requiring 

Sustained Attention, ERGONOMICS, 16:141-152, 1973. 

25. Thackray, R. I., K. N. Jones, and R. M. Touchstone: Personality and 

Physiological Correlates of Performance Decrement on a Monotonous Task 

Requiring Sustained Attention, BRITISH JOURNAL OF PSYCHOLOGY, 

65:351-358, 1974. 

14