·' ·' 

FAA-AM-82-16 

PERFORNANCE OF 40- TO 50-YEAR-OLD SUBJECTS ON A RADAR 

MONITORING TASK: THE EFFECTS OF WEARING BIFOCAL GLASSES 

AND INTERPOLATED REST PERIODS ON TARGET DETECTION TIME 

Richard I. Thackray and R. Nark Touchstone 

Civil Aeromedical Institute 

Federal Aviation Administration 

Oklahoma City, OK 73125 

April 1982 

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 Nedicine 

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 contents or use thereof. 
1. Report No. 2. Government Accession No. 

FAA-AM-82-16 

4. Title ond Subtitle 

PERFORMANCE OF 40- TO 50-YEAR-OLD SUBJECTS ON A RADAR 

MONITORING TASK: THE EFFECTS OF WEARING BIFOCAL 

GLASSES AND INTERPOLATED REST PERIODS ON TARGET 

Technical Meport Documentation Page 

3. Recipient's Cotolog No. 

5. Report Dote 

APRIL 1982 

6. Performing Orgonizotion Code 

1--:::-D_E:-T-:-E_C"":'T-:I_O_N_T_I_ME_ • ---------------------! 8. Performing Orgoni zotion Report No. 

7. Author( s) 

Richard I. Thackray and R. Mark Touchstone 

9. Performing Orgonizotion Nome ond Address 

FAA Civil Aeromedical Institute 

P.O. Box 25082 

Oklahoma City, OK 73125 

~~---------------------------~ 12. Sponsoring Agency Nome ond Address 

Office of Aviation Medicine 

Federal Aviation Administration 

800 Independence Avenue, S.W. Washin~Zton D C 20591 

15. Supplementary Notes 

Work was performed under task AM-C-81/82-PSY-88. 

16. Abstract 

10. Work Unit No. (TRAIS) 

11. Contract or Gront No. 

13. Type of Report ond Period Covered 

14. Sponsoring Agency Code 

The present study examined the effects of wearing bifocal glasses and interpolated rest periods on the performance of 40- to 50-year-old subjects on a radar 

monitoring task. The visual display was designed to resemble an air traffic control 

radar display containing computer-genera~ed alphanumeric symbols. Forty men and 

women were divided into four equal-sized groups, with each group consisting of one 

of the four possible combinations of bifocal/no-bifocal and rest/no-rest conditions. 

All subjects were tested over a 2-hour session. Rest periods (a 5-minute break 

every 30 minutes) significantly reduced the performance decrement of .40- to 50-yearold subjects, bringing performance to a level approximating that of 18- to 29-yearold subjects without rest periods. The wearing of bifocal glasses did not contri~ . 

bute to visual strain or somati.c discomfort. 

17. Key Words 

Air Traffic Control 

Age 

Attention 

Bifocal Glasses 

Monitoring 

19. Security Clossif. (of this report) 

Unclassified 

Form DOT F 1700.7 <8-72) 

18·. Distribution Statement 

Rest Periods 

Video Display 

Terminals 

Vigilance 

Document is available to the public 

through the National Technical Information 

Service, Springfield, Virginia 22161 

20. Security Clossif. (of this poge) 21. No. of Poges 22. Price 

Unclassified 11 

Reproduction of completed page authorized 
PERFORMANCE OF 40- TO 50-YEAR OLD SUBJECTS ON A RADAR 

MONITORING TASK: THE EFFECTS OF WEARING BIFOCAL GLASSES 

AND INTERPOLATED REST PERIODS ON TARGET DETECTION TIME 

Introduction. 

Previous studies of simulated air traffic control (ATC) monitoring 

performance conducted in our laboratory have generally found that target 

detection times remain relatively constant, or even decline, over the first 

hour of a 2-hour session, but increase significantly during the second hour 

(7,9,11,12,13,15). We have found that this general pattern is not 

influenced by such variables as sex of the subject (11), presence or absence 

of a sweepline (13), relatively high ambient noise level (7), or previous 

ATC experience (15). 

Results from one of our more recent studies, however, suggest that this 

pattern is significantly altered by age (14). The earlier onset and greater 

magnitude of performance impairment found for 40- to 50- and 60- to 

70-year-old subjects revealed a pattern that was quite different from the 

pattern obtained in the studies mentioned above, virtually all of which used 

younger (18- to 29-year-old) subjects. Of the many possible explanations 

for the greater performance impairment of older subjects, it was concluded 

that some form of fatigue process related to age was one of the more 

plausible hypotheses. A fatigue concept seemed reasonable, since there were 

no performance differences between the age groups initially. Further, it 

appeared likely that this fatigue was related to the visual search 

requirement of the task. 

One of the obvious differences between young and old subjects that 

might contribute to the development of visual search fatigue involves the 

increased need for bifocal or trifocal visual correction with age. Most of 

the 40- to 50-year-old and all of the 60- to 70-year-old subjects used in 

our age study required such visual correction. At least one manufacturer of 

video display terminals (VDT's) has cautioned that the wearing of bifocal 

glasses during prolonged viewing of such displays may contribute 

significantly to fatigue because of the tilted head posture often required 

for clear vision (5). Moreover, because bifocal glasses are typically 

fitted to bring objects into focus at a reading distance of approximately 13 

inches, an individual wearing such glasses and viewing a visual display may 

be required not only to tilt the head back, but also to lean forward in 

order to bring the image on the screen into focus. This body posture could 

result in painful discomfort in the back as well as the neck (1). It is 

certainly conceivable that various visual and bodily strains associated with 

the wearing of bifocal glasses could adversely affect visual search 

patterns, and this may have contributed to the greater performance 

impairment of older subjects found in our earlier study. A primary purpose 

of the present study, then, was to investigate this possibility. 

1 
A second purpose of the study was to examine the relative benefit of 

rest periods on the performance of older subjects with and without bifocal 

correction and to investigate possible reasons for any differences and 

benefits that occurred. Vigilance studies and studies of industrial 

inspection have generally found that brief rest periods materially improve 

performance and frequently assume that this benefit occurs because rest 

periods serve to relieve task monotony (2,3). Articles and studies dealing 

more specifically with video display usage, however, have tended to place 

greater emphasis on the benefits of rest periods in reducing visual fatigue 

and visual discomfort (1,4,5,6). While it ,was expected that rest periods 

would generally improve performance, if older individuals monitoring a 

simulated radar display experience considerable visual fatigue, then 

subjects wearing bifocals might be expected to benefit less from rest 

periods than those without bifocals, if the wearing of such glasses 

contributes to additional forms of fatigue. 

In addition to studying performance, a number of selected subjective 

and physiological measures were included. These consisted of questionnaire 

items relating to visual discomfort, general fatigue, attentiveness, 

tension, boredom, and monotony, along with physiological measures of heart 

rate, heart rate variability, and gross body movement. Heart rate 

variability has shown a relationship to reported attentiveness and 

performance impairment in a number of our previous studies (8,10); heart 

rate was included to provide a general measure of arousal, and body movement 

was included to assess restlessness resulting from general physical 

discomfort. 

Method. 

Subjects. Forty paid volunteer subjects (17 males and 23 females), 

ranging in age from 40 to 50 years and selected from the general population, 

participated in the study. The 40- to 50-year-old range was chosen because 

it corresponded to the age range of one of the older groups in our previous 

age study, thus enabling comparisons, and because it was felt that we could 

reasonably expect to find subjects within this age range that either 

required or did not require bifocal correction. Educational backgrounds 

varied ·from several years of high school to some graduate study, with 90 

percent of the subjects having at least a high school education. The 40 

subjects were randomly assigned, in approximately equal male-female 

proportions, to four groups of equal size: (i) With Rest Periods - With 

Bifocal Correction (~~-WB), (ii) With Rest Periods- Without Bifocal 

Correction (WR-WOB), (iii) Without Rest Periods- With Bifocal Correction 

(WOR-WB), and (iv) Without Rest Periods- Without Bifocal Correction 

(WOR-WOB). Subjects were administered a vision test prior to the 

experiment, and it was determined that all had normal near-point visual 

acuity (corrected to 20/20 in the case of subjects wearing bifocals). 

Apparatus and Design. All task programing and recording of responses 

were accomplished using a Digital Equipment Corporation PDP-11/40 computer, 

interfaced with a 17-inch cathode-ray tube that served as the subject's 

2 
display. The stimuli (targets) consisted of small rectangular "blips" 

representing the locations of given aircraft. Adjacent to each target was 

an alphanumeric data block that identified the aircraft and gave its 

altitude and speed. A simulated radar sweepline 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. Sixteen targets were present 

at all times; as one left, another appeared on the screen. The critical 

stimulus or signal to which the subject 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 stimuli appeared in 

each 30-minute period; five occurred in the first 15 minutes and five in 

the second. The subject's response to a critical stimulus consisted of 

pressing 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 subject failed to detect a 

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

its previous value. All performance data were recorded by the computer for 

subsequent processing. 

A Beckman Dynograph was used in recor di ng heart rate, heart rate 

variability, and gross body movement. Heart rate was obtained from chest 

electrodes with leads connected to a cardiotachometer coupler. Gross body 

movement was obtained from a crystal finger-pulse transducer attached to the 

chair directly below the seat. This location gave an output from the 

transducer that was approximately proportional to the magnitude of body 

movement. The transducer·s output was amplified and integrated (using a 

pulse integrator) and both the analog and integrated outputs recorded on the 

Dynograph. Outputs from the cardiotachometer and pulse integrator also lead 

to the computer for subsequent analysis of heart rate, heart rate 

variability, and body movement. 

The computer and other recording apparatus were located in an adjacent 

room from which the subject was monitored via closed-circuit TV. Indirect 

lighting was used in the subject's room, and the level of illumination at 

the display was 21.5 meter-candles. This level approximates that used in 

operational air traffic control environments. 

Procedure. After chest electrodes were attached, the subject was 

seated in a straight-backed chair directly facing the visual display. A 

9-point subjective rating scale was then administered, dealing with present 

feelings of attentiveness, tension boredom, and fatigue. Following task 

instructions, all subjects were administered a 4-minute practice period 

containing six critical stimuli. Those subjects assigned to one of the rest 

conditions were told that 5-minute rest periods would be given at various 

times during the task session. During these periods, they were to shift 

3 
their eyes away from the screen, look around the room, roll the head around 

to relieve any neck tension, and generally relax. The 2-hour task session 

was interrupted after every 30 minutes of performance for those subjects 

assigned to one of the rest conditions. During the rest periods, the 

display program was turned off and a tape recording of quiet background 

music was played. 

After 2 hours of task performance, the subjects completed a second form 

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

that they were asked to rate the items plus one additional item dealing with 

task monotony, the basis of how they felt near the end of the task session. 

Subjects also completed a questionnaire that asked them to indicate which, 

if any, of the following they had experienced during task performance: 

smarting or burning eyes, itching eyes, eye strain, headache, neck fatigue, 

squinting, watering eyes, blurred images, and quivering images. 

Measurement of the Performance and Physiological Data. Times from each 

critical stimulus onset to the button press were computer processed and mean 

values obtained for successive 30-minute scoring periods. In addition, the 

program tabulated all critical stimuli not detected in each of these 

periods. 

The computer program described in a previous report (10) was used to 

obtain 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. 

Results. 

Performance Data. Figure 1 shows mean detection times across 30-minute 

periods for the four groups. Analysis of variance applied to these data 

revealed a signficant difference (.E. < .OS used throughout) between 30-minute 

periods (F(3/108) = 7.71) and a significant main effect for rest periods 

(I(1/36) ~ 4.38). The main effect for the bifocal comparison was not 

significant nor were any of the interactions. Although Figure 1 suggests 

that the bifocal group without rest was the group largely responsible for 

the significant main effect for rest periods, none of the comparisons (using 

the studentized range statistic) between this group and the other three 

groups at each of the four 30-minute measurement periods was significant. 

Because there were no significant differences in performance 

attributable to the wearing or not wearing of bifocal glasses, bifocal and 

no bifocal groups within each condition of rest were combined. Mean 

performance of rest and no-rest groups is shown in Figure 2. These data 

reveal a relative superiority in performance of the rest period group that 

occurred following the first rest break and that was maintained throughout 

the remainder of the .2-hour session. Newman-Keuls comparisons of the first 

with subsequent 30-minute periods in each group revealed no differences 

between the first period and any of the subsequent periods for the group 

4 
16 

14 

12 

~ 10 

z 

0 8 c.> 

~ 6 

4 

2 

----· ---l:l ·-- --·:--- ~ ---·------- .••••. -A-:;;•------- -...::.;..._____ A •••••••"" --0 

--....._ ·-·-u····• ---- --o---• • No Rest- With Bifocals 

.t.--.t. No Rest - No Bifocals 

0-·-0 With Rest- With Bifocals 

A------A With Rest- No Blfoca Is 

o~~-----------L------------~----------~ 2 3 4 

30- MINUTE PERIODS 

Figure 1. Mean target detection times for the four experimental 

groups. 

IJ) 

0 

z 

16 

14 

12 

0 8 c.> 

~ 6 

4 

Without rest 

.......... .....-------- ---..._ o----

With rest 

o---- _o 

--

2 

oLL------------~2------------=3----------~4 

30- MINUTE PERIODS 

Figure 2. Mean target detection times for rest and no-rest 

groups. 

5 
administered rest breaks. Although detection times of the group without 

rest breaks increased throughout the session, only the last 30-minute period 

differed significantly from the first. 

With regard to missed stimuli, neither the wearing of bifocals nor the 

inclusion of rest periods appeared to have any apparent effect on omission 

errors. The number of subjects missing one or more critical stimuli in 

groups WR-WB, WR-WOB, WOR-WB, and WOR-WOB were 5,6,7, and 7 respectively. 

None of the chi-square comparisons for these groups was significant. 

Physiolodcal Data. Analyses of variance applied to the physiological 

data revealed significant differences between 30-minute periods for heart 

rate (!(3/108) • 90.63), heart rate variability (!(3/108) • 12.47) and body 

movement (!(3/108) • 22.09). However, there were no other significant 

effects (main effects or interactions) for any of the three physiological 

variables. Because of this lack of significance, the data were comb:f.ned 

across groups. It is apparent in the combined data of Figure 3, that heart 

rate declined during the session, while heart rate variability and body 

movement increased. This general pattern was expected and has been found in 

several of our previous studies (8,9). 

1'1 

141 

1- 14 :I 

z 

i 1'2 

a: 10 141 

IL II 

C/1 

~ 5.1 

llju 

4.1 

0 

_,..,. 6,.,. ,-"' / locly movement ----0 """""'/ o-------

111"" , ... o.---... ------ -------- Heart raft variability 

1510 

1500 C/1 

1420 Ill 

...j 

1!40 ~ 

1210 a: 

1110 ~ 

1100 m 

1020 ~ z 140 -

~._----------~--------_.----------~~0 2 ! 4 

50- MINUTE PERIODS 

Figure 3. Heart rate, heart rate variability, and body movement combined 

across groups. 

6 
Subjective Data. Analyses of variance were also applied to the 

subjective ratings of attentiveness, tension, boredom, and fatigue. 

Significant differences between measurement periods were obtained for 

attentiveness (!(1/36) • 24.22), boredom (!(1/36) • 32.92), and fatigue 

(!(1 /36) • 23.11). The increase in tension was nonsignificant. No 

significant main effects for bifocal glasses or rest periods and no 

significant interactions were. obtained for any of the above variables. 

Separate ~ tests conducted on the monotony data collected only at the end of 

the experiment likewise revealed no evidence of significant differences 

between groups. Statements on the scales corresponding to the mean ratings 

at the completion of the task period suggested that the subjects were 

moderately interested and attentive during the experiment, but felt the task 

to be rather monotonous and tiring. Actual mean values obtained on the 

scales are not presented, since they would add nothing to the verbal 

descriptions just given. 

Symptoms relating to visual c.omplaints and to general discomfort were 

treated as follows: for visual symptoms (i.e., smarting or burning eyes, 

itching eyes, eye strain, squinting, watering eyes, blurred images, and 

quivering images), all items checked were summed for each subject and means 

computed for each of the four groups. Means for groups WR-WB, WR-WOB, 

WOR-WB, and WOR-WOB were 2.5, 1.8, 1.6, and 1.8. Separate~ tests revealed 

no significant differences between groups. Reports of headache were 

extremely uncommon, with one subject reporting a task-induced headache in 

each of groups WOR-WOB and WR-WOB and no reports of headache in the other 

two groups. Fifty percent of the subjects in three of the groups reported a 

slight degree of neck fatigue, while in one group (WR-WOB), 30 percent 

reported this symptom. It is evident that there were no differences between 

the groups in the report~d frequency of either headache or neck fatigue. 

Discussion. 

The present study found no evidence that the wearing of bifocal glasses 

impaired visual monitoring performance in any way. Nor did the wearing of 

such glasses contribute to reported visual discomfort, headache, neck muscle 

fatigue, or general fatigue. Although these findings might suggest that 

previously mentioned concerns over fatfgue and discomfort among bifocal 

users of VDT's are unwarranted, such a conclusion should be treated with 

caution. In most VDT applications, the operator is required to sit close to 

the screen because the attached keyboard requires it. Thus, individuals 

with bifocal visual correction must often assume a tilted head position to 

view the screen clearly, and this could result in various forms of fatigue 

or discomfort. An air traffic control specialist (ATCS), however, is not 

required to be in close proximity to the radar screen at all' times and 

frequently will sit several feet away from the screen or even observe it 

while standing. The present study was designed with this in mind, and 

subjects were allowed to view the screen from whatever distance they found 

to be most comfortable. Thus, the study sought to determine whether the 

wearing of bifocal glasses might in any way contribute to performance 

impairment or visual discomfort of subjects performing a visual monitoring 

7 
task similar to that of actual ATCS's. The results strongly suggest that 

the wearing of bifocal glasses during radar control work should neither 

degrade performance nor contribute to visual discomfort. 

With regard to the effect of rest periods, subjects not administered 

such breaks showed a progressive i.ncrease in target detection time that was 

quite similar to the performance pattern found for 40- to 50-year-old 

subjects in our previous age study (14). Those receiving rest breaks, 

however, showed no significant increase in target detection time across the 

2-hour session. The general performance pattern of subjects administered 

rest periods was almost identical in form and magnitude to the pattern 

obtained for younger (18- to 29-year-old) subjects in our age study, as well 

as in our previous studies in which no rest periods were incorporated and in 

which younger subjects have. been used (7,9,11,12,13). 

While the results suggest that brief rest periods significantly improve 

monitoring performance of older subjects, bringing performance to a level 

approximating that of younger individuals performing without rest breaks, 

efforts to account for why rest periods produced this benefit were 

unsuccessful. There was no indication that rest periods reduced visual 

discomfort, general restlessness, or reported fatigue, and no evidence that 

attentiveness or arousal level was increased as a function of rest. If, as 

hypothesized earlier, the greater performance impairment of older subjects 

is related to some fatigue process, the present study failed to provide 

information as to the nature of this fatigue. Perhaps the various 

subjective and physiological measures were inadequate in assessing fatigue 

related to the visual search process. Or perhaps rest periods benefited 

performance not because they reduced fatigue, but because they provided some 

form of change or relief from the monotony of an otherwise rather repetitive 

task situation, an explanation frequently advanced in the vigilance 

literature to account for rest period effects (2,3). If this were the case, 

however, one might have expected to see some difference between the rest and 

no-rest groups in their ratings of boredom and monotony. It appears evident 

that, while brief rest periods clearly improve monitoring performance, the 

reasons for this improvement are not easily determined. 

Conclusions. 

The present study found no evidence to suggest that the wearing of 

bifocal glas~es contributes to performance impairment or experienced 

fatigue/discomfort during simulated radar monitoring over a 2-hour period. 

Target detection times for 40- to 50-year-old subjects not receiving rest 

breaks increased significantly during the task session, while detection 

times for those receiving 5-minute breaks every 30 minutes showed no 

significant change. Although the results suggest that 40- to 50-year-old 

individuals administered brief rest periods perform at a level approximating 

that of younger (18- to 29-year-old) subjects without breaks, it is not the 

intent of this study to suggest that rest periods of this duration or 

frequency would necessarily facilitate the performance of older ATCS's in 

operational environments. The present study sought only to examine one 

8 
possible reason for the greater performance impairment of older individuals 

on a simulated radar mor ; t oring task and one way in which this impairment 

might be reduced. Recommendations for optimal work-rest periods of ATCS's, 

regardless of age, would require a much larger study performed under field 

conditions, taking into consideration the job constraints of ATCS's in their 

operational environment. 

9 
REFERENCES 

1. Bergman, T. 1980. Health effects of video display terminals. 

Occupational Health and Safety. 49: 24-55. 

2. Bevan, w., L. L. Avant, and H. G. Lankford. 1967. Influence 

3. 

of interpolated periods of activity and inactivity upon the vigilance 

decrement. Journal of Applied Psychology. 51: 352-356. 

Colquhoun, w. P. 1959. 

efficiency. Ergonomics. 

The effect of a short rest-pause on inspection 

2: 367-372. 

4. Mourant, R. R., R. Lakshmanan, and R. Chantadisai. 1981. Visual 

fatigue and cathode-ray tube display terminals. Human Factors. 

23: 529-540. 

5. Ostberg, o. 1975. CRT's pose health problems for operators. The 

International Journal of Occupational Health and Safety. 44: 24-46. 

6. Tabor, M. 1981. Video display terminals: The eyes have had it. 

Occupational Health and Safety. 50: 30-39. 

7. Thackray, R. I. 1982. Some effects of noise on monitoring performance 

and physiological response. Academic Psychology Bulletins. 4: 73-81. 

8. Thackray, R. I., J. P. Bailey, and R. M. Touchstone. 1977. 

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, 

~ Physiological Correlates. New York: Plenum Publishing Company. 

9. Thackray, R. I., J. P. Bailey, and R. t-f. Touchstone. 1979. The 

effect of increased monitoring load on vigilance performance using a 

simulated radar display. Ergonomics. 22: 529-539. 

10. Thackray, R. I., K. N. Jones, and R. t-1. Touchstone. 1974. Personality 

and physiological correlates of performance decrement on a monotonous 

task requiring sustained attention. British Journal of Psychology. 

65: 351-358. 

11. Thackray, R. I., R. M. Touchstone, and J. P. Bailey. 1978. Comparison 

of the vigilance performance of men and women using a simulated radar 

task. Aviation, Space, and Environmental Medicine.. 49: 1215-1218. 

12. Thackray, R. I., and R. M. Touchstone. An exploratory investigation 

of various assessment instruments as correlates of complex visual 

monitoring performance. FAA Office of Aviation Medicine Report 

No. AM-80-17. 

10 

• 
13. Thackray, R. I., and R. M. Touchstone. 1980. Visual search 

performance during simulated radar observation with and without a 

sweepline. Aviation, Space, and Environmental Medicine. 

51: 361-366. 

14. Thackray, R. I., and R. M. Touchstone. 1981. Age-related differences 

in complex monitoring performance. FAA Office of Aviation Medicine 

Report No. AM-81-12. 

15. Thackray, R. I., and R. M. Touchstone. Performance of air traffic 

control specialists (ATCS's) on a radar monitoring task: An 

exploratory study of complacency and a comparison of ATCS and non-ATCS 

performance. FAA Office of Aviation Medicine Report, 1982, in press. 

11