1. Report No. 2. Government Accession No . 

FAA-AM-81-12 

4. Title and Subtitle 

AGE-RELATED DIFFERENCES IN COMPLEX MONITORING 

PERFORMANCE 

Technical ~eport Documentation Page 

3. Recipient's Catalog No. 

5 . Report Date 

APRIL 1981 

6. Performing Organization Code 

1--::---:--:---,--,;------------------------------j 8 . Performing Organi zati an Report No . 

7. Author' s) 

RICHARD I. THACKRAY AND R. MARK TOUCHSTONE 

9. Performing Organization Name and Address 

FAA Civil Aeromedical Institute 

P.O. Box 25082 

Oklahoma City, Oklahoma 73125 

10. Work Unit No. (TRAIS) 

11. Contract or Gront No . 

13. Type of Report and Period Covered 

~~------------------~~--------------------------------~ 12. Sponsoring Agency Name and Address 

Office of Aviation Medicine 

Federal Aviation Administration 

800 Independence Avenue, SW. 

Washington, D.C. 20591 

15 . Supplementary Notes 

14. Sponsoring Agency Code 

Work was performed under task AM-C-81-PSY-84. 

16. Abstract 

The present study examined the effect of age on the ability to sustain attention to 

a complex monitoring task. The visual display was designed to resemble an air 

traffic control radar display containing alphanumeric symbols. Subjects in age 

groups 18-29, 40-50, and 60-70 years were tested over a 2-hour session. Sixteen 

s~nbols appeared on the screen at all times, with 10 critical stimuli (a designated 

change in the alphanumerics) occurring during each half-hour. Performance decrement 

was significantly related to age, with performance declining earlier in the session 

in the oldest group of subjects. Recordings of eye movement activity, skin 

conductance level, and subjective assessments of fatigue, boredom, monotony, and 

attentiveness generally failed to reveal any clear reasons for the greater performance decrement of older subjects. Possible directions for further research are 

discussed. 

17. Key Words 

Age 

Air Traffic Control 

Arousal 

Attention 

19. Security Classif. (of this report) 

Unclassified 

Form DOT F 1700.7 (8-72) 

18. Distribution Statement 

Fatigue 

Performance 

Vigilance 

Visual Search 

Document is available to the public 

through the National Technical Information 

Service, Springfield, Virginia 22161 

20. Security Clossif. (of this page) 21. No. of Pages 22. Price 

Unclassified 13 

Reproduction of completed page authorized 
AGE-RELATED DIFFERENCES IN COMPLEX MONITORING PERFORMANCE 

Introduction. 

Effective monitoring of complex visual displays, such as the cathode-ray 

tube (CRT) displays used in air traffic control radar systems, requires the 

ability to detect relevant stimulus changes rapidly, usually in the presence 

of many irrelevant and/ or competing stimuli (selective attention), and the 

ability to maintain a high level of attention to the visual display over 

prolonged periods of time (sustained attention). It has recently been 

argued that these two forms of attention, i.e., selective and sustained, are 

not only quite different conceptually, but may reflect brain processes that 

are neurologically distinct (8). 

Evidence is accumulating that selective attention, as it relates to the 

ability to detect task-relevant visual information in the presence of taskirrelevant information, is significantly impaired as a function of age. In 

a study by Rabbitt (14), for example, subjects (Ss) in their 20's or 70's 

were required to sort cards containing either an A or a B into two separate 

stacks. Four levels of difficulty were employed in which different packs of 

cards contained 0, 1, 4, or 8 additional irrelevant letters. Sorting time 

was found to increase with age as the number of irrelevant stimuli increased. 

In addition, a significant age by packs interaction indicated that sorting 

times for older Ss increased more sharply than did times for younger Ss as 

the number of irrelevant stimuli increased. 

Farkas and Hoyer (4) have recently examined the age-related effects of a 

different variable, perceptual grouping, on selective attention. Ss in three 

groups with mean ages of 22.13, 46.75, and 69.38 years were required to sort 

decks of cards containing either no irrelevant information, contrasting 

irrelevant information, or similar irrelevant information. Each card contained either an upright or inverted letter T (target item) located in one 

of the four squares of an imaginary 2 x 2 matrix. The other three squares 

were either empty (no irrelevant information), contained the letter I's that 

were all in the same vertical plane as the T (similar irrelevant information) 

or contained I's that were displaced 90° relative to the T (contrasting 

irrelevant information). Ss sorted cards from each of the three types of 

packs into separate stacks according to the orientation of the target letter. 

Sorting time was greater for all age groups in the presence of similar than 

in the presence of contrasting, irrelevant information. In general, however, 

the oldest group was impaired the most by both types of irrelevant information. 

The above studies of selective attention clearly reveal age-related 

impairments in the ability to detect well-defined task-relevant information 

in the presence of task-irrelevant information. Evidence supporting an agerelated change in sustained attention (vigilance), however, is far more 

equivocal. In one of the earliest studies of age and vigilance performance, 

York (20) failed to find any differences between groups with mean ages of 30, 

50, and 70 years in their performance of a simple visual vigilance task 

l 
involving the detection of infrequent double flashes. Likewise, Griew and 

Davis (7) found no differences between groups with ages of 19 to 31 and 45 to 

60 years in the number of correct detections using an auditory vigilance task 

requiring the detection of occasional sequences of three consecutive odd 

digits. A number of subsequent studies using essentially the same auditory 

task employed by Griew and Davis and employing comparable age groups also 

failed to find any differences in correct detections as a function of age 

(2,6,12,18,19). 

On the basis of the above findings, one might reasonably conclude that 

sustained attention, as measured in conventional vigilance experiments, is 

not influenced by the aging process. Yet in 1964, Surwillo and Quilter (16) 

reported a vigilance study in which clear age-related differences were 

obtained. Ss ranging in age from 22 to 82 years monitored a Mackworth clock 

(11) for a period of 1 hour. This task requires Ss to detect occasional 

double jumps of a pointer that normally rotates in discrete, once-per-second 

jumps about a plain white clock face. Older (> 60 years) Ss detected significantly fewer signals than did the younger (< 60 years) Ss in this study. 

More interesting, however, was the fact that differences in vigilance performance were not present at the beginning of the task. The significant 

difference between age groups was the result of a much greater performance 

decrement among the older Ss toward the end of the session. A subsequent 

replication of this study confirmed these findings (15). 

A possible explanation for the age-related differences reported by 

Surwillo (15) and Surwillo and Quilter (16) involves the vigilance task 

employed. Davis and Tune (3) have hypothesized that effective performance on 

the Mackworth clock requires continuous search, and the aaditional load 

imposed by this requirement may have affected older ~ more adversely than 

younger ones. If the greater performance decrement obtained for older Ss in 

the Surwillo studies resulted from this rather minimal search requirement, 

then vigilance tasks involving much greater visual search should show even 

more pronounced age differences in sustained attention. 

The present study was conducted to investigate this possibility. The 

task employed resembled a contemporary air traffic control radar display and 

contained a constant number of alphanumeric data blocks. Ss from different 

age groups monitored the display over a 2-hour period for occasional "critical 

stimuli" consisting of designated changes in the alphanumerics. On the basis 

of the studies of selective attention reviewed earlier, it was expected that 

overall (mean) detection times on this task would be greater with advancing 

age. However, the results obtained by Surwillo would suggest that agerelated differences in target detection time may occur only after some time 

period of task performance, with the onset of performance impairment occurring 

earlier among older Ss than among younger ones. 

Besides studying performance effects, certain physiological and subjec t i ve 

measures were included in the present study. Skin conductance level was 

recorded to provide estimates of differences in arousal level, and electro -

occulographic (EOG) recordings of horizontal eye movement activity were 

2 
obtained to assess scanning activity. A third measure, average dominant 

electroencephalographic (EEG) frequency, was recorded but not analyzed 

because of instrumentation problems. The subjective measures included selfreported feelings of fatigue, attentiveness, annoyance, boredom, and 

monotony. It was felt that measures of subjective fatigue and boredom were 

particularly important to include, since it has been suggested that the agerelated performance decrement reported by Surwillo (15) and Surwillo and 

Quilter (16) might have been due to greater task boredom and fatigue experienced by older Ss (5). 

Method. 

Subjects. Forty-five paid volunteer Ss (19 males and 26 females) 

participated in the experiment. Three age groups were represented (18-29, 

40-50, and 60-70 years), with 15 Ss in each group. Mean ages within the 

groups were 21.7, 45.7, and 63.1 years respectively. Educational backgrounds 

varied from several years of high school to some graduate study. The most 

common "occupations" were college student, housewife, clerical-sales, and 

retired military/civil service. All Ss were in good health as determined 

from an interview that included questions concerning any regular usage of 

prescribed drugs or medication. In addition, all had normal visual acuity 

(corrected to 20/20 if necessary). 

Design and Task Apparatus. All task programing and recording of responses were accomplished using a Digital Equipment Corporation PDP-ll/40 

computer interfaced with a 17-inch (43 em) CRT. The CRT was located in a 

console resembling an air traffic control radar unit. The stimuli (targets) 

consisted of small rectangular "blips" representing the locations of given 

aircraft. Adjacent to each target was an alphanumeric data block. Data 

blocks comprised two rows of symbols: the top row, consisting of two letters 

and three numerals, identified the aircraft, while the bottom row of six 

numerals indicated its altitude and speed. The first three of these numerals 

gave altitude in hundreds of feet and the last three gave groundspeed. 

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 S was instructed to respond consisted of a change in a target's 

displayed altitude to a value greater than 550 or less than 150. The values 

of the increases or decreases in altitude were randomly determined, except 

that the changed altitude value could not be greater than 599 or less than 

100. Ten such critical stimuli appeared in each 30-minute period; five 

occurred in the first 15 minutes and five in the second. The S's response 

to a critical stimulus consisted of 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 

3 
the ~ 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. 

Physiological Recordings and Instrumentation. Beckman miniature biopotential electrodes were attached directly above and below the right eye and 

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

pairs of electrodes were connected to two separate channels of a Beckman 

Dynograph and recorded with a 3.0-second time constant. These channels 

served as the two primary EOG channels and, because of the relatively long 

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

extract only the faster saccadic movements for computer processing, the output of the primary horizontal channel was recorded on a third channel by 

differentiating the EOG with a time constant of 0.03 seconds. (Only horizontal movements were computer-processed because of eyeblink artifacts in the 

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

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

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

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

Dynograph. 

Beckman biopotential electrodes filled with a saline paste (10) and 

attached to the volar surfaces of the index and middle fingers of the S's 

left hand were used for measuring conductance level. Leads from these 

electrodes led to a Beckman Type 9844 coupler that recorded conductance 

directly. 

The computer and other recording apparatus were located in an adjacent 

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

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

21.5 meter-candles. This level approximates that used in op erational air 

traffic control environments. 

Eye Movement Calibration. The gain of the primary hori zontal channel o n 

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

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

controls of the Schmidt triggers were then adjusted to just fire at the p eak 

of each positive a nd negative excursion of the calibration signal. Fo l l owing 

this, each S's horizontal as well as v e rtical e y e movements were c alibrate d 

using a n optical table with chi nrest supp ort. Ss were instructed to fix a te 

points at 90° and 2 70° on the circumference of a-22-cm c ircle which subtended 

a visual angle of 20° . A similar p rocedure was fol lowed for v ertical e ye 

movements, except tha t p oints at 180° and 360° were u sed . The gain controls 

of the primary hori zontal and vertical channels were adjusted to y i e l d peakto-peak deflections of 2 em as the e y es were def l ect e d t o t h e extremes of t h e 

circle. Thus 1 rom of p e n defl ection equa led 1° o f e y e movement. Any horizonta l saccadic moveme nt equa l t o or g rea ter t h an thi s v a lue caused an out put 

from one of the Schmidt triggers. 

4 
Procedure. On arrival the ~ was taken to the experimental room, 

orientation instructions were given, the ~was instrumented for physiological 

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

rating scale was administered dealing with present feelings of attentiveness, 

fatigue, annoyance, and boredom. 

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

The circular display area of the screen subtended a visual angle of approximately 20° at the S's viewing distance. The minimum separation of alphanumeric targets at this distance was approximately 2.4°. Although a rigidly 

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

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

task session. Instead, each ~ was instructed to sit straight in the chair 

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

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

movements indistinguishable from true eye movements, it was expected that 

error resulting from head movements would be randomly distributed across 

conditions and within Ss. Periodic observations revealed that virtually all 

Ss complied with instructions to keep gross head movements to a minimum. 

The task instructions emphasized the necessity of pressing the button 

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

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

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

values of the targets. It was explained that occasional large changes in 

altitude would not normally occur in an actual radar system, but that this 

departure from normal conditions was necessary to insure that all targets 

would be given equal priority in scanning. Following the taped instructions, 

the~ was given a 4-minute practice period containing six critical stimuli. 

After the 2-hour task session, 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 were obtained on each 

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

critical stimulus onset to the button press): 

(i) 

(ii) 

(iii) 

Mean response latencies to critical stimuli correctly 

identified. 

Number of button presses without a critical stimulus. 

Number of critical stimuli missed. 

5 
For eye movements, the computer identified each correct response (button 

press) and then determined mean fixation duration from the inter-saccadic 

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

response. (Mean fixation duration can also be considered an index of fixation frequency. Consequently, although the data were analyzed only in terms 

of mean fixation durations, subsequent discussions may refer to mean fixation duration and frequency of fixations interchangeably.) If a critical 

stimulus was missed, the 30-second interval prior to the time the stimulus 

timed out was analyzed. Average values derived from the above 30-second 

intervals were obtained for each 30-minute period. To eliminate various 

forms of electronic and/or physiological noise from the data, all apparent 

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

Conductance levels were measured directly from the Dynograph recordings 

at half-hour intervals. 

Results. 

Performance Data. Figure l shows mean detection latencies across 30-

minute periods for the three age groups. Analy sis of variance ap plied to 

these data revealed significant (£ < .05 throughout) main effects for age 

groups, F(2/42) = 6.05, for 30-minute periods, F(3/l26) = 12.01, and for the 

age by periods interaction, F(6/l26) = 3.06. To clarify the nature of the 

significant interaction effect, further comparisons were made. Newman-Keuls 

16 

14 

(/) 

~ 12 

0 

u 

w 

(/) 

10 

8 

0 

Age Group 18-29 

2 3 

30- MINUTE PERIODS 

Figure l. Mean target detection times for the three 

age groups. 

4 

tests revealed that there were no significant differences between t he three 

age groups in mean d e tection latency dur ing the fir s t 3 0-minute p e r iod . 

Comparisons of the firs t with subsequent periods revealed a significant 

6 
increase in target detection time during the second and fourth 30-minute 

periods for the oldest group of Ss. For the middle-aged and youngest 

groups, there was no significant performance impairment until the last 

30-minute period. Although Figure 1 shows an apparent improvement in the 

performance of the oldest group during the third measurement period, this 

decrease in detection time was not significantly different from the times 

for the two adjacent periods. 

TABLE 1. Mean Number of Omission and Commission Errors 

for Each Age Group 

Age Errors 

Groups Omission Commission 

18-29 0.27 0.53 

40-50 1.07 l. 33 

60-70 2.47 3.13 

Because of relatively low frequency of occurrence of failures to 

respond to critical stimuli (errors of omission) and responses to noncritical 

stimuli (errors of commission), errors in each of these two categories were 

summed for each ~ across all four 30-minute periods. Table 1 shows an 

increase in both types of error with age. Analyses of variance revealed 

that these differences between age groups were significant for both errors 

of omission, F(2/42) = 6.12 and errors of commission, F(2/ 42) = 5.79. 

Physiological Data. Mean eye fixation durations for the three age 

groups are shown in Figure 2. The data reveal similar patterns of change 

540 

520 

(/) 

0 500 

z 

0 

u 

w 480 

(/) 

...J 

...J i 460 

440 

0 

Ac;~e Group 18- 29 

2 3 4 

30-MINUTE PERIODS 

Figure 2 . Mean visua l fixation durations for the three age groups. 

7 
in all groups across the two-hour session and suggest differences between 

groups in fixation duration. Analysis of variance revealed a significant 

main effect for 30-minute periods (F(3/l26) = 7.50). However, neither the 

difference between age groups (F(2/42) = 2.25) nor the age by periods interaction (F(6/126) = 1.24) was significant. 

Mean conductance levels are shown in Table 2. Because of instrumentation problems, usable data were not available for all S's in two of the 

groups (this is indicated in Table 2). Consequently, an analysis of variance 

using an unweighted means solution was employed. Both the main effect for 

TABLE 2. Mean Conductance Level (in Micromhos) at the End of 

Successive 30-Minute Periods for Each Age Group 

Age Periods 

Groups 

18-29 

(N=8) 

40-50 

(N=lS) 

60-70 

(N=l3) 

1 

9.65 

6.46 

4.69 

2 

8.76 

5.89 

3.98 

3 4 

8.80 7.70 

5.56 5.29 

3.93 3.52 

age, F(2/33) = 9.64 and the main effect for 30-minute periods, F(3/99) = 23.41 

were significant. There was no significant interaction. As is evident from 

the values displayed in Table 2, conductance level is inversely related to 

age and shows a general decline during the task session. 

Subjective Data. Table 3 shows the mean values obtained for each of the 

variables contained in the rating scale administered prior to and following 

task performance. Analyses of variance revealed no significant differences 

between the age groups on any variable at the beginning o£ the task session. 

At the end of the session, the age groups differed in their ratings on only 

two scales. The youngest group felt itself to be less attentive than did 

either of the two older groups, while the oldest group remained more 

interested in the task than did either the youngest or the middle-aged groups. 

All groups were equally tired at the end of the session, and all felt the 

task to be "moderately monotonous." 

Discussion. 

The requirement to detect critical altitude changes in the presence of 

many similar, competing stimuli resulted in more errors of omission and cornmission and longer average detection times among older Ss than among younger 

ones. Moreover, it was found that age differences in detection time did not 

8 
\.0 

TABLE 3. Mean Values Obtained for Each Rating Scale Variable for the Three Age Groups 

Variable 

Fatigue 

Annoyance 

Attentiveness 

Boredom 

Monotony 

Age Group 

18-29 

40-50 

60-70 

18-29 

40- 50 

60-70 

18-29 

40-50 

60-70 

18-29 

40-50 

60-70 

18-29 

40-50 

6Q-70 

Mean Values with Corresponding 

Rating Scale Descriptors 

Pre Task 

4.80 (Usual level) 4.27 4.33 

" 

1.33 (Not annoyed) 

1.07 

1. 33 " " 

7.07 (Quite attentive) 

6.67 " " 

7 . 40 " 

2.07 (Extremely interested) 

1.60 

2.00 " " 

Post Task 

7.13 (More tired than usua

6.93 H It II II 

6.33 II " 

2.40 (Not annoyed) 

1.73 1.67 " 

" 

3. 67 (Inattentive) 

5.53 (Attentive) 

5.80 (Attentive) 5.87 (Indifferent) 5.27 " 

" " 

3.07 (Moderately intereste

6.33 (Moderately monotonou

5.93 

4.47 

" 

" " 

a The F values shown are for those post-task comparisons in which significant differences wer

age groups. There were no significant differences between groups on any pre-task measure. 

were based on 2/42 df. 
exist initially; they became apparent only after some period of task performance, with the oldest group showing evidence of performance impairment much 

earlier in the session than was the case with either of the two younger 

groups. This latter finding is in general agreement with the results reported 

by Surwillo (15) and Surwillo and Quilter (16) in their vigilance studies 

comparing the performance of Ss above and below 60 years of age. These resul~ 

appear to suggest that age-related differences in selective attention during 

prolonged monitoring are time dependent, i.e., they become manifest or more 

pronounced only after the passage of some period of time. In the present 

study, both the time of occurrence and the magnitude of performance impairment were related to age. 

Although most studies of selective attention have not been specifically 

interested in time effects (sustained attention), Farkas and Hoyer (4) did 

report a significant age by trials interaction with results that resembled 

those obtained in this study. Mean card-sorting time from the first to the 

fourth block of trials generally increased for their elderly group of Ss, 

while sorting times for the middle-aged and young groups remained the same 

or decreased. 

In attempting to account for their significant age by trials interaction, 

Farkas and Hoyer speculated that fatigue and/ or boredom must have been greater 

among the elderly Ss. The same explanation was offered by Fozard et al . (5) 

to account for the age-related decrement found by Surwillo and Quilter (16). 

In the present study, however, subjective reports following task performance 

revealed no evidence to indicate that the oldest Ss felt the task to be more 

monotonous, boring, or tiring than did the two younger groups. On the contrary, the oldest group felt less bored at the end of the session than did 

either the middle-aged or youngest group, and all groups felt equally tired. 

Decreased visual search activity is another possible explanation for the 

earlier as well as greater performance decrement exhibited by the older Ss. 

Yet no significant differences were found between age groups in mean frequency 

of eye fixations or in change in frequency over time. While the electroocculographic method employed yielded only a gross estimate of total visual 

scanning activity, its use in a previous study (17) revealed it to be quite 

reliable and to correlate highly with hand-scored measures of eye movement 

activity. Unfortunately, frequency of visual fixations per se provides no 

information with respect to the type or adequacy of the search patterns 

employed. Further research using more adequate techniques for assessing 

scanning activity would be required before it could be determined with 

certainity that the age-related performance differences found in this study 

were independent of scanning patterns. 

Skin conductance level differed significantly among the age groups, but 

the decline in conductance over time was unrelated to age. These same findings were also reported by Surwillo (15). If it could be assumed that a 

lower conductance level with increasing age implied a lower arousal level, 

this might account for the obtained differences in performance. However, 

this assumption has recently been questioned by Catania, Thompson, Michalewski, 

10 
and Bowman (1) who report that low conductance levels in aged ~may be the 

result of a low density of active eccrine sweat glands in this age group and 

not necessarily an indication of lower autonomic arousal. If this is the 

case, then the negative correlation of conductance level with age found in 

the present study may be more of a manifestation of peripheral factors than 

a reflection of differences in arousal or activation level. 

In spite of the fact that reported fatigue (tiredness) was no greater 

among the oldest ~than among younger ones, the most plausible explanation 

for the age-related decline in performance would still seem to involve some 

form of fatigue process. Simple habituation concepts do not appear adequate, 

since numerous vigilance studies using auditory tasks, or visual tasks lacking 

a scanning requirement, have failed to show any age-related change in performance (3). Clearly, fatigue must be tied to the visual search requirement. 

Although it is well known that visual accommodation declines with age, 

the possibility that this factor alone could account for age-related changes 

in selective attention has been largely ignored (9) . Yet this factor must be 

taken seriously, since the decline in accommodation with age results in an 

increased need for visual correction, especially bifocal correction. At 

least one manufacturer of CRT displays has cautioned that the wearing of 

bifocal glasses during prolonged viewing of these displays may contribute 

significantly to fatigue because of the tilted head posture often required 

for clear vision (13). This form of fatigue may not have been adequately 

assessed by the subjective scales employed in the present study , and it is 

certainly conceivable that such fatigue could have adversely affected the 

search patterns of older Ss. 

Research in progress will evaluate the extent to which the wearing of 

bifocal corrective lenses may contribute to the age-related decline in 

performance found in this study. Peripheral £actors such as this should be 

carefully examined before more esoteric central processes are invoked to 

explain age-related changes in sustained attention during complex monitoring 

performance. 

11 
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11 II 

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12 
14. Rabbitt, P.: An Age-Decrement in the Ability to Ignore Irrelevant 

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