Table Of ContentVigilance :
the Problem of
Sustained Attention
DR CARL M. STROH
Road and Motor Vehicle Traffic Safety,
Canadian Ministry of Transport,
Ottawa, Canada
P E R G A M ON PRESS
OXFORD · NEW YORK · TORONTO
SYDNEY · BRAUNSCHWEIG
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Copyright © 1971 Carl M. Stroh
All Rights Reserved. No part of this publication may be reproduced,
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First edition 1971
Library of Congress Catalog Card No. 76-157656
Printed in Great Britain by A. Wheaton & Co., Exeter
08 016711 X
CHAPTER ONE
INTRODUCTION
MAN is basically a cognizant being: all that we are and all that we ever
shall be is determined by our knowledge of the world around us. Were it
not for our ability to think and to know, we would still be in the jungles
and forests of our lost beginnings.
Knowledge is derived from the analysis of physical properties of our
environment—how things look, feel, smell, sound and taste. The employ-
ment of physical characteristics in the assimilation of knowledge depends
upon our ability to discriminate among stimuli. The process of dis-
criminating among stimuli is what we call perception. Because of the vast
number of stimuli which are impinging on the sense organs at any given
time, it is impossible to attend to all the aspects of the many stimuli with
the same degree of awareness. Our perception must, therefore, be selective;
some stimuli must be given more attention than others.
We must conclude that there would be no knowledge, no civilization,
if man did not have the ability both to focus and to maintain attention.
Concerning the first of these abilities, the ability to focus attention, we
have found that those stimuli which will have the greatest likelihood of
being attended to are the ones which: (1) are the largest (in intensity, size
and time); (2) are novel; (3) involve movement; (4) fulfil a need; (5) are of
particular interest to the individual; or (6) conform to expectancy.
The second of these abilities, the ability to sustain attention, is what we
call vigilance. Vigilance, then, can be defined as the process of maintaining
attention. Although attention shifts constantly, the shifting is not random;
it is guided by certain definite conditions of the external and internal
environment. These conditions, and the extent of their influence, will be
discussed in the next chapter.
Interest in the area of vigilance research resulted, initially, from the
introduction of assembly-line methods into industry. Wyatt and Langdon
(1932) used a visual inspection task to study performance trends in work
1
2 VIGILANCE
which involved no accuracy feed-back to the subjects. They found that
performance deteriorated after about 40 minutes, then showed an irregular
recovery during the last 2 hours of a 4-hour session. Up to this point,
research had been rather sparse and for the most part more philosophic-
ally than research oriented. With the onset of World War II, and the devel-
opment of radar, the area of vigilance research emerged from seminal
pre-potence into a rather boisterous childhood. Ditchburn (1943), investi-
gating the performance of ships' look-outs, found that performance
deteriorated after only a few minutes. Then, following reports of missed
targets by radar operators, Anderson et al. (1944) employed a synthetic
radar display to study operator performance. As a result of their research,
they suggested that daily sessions last not longer than 40 minutes, although
occasional periods of up to 4 hours did not show marked deterioration.
Working at the same time, Lindsley (1944) employed eight subjects on a
radar-like task for seventeen consecutive days, but his results cannot be
applied to conventional vigilance research as he used five signals per
minute.
The previously cited researches, although of historical interest, did not
contribute much to the body of knowledge about vigilance behavior. They
did, however, create interest, the fertile soil from which later, more fruit-
ful, research sprang. What was probably the first major piece of research
in this area was carried out by H. N. Mackworth in 1948. The task of his
subjects was to detect a double-jump of a clock hand which jumped once
every second. For the ordinary single-jump, the tip of the pointer moved
0.3 inch, and for a double-jump the tip moved 0.6 inch, one hundred
single-jumps constituting a complete revolution of the clock pointer.
Mackworth concluded: (1) That efficiency at signal detection was lower
when subjects had been watching the display for about half an hour.
(2) "A half-hour rest following a half-hour watch was sufficient to allow
a further half-hour spell to be undertaken straight away with no detri-
mental effect on accuracy." (3) "A sudden short telephone message in
the middle of a 2-hour watch produced a temporary improvement
lasting only half an hour, but previous instructions that a particular
period of the watch would be likely to contain rather more signals than
usual had no demonstrable effect on performance."
Since 1948, a good deal of vigilance research has been rationalized on
the grounds that it is an area which is strictly applicable to radar-scanning,
various military observation tasks, the safe operation of all vehicles
INTRODUCTION 3
(automobiles, ships, aircraft, spacecraft, etc.), as well as many military
and industrial situations. As recently as 1963 it was claimed that 'Our
defence against all-out nuclear attack depends ultimately upon the vigi-
lance of the men observing the displays in our early-warning stations ..."
(Buckner and McGrath, 1963b). While this may have been true in the past,
radar-scanning may no longer be cited as the major raison d'être for vigi-
lance research. Modern radar systems employ automatic signal detection
devices which call the attention of the radar operators to any unexpected
or unfamiliar blip on the screen. The problem of modern radar observers
is no longer one of signal detection, but rather of signal interpretation.
This fact, plus the work of Brown et al. (1967), which showed that a
virtually continuous 12-hour period of driving during the normal working
day need not effect either motor or perceptual skills adversely, indicate
that it is likely that vigilance research is not so readily applicable to every-
day situations as some workers would have us (or rather their sponsoring
organizations) believe.
Kibler (1965) points out that "modern technology has been redirecting
the human monitoring function away from simple tasks (that show the
typical vigilance decrement) toward greater display and response com-
plexity." Kibler also delineates several task characteristics which appear
to be changing in contemporary monitoring tasks: (1) Weak, brief
duration signals are rarely encountered in applied monitoring tasks. (2)
Human monitors are typically required to keep watch over multiple
information sources, and frequently more than one type of target or
information class is the object of their vigil. (3) Usually, determining the
appropriate response to a signal event entails a decision process much
more complex than those required in laboratory vigilance studies. Buckner
and McGrath (1963a) have expressed the opposite view that "The increased
sophistication of electromechanical systems in modern society tends to
reduce man's role as an operator and to increase his role as a monitor."
This point could be argued convincingly for both sides, but it is con-
sidered by the present writer to be, at least partially, irrelevant, since the
major contribution of vigilance research has been towards the under-
standing and testing of related theories, rather than to any particular
military or industrial application. The performance of subjects in a
vigilance task is of particular interest to the proponents of various theories
of arousal and personality which make predictions of behavior under
monotonous conditions.
4 VIGILANCE
Whether or not there are strict military or industrial applications,
vigilance research continues to receive a considerable amount of attention
Although researchers have investigated a wide variety of vigilance tasks,
these five tasks have been the most frequently employed: (1) The "Mack-
worth clock" test; (2) detection of unusual flashes of light in a series of
uniform flashes; (3) monitoring a number of fluctuating dials in order to
detect an unusual fluctuation in one of them; (4) listening for a particular
sequence of digits in a series of digits; (5) detecting some faint auditory
signal.
Although a wide variety of techniques have been employed in the study
of vigilance performance, most researchers are in agreement as to the
essential characteristics of a vigilance task—a vigilance task is one that
requires subjects to respond in some way to the occurrence of relatively
infrequent and unpredictable (in time, space, or both) stimulus changes
over relatively long periods of time.
The innumerable techniques have produced an even more confusing
array of disharmonious and conflicting results. The effect of more than
twenty different variables on vigilance performance has been investigated,
but inexpugnable conclusions can be drawn for only one or two of these
variables. A few of these results, and a considerable number of the more
important variables, will be discussed in the next chapter.
CHAPTER TWO
FACTORS INFLUENCING VIGILANCE
PERFORMANCE
A. SIGNAL FREQUENCY
One area of vigilance research which has been investigated rather
extensively is the effect on vigilance performance of both signal and non-
signal events. The first of these variables, signal frequency, will be discussed
in this section, while the second variable will be dealt with in the following
section.
Researchers wanted to know if, by increasing the number of events to
be detected, they could improve vigilance performance. Up until 1962,
almost all of the research done in the area supported the hypothesis that
performance could be improved by increasing the number of signals to be
detected (Browne, 1949; Jenkins, 1953; Deese and Ormond, 1953; Deese,
1955; Bowen and Woodhead, 1956; Bowen, 1956; Nicely and Miller,
1957; Jenkins, 1958; Baker, 1958b, 1959a; Pollack and Knaff, 1958a;
Jerison, 1959b; Kappauf and Powe, 1959). The bulk of the more recent
research has also tended to support the position that increasing the
number of signals tends to improve performance (Howell et al, 1966;
McDonald and Burns, 1964; Bowen, 1964; Broadbent and Gregory, 1965;
Hawkes and Loeb, 1961, 1962; Pope, 1962; Johnston et al, 1966; Martz,
1965, 1966; Smith et al, 1966; Wiener, 1963; Corcoran, 1963; Loeb and
Binford, 1968), although some workers have reported the opposite
tendency (Baker et al, 1962), and several well-planned experiments have
revealed no significant effect of signal frequency on vigilance performance
(Adams and Boulter, 1960; Wilkinson, 1964; Stern, 1966; Taub and
Osborne, 1968; York, 1962; Eason et al, 1965; Mack worth and Taylor,
1963; Stroh, 1969).
Interpretation of many of the earlier findings is difficult in view of the
fact that many of them employed an extreme range of signal frequencies,
and presented their results in terms of per cent detections. Jenkins (1958),
5
6 VIGILANCE
for example, had his subjects detect movements of greater than standard
amplitude on a voltmeter. Signals occurred at from 7.5 to 480 times per
hour against a background event rate of one deflection per second.
Jenkins's conclusion that the extent of detection decline increases with
lower signal rates is somewhat tenuous in view of the fact that the only
signal rate that was significantly different from the others was 480 signals
per hour. Using per cent detections, as Jenkins does, it can be seen that
subjects must make a great many misses under the 480 signals per hour
condition before their apparent performance will fall to the level of the
next group (60 signals per hour). A similar criticism could be levelled
against the work of Deese and Ormond (1953), who found that 46, 64,
83 and 88 per cent of the signals (10, 20, 30 and 40 per hour respectively)
presented on a simulated radar display were detected by subjects during a
3-hour watch. Once again, the use of per cent is perhaps misleading. If
we were to assume that 5.6 signals per hour were missed (regardless of the
total number of signals presented), then the resultant percentages would
be 44, 72, 81 and 86. These figures are not much différent from the ones
obtained by Deese and Ormond. Therefore, the actual number of errors
made by subjects in this experiment was nearly the same under each
condition. Increasing the signal frequency in many of the earlier experi-
ments did not prevent a vigilance decrement, it merely made it more
obscure.
Improved performance following an increase in the signal presentation
rate has been observed under a wide variety of conditions, and over a
broad signal frequency range. Bowen's (1964) subjects were required to
detect a light spot which appeared at the same location of a noisy simu-
lated radar display at the rate of 1,10 or 20 times per hour. He used, as his
dependent variable, the number of signal repetitions required before a
response was given, and found that there was no general loss of vigilance
with time. He did, however, note that the over-all performance of subjects
under the 20 signals per hour condition was superior to that of subjects
under the 10 signals per hour condition. A suggested reason for this lack
of vigilance decrement is the extensive (one hour) practice period which
preceded the vigilance session. Many of the vigilance experiments which
have reported a performance decrement with time have given subjects
sparse and inappropriate training. Broadbent and Gregory (1965) used a
series of regular intensity light flashes (100 ft lamberts), which included
occasional flashes of greater intensity (150 ft lamberts). They used signal
FACTORS INFLUENCING VIGILANCE PERFORMANCE 7
rates of 80 and 240 per 70-minute period, and supported the findings of
Bowen (1964). This result, however, is difficult to interpret in view of the
questionably high signal rate of 240 per 70-minute period, which might not
be generally accepted as falling within the confines of a vigilance task.
Martz (1966) presented subjects with trains of twelve successive tones
ranging from 14 db below to 10 db above the average subject's threshold,
in 2 db steps. The subject's task was to press a microswitch when he
detected the tones. Martz concluded that there was a positively accelerat-
ing linear relationship between auditory detection and log signal rate
(signal rates of from 1 to 15 per hour). Smith et al. (1966) used response
time as their dependent variable, and also obtained a linear function of
logarithmic increases in signal density. Howell et al. (1966) presented their
subjects with deletions or additions of alpha-numeric symbols appearing
in sixty-four spatial locations contained within an 8 χ 8 matrix. The task
was a complex one, subjects being required to (1) detect the signals, (2)
give the location of the signals, and (3) identify the signals. With displays
of 4, 8, 16 and 32 stimuli, and signal frequencies of 30 and 75 per hour,
they found that the poorest performance was with low signal frequency
and high stimulus density displays.
The only notable study to find a deleterious effect of increased signal
frequency on vigilance performance is that of Baker et al. (1962). Six
groups of twenty subjects monitored brief interruptions of a continuously
presented sound or light source for a 3-hour period. A greater percentage
of signals was detected with a signal rate of 12 per hour than with a signal
rate of 24 per hour.
There are, however, several experiments which have found no signi-
ficant effect of signal frequency on vigilance performance. The first of
these studies had subjects detect double-flashes against a background
single-flash rate of 1 per second (York, 1962). Signals were presented at
rates of 3, 6 and 90 per 15-minute period. Although York found no
significant effect of signal frequency on performance, his results cannot be
given much weight since both the large number of signals under the 90
signals per 15 minutes condition and the relatively short task (15 minutes)
are not within the bounds of what is usually considered a vigilance task.
Eason et al. (1965) found no significant effect of signal frequency on either
performance or physiological variables such as heart-rate and skin
conductance. The background stimulus was a light which flashed on for
0.5 second every 3 seconds, and the signal to be detected was a flash of
8 VIGILANCE
longer duration (0.8 second). Signal rates of 2 per minute and 1 per 2
minutes were used during a 1-hour session. By virtue of a sounder experi-
mental design, we may assign more weight to these results than to those of
York. Nevertheless, it should be remembered that we are still dealing with
a rather small segment of the "signal-per-hour" distribution, both 120
and 30 signals per hour being rather a lot. Stern (1966) also used rather
high signal frequencies of 120 and 60 signals per hour, and concluded that
signal frequencies had no effect on detection performance. He did find,
however, that the 60 signals per hour group made a significantly greater
number of false alarms than the 120 signals per hour group. What is
perhaps the most convincing contradiction of the general finding that
performance is improved by an increase in signal frequency, arises from
the well designed experiments of Taub and Osborne (1968). Using a
Mack worth clock test, and signal frequencies of 27, 54, 108 and 216
signals per 54-minute session, they found that signal frequency had no
effect on performance.
Additional support is provided by the more recent work of Stroh
(1969). He had seventy-two subjects take part in a 1-hour visual vigilance
task (flashing light) which involved the detection of either 6, 18 or 60
signals against a background stimulus rate of 360, 1200 or 3600 per hour.
Correlational analysis, as well as analysis of variance techniques, revealed
no significant effects of signal frequency on either d' (visual sensitivity) or
β (cautiousness).
Conclusion
At first glance, most of the research would seem to support the position
that vigilance performance can be improved by increasing the number of
signals to be detected. It has been shown that much of the research upon
which this conclusion is based is difficult to interpret because of the
misleading use of "per cent detections" as performance criterion. Further-
more, it is apparent that many of the studies which have lent support to
this position have employed excessively high signal frequencies. When
signal frequency is raised beyond a reasonable level (60-90 per hour),
performance might be improved, but then it is doubtful that we are still
dealing with a true vigilance situation (see Fig. 1). Obviously, when we are
dealing with a continuous (or nearly continuous) task, attention will be
maintained much better than in a task which involves comparatively long
periods of inactivity.
