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VIGILANCE AND SUSTAINED ATTENTION
Instructor: P.A. Hancock
Introduction
Vigilance, or sustained attention, refers to the ability to monitor displays for stimulus events over
prolonged periods of time. The term ‘vigilance’ as applied to human behavior was coined by Sir Henry
Head (1923), who referred to it as a state of maximum physiological and psychological readiness to react.
However, the origin of modern vigilance research, as in many other areas of human factors, was in the
Second World War. The British Royal Air Force were employing a new technology, RADAR, in their
aircraft to scan for German U-boats sinking allied shipping and impairing the Anglo-American war effort.
These were highly motivated and skilled operators, and yet they were missing targets. Norman H.
Mackworth was assigned the task of determining why motivated and skilled airmen were missing signals
(although see Ditchburn, 1943). He devised the ‘Clock Test’ which consisted of a blank-faced clock and a
black pointer that would make movements of .3 inches per second. Observers were to monitor this display
for the occurrence of ‘double jumps’ of .6 inches. The length of the session of the original experiment was
2 hours. The figure in the second PowerPoint slide shows the results he observed (Mackworth, 1948).
Observers’ ability to detect signals declined over the course of the vigil, with the steepest decline early in
the watch. This progressive decline in the quality of performance over the course of a watch keeping
session is one of the most ubiquitous effects in vigilance (See, Howe, Warm, & Dember, 1995), and is
referred to as the ‘decrement function’ or the ‘vigilance decrement’ (Davies & Parasuraman, 1982). The
decrement has been observed in both laboratory and field settings, across a variety of domains. These
include RADAR, SONAR, cytological screening, anesthesia gauge monitoring, nuclear power plant
operation, industrial quality control, baggage screening, and detection of criminal or insurgent activity
(e.g., friend/foe identification during long shifts). Vigilance is also a component of tasks requiring sustained
performance, such as shift work and driving.
Theories of Vigilance
Inhibition Theory. Since Mackworth’s (1948) original findings there have been several theories of
vigilance, and the progression of theoretical change has mirrored general trends in psychology. Thus, the
earliest explanations for vigilance relied on behaviorist theories, particularly the Hullian (Hull, 1943) theory
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dominant at that time. The central construct for this view was inhibition. Inhibition was conceived as a
force opposing the tendency of organisms to expend energy through behavior. Inhibition can be overcome
by either stopping the behavior for some period of time (e.g., a rest break), allowing the inhibition to
dissipate, or by providing reinforcements for the behavior. The process of inhibition was therefore similar to
that observed in extinction in conditioning experiments. According to this perspective, vigilance declines
because of a progressive rise in response inhibition. Data were reported that were consistent with this view,
that knowledge of results (‘reinforcement’) increases detection responses, and that performance improves
after brief rest periods, presumably a reflection of dissipation of inhibition. In addition, introducing a novel
stimulus event can re-establish vigilance levels to levels obtained at the beginning of the watch, a process
known as dis-inhibition.
However, inhibition theory cannot explain why in some cases performance never declines during
the watch-keeping session, while in other cases performance is poor from the very beginning of the vigil. In
addition, the operator’s responses may not always decline over time. In other words, sustained attention
may decline over time, but the number of responses may remain relatively stable. However, one of the most
problematic empirical findings for this theory was that increasing signal probability, which should increase
inhibition and thereby decrease performance, actually improves vigilance.
Expectancy Theory. The decline of behavioristic and drive-based theories of behavior led to a
‘cognitive revolution’ in psychology (Dember, 1974). The information processing theories of the 1950’s
were manifestations of this shift. Although Broadbent (1958) applied his attentional filter theory to
vigilance, the evidence has not strongly supported his assertions (see Craig, 1985; but see also Broadbent,
1971). During this same time period, the expectancy theory of vigilance emerged. Expectancy theory was
not intended to explain the performance decrement, but rather to account for findings problematic for
inhibition theory: maintenance of a low but persistent level of responding, and the increase in responding as
a function of signal probability. The theory, first presented by Baker (1959), asserts that observers’
expectancies regarding signal events often differ from reality, and this discrepancy accounts for the response
patterns observed in vigilance experiments. Thus, observers adjust their level of responses (and thereby
their level of detections) according to the perceived signal frequency and their prior experiences on the task
or similar tasks. Expectancy can also account for the increase in conservatism with time on watch, since
observers may learn that the signals they are looking for are rare and adjust their responding accordingly
(Craig, 1978). Although this theory still has utility in explaining response bias effects and responses by
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observers to changes in signal and event rate, it does not provide a complete explanation for vigilance
effects. An attempt to more fully explain vigilance was emerged with the application of arousal theory.
Arousal Theory. This theory draws on the ‘inverted-U’ model of arousal attributed the Yerkes and
Dodson (1908) but actually presented in its most recognized form by Hebb (1955). According to this view,
an individual’s level of vigilance depends on their arousal, and the performance decrement results from
under-arousal resulting from the under stimulating environment of the vigilance task. While there is some
evidence for this contention (e.g., electro-cortical activity and autonomic activity tends to decline during a
vigil; see Davies & Parasuraman, 1982), arousal theory cannot account for the high stress levels associated
with vigilance. For instance, vigilance is associated with increased levels of catecholamine (e.g., see
Frankenhaeuser, Nordheden, Myrsten, & Post, 1971), which is associated with the endocrine system’s
response to stress, and observers report more symptoms of stress before a vigil than prior to its start (for a
review, see Szalma, 1999).
In addition, because arousal theory argues that vigilance tasks are
underarousing, the mental workload associated with such tasks should be low. However, it has been well
established that the workload of vigilance is, in fact, quite high (Warm, Dember, & Hancock, 1996).
Resource Theory. The problems associated with the arousal theory of vigilance coincided with the
problem of unitary arousal theories in general. Resource theory (Kahneman, 1973; Moray, 1967; Navon &
Gopher, 1979; Norman & Bobrow, 1975) emerged as an alternative to arousal theory, and it was adopted
as a theory of vigilance. Thus, according to resource theory, an individual’s vigilance depends on the mental
capacities or ‘resources’ that can be allocated to the task. The concept of resources draws on economic and
thermodynamic metaphors, utilizing the notions of supply and demand for the former and ‘pools’ of energy
or processing units available for the latter. According to this theory, the performance decrement occurs
because individuals expend resources for maintaining attention at a rate faster than they can be replenished
(Parasuraman, Warm, & Dember, 1987). This theory is consistent with empirical findings related to tests
of the vigilance taxonomy (Warm & Dember, 1998) and with the findings in regard to workload (Warm,
Dember, & Hancock, 1996) and stress (Szalma, 1999). However, as extensive as the favorable evidence is,
it is still difficult to argue that the data support resource theory. This is because it is very difficult (and may
be impossible; but see Szalma & Hancock, 2002) to test the concept of resources itself. Like other
constructs in psychology, resources are difficult to define operationally, and therefore difficult to
empirically test. The future of this theory in general and as a theory of vigilance rests on current and future
efforts to understand what ‘resources’ are.
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The Vigilance Taxonomy
By the early 1970s it was clear that whether performance decrements were observed depended in
part on task characteristics. However, it was unclear what task properties were important determinants.
Parasuraman and Davies (1977) noted that although the correlations in performance across task types were
low, they were not zero. They derived a taxonomic scheme for classifying vigilance tasks in categories
according to sensory modality, number of sources to be monitored (source complexity), the rate of
stimulus events (event rate), and memory load. For the last category Parasuraman and Davies (1977)
distinguished between tasks that required observers to compare a stimulus to a standard held in memory
(successive tasks requiring absolute judgment) from those tasks in which the standard and the test stimulus
were both presented (simultaneous tasks requiring comparative judgment). In an extensive review of the
literature they observed that the vigilance decrement only occurred for certain categories of task.
Specifically, they found that performance decrements were most likely to occur with successive tasks at
high event rate (event rates> 24 events per minute) with multiple sources to be monitored. In regard to
modality, visual tasks elicited lower performance levels than auditory tasks (but see Hatfield & Loeb, 1968).
Later meta-analytic work (See, Howe, Warm, & Dember, 1995) confirmed that event rate was a major
determinant to vigilance performance, but they also found that there was evidence that the decrement can
be obtained for both successive and simultaneous tasks. In addition, they also reported differences between
tasks that required primarily sensory discrimination from those that imposed cognitive demands (e.g.,
mathematical calculations), and suggested that the sensory/cognitive distinction be added as an additional
taxonomic category.
The Psychophysics of Vigilance
In addition to the taxonomic categories, there are psychophysical variables that influence
performance in vigilance. These include modality and complexity, which are also categories of the
taxonomy, as well as signal salience (conspicuity), event rate, signal rate, and spatial and temporal
uncertainty. In class we will review the effects of each of these categories on vigilance performance, as well
as the influence of feedback and cueing on performance and transfer of training (but see also Warm &
Jerison, 1984).
Current Learning Objectives
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The objective for the unit on vigilance is to provide you with an introduction to the phenomenon,
its importance in human factors, the relevant characteristics of vigilance tasks, and theories of vigilance.
Thus, if you understand the taxonomy and the theoretical issues surrounding vigilance, and the factors that
influence performance, workload, and stress in vigilance, you will gain an understanding of how to avoid
the problem in design and how to account for vigilance effects that may occur in your research. In addition,
it will be important for you to understand the relations among measures of performance, workload, and
stress (i.e., associations/dissociations).
References
Baker, C.H. (1959). Attention to visual displays during a vigilance task: II. Maintaining the level of
vigilance. British Journal of Psychology, 50, 30-36.
Broadbent, D.E. (1958). Perception and communication. London: Pergamon Press.
Broadbent, D.E. (1971). Decision and stress. New York: Academic Press.
Craig, A. (1978). Is the vigilance decrement simply a response toward probability matching?
Human Factors, 20, 441-446.
Craig, A. (1985). Vigilance: Theories and laboratory studies. In: S. Folkard and T.H. Monk (Eds.), Hours
of work. (pp. 107-121).Chichester, UK: Wiley.
Davies, D.R., & Parasuraman, R. (1982). The psychology of vigilance. London: Academic Press.
Dember, W.N. (1974). Motivation and the cognitive revolution. American Psychologist, 29, 161-168.
Ditchburn, R.W. (1943). Some factors affecting the efficiency of work by lookouts (Report No.
ARC/R1/84/46/0). Admiralty Research Laboratory.
Frankenhaeuser, M., Nordheden, B., Myrsten, A.L., & Post, B. (1971). Psychophysiological
reactions to understimulation and overstimulation. Acta Psychologica, 35, 298-308.
Hatfield, J.L., & Loeb, M. (1968). Sense mode and coupling in a vigilance task. Perception &
Psychophysics, 4, 29-36.
Head, H. (1923). The conception of nervous and mental energy. II. Vigilance: A physiological
state of the nervous system. British Journal of Psychology, 14, 126-147.
Hebb, D. O. (1955). Drives and the CNS (conceptual nervous system). Psychological Review, 62, 243254.
Hull, C.L. (1943). Principles of behavior: An introduction to behavior theory. New York: Appleton.
Kahneman, D. (1973). Attention and effort. Englewood Cliffs, NJ: Prentice Hall.
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Journal of Experimental Psychology, 1, 6-21.
Moray, N. (1967). Where is capacity limited? A survey and a model. Acta Psychologica, 27, 84-92.
Navon, D., & Gopher, D. (1979). On the economy of the human information processing system.
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Norman, D., & Bobrow, D. (1975). On data-limited and resource-limited processing. Journal of Cognitive
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Szalma, J.L. (1999). Sensory and temporal determinants of workload and stress in sustained attention.
Unpublished doctoral dissertation, University of Cincinnati, Cincinnati, OH
Szalma, J.L., & Hancock, P.A. (2002). On mental resources and performance under stress. Unpublished
white paper, MIT2 Laboratory, University of Central Florida. Available at www.mit.ucf.edu
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(pp. 87-112). Washington, DC: American Psychological Association.
Warm, J.S., Dember, W.N., & Hancock, P.A. (1996). Vigilance and workload in automated systems.
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applications (pp. 183-200) Hillsdale, NJ: Erlbaum.
Warm, J.S., & Jerison, H.J. (1984). The psychophysics of vigilance. In: J.S. Warm (Ed.), Sustained
attention in human performance. (pp. 15-59). Chichester, United Kingdom: Wiley.
Yerkes, R.M., & Dodson, J.D. (1908). The relation of strength of stimulus to rapidity of habit-formation.
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