Sustained Attention in the Mentally Retarded: The Vigilance Pradigm

Sustained Attention in the Mentally Retarded: The Vigilance k a d i g m JOEL S. WARM AND DANIEL B. BERCH' DEPARTMENT OF PSYCHOLOGY AND 'CINCINNATI CENTER FOR DEVELOPMENTAL DISORDERS UNIVERSITY OF CINCINNATI CINCINNATI, OHIO

Introduction ............................................................ A. Short-Term Attention.. ............................................. , B. Sustained Attention ................................................. 11. An Overview of Vigilance Theory and Research.. ........................... A. The Vigilance Paradigm.. ............................................ B. The Psychophysics of Vigilance.. ................................ , .... C. Theories of Vigilance.. .............................................. 111. Sustained Attention in the Mentally Retarded ............................... A. The Continuous Performance Test,. ................................... B. Developmental Changes in Vigilance Performance. ...................... C. Monitoring Efficiency and IQ Level in the Intellectually Normal D. Vigilance Studies with the Mentally Retarded.. ......................... IV. Summary and Conclusions ............................................... References. ............................................................. I.

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INTRODUCTION

Short-Term Attention

The concept of attention occupies a central role in modern psychological research. The essence of this concept is the fdcusing of awareness on a particular source of stimulation to the exclusion of others-a process by which an organism trades off a gross loss of information for the opportunity t o deal with a small fraction of the information that may be important to it at a given time (Dember & Warm, 1979; Egeth & Bevan, 1973; James, 1890). INTERNATIONAL REVIEW OF RESEARCH IN MENTAL RETARDATION, Vol. 13

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Copyright 0 1985 by Academic Press, Inc. All ri~htsof reproduction in any form reserved.

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Much of the research on the nature of attention has been concerned with the manner in which individuals select information for processing over brief periods of time, particularly when they are being exposed to more information than they can handle (cf. Eysenck, 1982). This line of research has been particularly attractive to experimental psychologists in the areas of cognition and information processing because it permits an exploration of the way individuals control the choice of stimuli which, in turn, influence their behavior (Kahneman, 1973). As Nettelbeck and Brewer (1981) have pointed out, the concept of short-term attention has also been useful for students of mental retardation in their attempts to account for the comparatively low performance levels typically exhibited by the intellectually handicapped on a variety of learning and information-processing tasks (cf. Furby, 1974; O’Connor & Hermelin, 1963, 1971; Zeaman & House, 1963, 1979).

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Sustained Attention

In addition to short-term selection, there is another aspect of the study of attention which has also spurred considerable research interest and is the main concern of this article. That aspect is sustained attention or vigilance-the ability of observers to maintain their focus of attention and to remain alert to stimuli over prolonged periods of time (Davies & Parasuraman, 1982; Warm, 1984). This line of investigation occupies a unique niche in psychology because it has simultaneous appeal for investigators in both basic research and more applied settings. The capacity to maintain some level of alertness during the activities of the day is a primary aspect of perceptual functioning (Dember & Warm, 1979; Jerison, 1977). Moreover, this capacity plays a vital role in the reliability of human performance in a variety of work situations. Among these are industrial inspection tasks, nuclear power plant operation, air-traffic control, seaboard navigation, and long-distance driving (Craig, 1984; Davies & Parasuraman, 1982; Drury & Fox, 1975; Mackie, 1977; Wiener, 1984). Indeed, the relevance of vigilance extends to any aspect of our automated society in which a good part of a human operator’s time is spent in the passive monitoring of dials, video screens, and other sources of information for occasional signals which demand decision and action (Warm, 1984). By virtue of comprising a laboratory analog of simple, repetitive work under relatively monotonous conditions (Stroh, 1971), the vigilance paradigm has also attracted the attention of researchers interested in mental retardation. As B. Locke, Byrd, Berger, and Childs (1982) have pointed out, the adoption of this methodology is not surprising, given the opinion of some professionals (cf. Kohn, 1977) that work viewed as boring by peo-

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pIe of average or higher intelligence may be acceptable and perhaps even challenging to mentally retarded individuals (i.e., as the old adage says, “dull minds for dull jobs”). From a contrasting perspective, the vigilance paradigm has also appealed to students of mental retardation because of its apparent utility for demonstrating that the intellectually handicapped may suffer from an “attentional deficit” (see Crosby & Blatt, 1968, for a review). A third reason for its popularity in this field is that the vigilance paradigm provides an excellent vehicle for empirically testing theoretical views of the greater inattentiveness characteristically attributed to the mentally retarded. Unfortunately, however, as Berch and Kanter (1984) have recently noted, there has been relatively little cross-fertilization of ideas between investigators who employ vigilance-type tasks for studying the attentional difficulties of special populations and basic researchers working on the fundamental aspects of sustained attention. Their paper was written primarily to make the latter individuals aware of the procedures, findings, and problems of the former. A major purpose of the present article is precisely the converse: to provide the investigator interested in the monitoring abilities of mentally retarded children and adults with information extracted from the basic research literature that we consider to be of critical importance for the successful design, execution, analysis, and interpretation of vigilance studies with the mentally handicapped. Our coverage includes a summary of major psychophysical principles along with information regarding recent empirical advances and theoretical developments in the experimental psychology of sustained attention. This overview of vigilance theory and research will set the stage for the second half of the article in which we present a detailed review and appraisal of contemporary vigilance research with mentally retarded observers. A discussion of evidence concerning developmental changes in the monitoring efficiency of intellectually normal children is also included because (1) this information can provide a normative baseline for assessing developmental disorders of attention, and (2) to date, other than the synopsis by Berch and Kanter (1984), no thorough, integrative review of the scattered and often disparate reports in this area has appeared in the literature.

II. AN OVERVIEW OF VIGILANCE THEORY AND RESEARCH Systematic study of sustained attention began during the Second World War. As noted by Davies and Parasuraman (1982) and by Warm (1984), it was stimulated by surprising fallibility in the performance of British radar

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observers while on patrol for enemy submarines. These individuals were often required to maintain continuous observation of their radar scopes during long flights over the Bay of Biscay watching for telltale “blips” that signaled the presence of enemy submarines in the sea below. Despite their extensive training and obvious motivation to perform well, the observers failed with increasing frequency to notice the critical signals displayed by their equipment as time on watch progressed. As a result, the submarines passed undetected. In response to a request from the Royal Air Force to study the problem, Norman Mackworth (1948, 1950/1961) initiated a series of ingenious experiments which formed the first systematic effort to bring the study of sustained attention into the controlled environment of the laboratory. He devised a simulated radar display called the “Clock Test” in which subjects were asked to view movements of a black pointer about the circumference of a blank-faced clock which contained no scale markings or reference points. Normally, the pointer moved in .3-inch increments around the face of the clock. Occasionally, it executed a “double jump” of .6 inches. Subjects responded by pressing a key whenever they spotted a movement of double length during a continuous 2-hour session. Using the Clock Test in this way, Mackworth was able t o uncover several factors which influenced sustained attention, thereby setting the stage for much of the research to follow.

A.

The Vigilance Paradigm

1. BASIC FEATURES

Since Mackworth’s early experiments, a host of other tasks have been employed in the study of vigilance (see Hancock, 1984; and J. F. Mackworth, 1970, for a catalog of tasks). Nevertheless, these tasks share several characteristics which were present in Norman Mackworth’s initial work and which, in sum, can be considered as the paradigmatic elements of vigilance situations (cf. Jerison, 1970; Warm, 1977, 1984). These characteristics are (1) the task is prolonged and continuous, often lasting for 30 minutes or more, (2) the signals to be detected are “weak” but clearly perceivable when the observer is alerted to them, (3) the signals to be detected occur without forewarning on an infrequent and aperiodic basis, and (4) the observer’s response typically has no influence on the probability of appearance of signals. It is worth noting that all vigilance experiments do not conform to these characteristics in a hard and fast manner. For the most part, however, they seem to capture the essential elements of most laboratory vigilance tasks.

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2. THE DECREMENT FUNCTION

One of the most dramatic findings in Mackworth’s pioneering work with the Clock Test was that the quality of sustained attention is often quite fragile-it wanes over time! Mackworth found that his subjects became progressively more inefficient at detecting signals as time on watch progressed, and that this inefficiency did not take long to develop. In general, the accuracy of signal detections declined from about 85% during the first 30 minutes of watch to about 75% during the second 30 minutes, and then showed a more gradual decline during the remainder of the 2-hour session. The progressive decline in performance over time noted in Mackworth’s early experiments has since been found in a large number of subsequent investigations. The effect takes the form of a drop in the accuracy of signal detections and/or a rise in the latency of detections (cf. Buck, 1966; Davies & Parasuraman, 1982). It has been labeled the “decrement function” or the “vigilance decrement” and is perhaps the most ubiquitous finding in vigilance studies. Many experiments, using a broad assortment of tasks, indicate that the decline in performance is complete from 20 to 35 minutes after the initiation of the vigil, and that at least half of the final loss is completed within the first 15 minutes (Teichner, 1974). Indeed, the decrement can, under certain conditions, appear within the very first few minutes of watch (Jerison, 1963; Nuechterlein, Parasuraman, & Jiang, 1983). As Dember and Warm (1979) have noted, the most striking aspect of this finding is that it seems to result merely from the necessity of looking or listening for a relatively infrequent signal over a continuous period of time. Still another interesting aspect of the decrement is that it may be related to hemispheric asymmetries in information processing (cf. Bradshaw & Nettleton, 1983). Diamond (1979a,b) has reported that commissurotomized patients detect more signals during a vigilance task when stimuli are directed to the right as compared to the left hemisphere, and Warm and his coworkers (Warm, Richter, Sprague, Porter, & Schumsky, 1980) have observed that among normal subjects the decrement occurs predominantly with signals delivered to the right hemisphere. Their findings led Warm and his colleagues to suggest a cooperative interaction model for hemispheric functioning in sustained attention (cf. Allen, 1983). According to this view, both hemispheres play a role in organizing vigilance performance, but the contribution of the right hemisphere predominates.

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The Psychophysics of Vigilance

1. AN EMPIRICAL EQUATION

At first glance, vigilance situations might seem to be rather simplistic affairs in which observers merely await the appearance of signals and then

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respond to them in a predetermined fashion. However, as Warm and Jerison (1984) have noted, this simplicity is deceiving. Upon analysis, it becomes clear that the observer’s behavior is quite intricate. The intricacies involved are due, in part, to the fact that performance efficiency is closely tied to the nature of the stimuli which demand attention. Consequently, the study of vigilance, like that of other perceptual phenomena, has profited from the precise determination of the stimulus conditions which influence performance. In summarizing these conditions, we shall follow the lead of Jerison (1959) and describe them within the framework of an empirically determined functional equation which takes the following form: P = f(Sm, Sc, Su, BER)

(1)

According to this relation, vigilance performance (P) is a function of the sensory modality of signals (Sm), the conspicuity of signals (Sc), stimulus uncertainty (Su), and background event rate (BER). a. Sensory Modality. Acoustic, tactual, and visual stimuli have been used in vigilance experiments. However, the sensory modality of signals is not a matter of indifference where the quality of sustained attention is concerned. Auditory tasks tend to be associated with a higher level of overall efficiency and with greater stability over time than their tactual and visual analogs (Davies & Parasuraman, 1982; Davies & Tune, 1969; Warm & Jerison, 1984). In addition, intersensory correlations are often low or nonsignificant (Hatfield & Loeb, 1968). Results such as these have meaning on a theoretical as well as on an empirical level. They imply that vigilance performance may not reflect the properties of a single central process, but instead, may be modality specific (Dember & Warm, 1979). Fortunately, evidence is available which affords a more inclusive view. This evidence indicates that audiovisual correlations can be increased by closely equating the types of discriminations required in the two modalities (Hatfield & Loeb, 1968; Parasuraman & Davies, 1977). and that experience in one modality can influence subsequent performance in another modality (Gunn & Loeb, 1967; Tyler, Waag & Halcomb, 1972). Particularly compelling evidence for a general process view of vigilance comes from studies using redundant displays in which analogous signals are presented simultaneously to the auditory and visual channels. Performance with these displays exceeds that with single-mode displays (auditory or visual), and the dual-mode superiority has been shown to stem from the integrative action of the sensory systems involved rather than from a fortuitous combination of their independent activities (Craig, Colquhoun, & Corcoran, 1976). Thus, while sensory considerations are clearly a factor

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in the maintenance of sustained attention, vigilant behavior can, within limits, be considered as a general characteristic of the observer. b. Signal Conspicuity. A common finding in psychophysical studies under alerted conditions is that signal detection is positively related to stimulus amplitude and duration. These factors are also important in the vigilance experiment. Several investigations have demonstrated that the overall quality of sustained attention can be enhanced and performance rendered more stable over time by increasing the amplitude or the signal-to-noise ratio of critical signals (Adams, 1956; Guralnick, 1972; Loeb & Binford, 1963; Wiener, 1973). Indeed, Corcoran and his co-workers (Corcoran, Mullin, Rainey, & Frith, 1977) have shown that it is possible to reverse the usual temporal course of performance by “turning up the gain” on the sensory channel being monitored. In their study, which required attention to acoustic pulses, an abrupt increment in the amplitude of the stimuli midway through the vigil bolstered the frequency of signal detections during the remainder of the session. In addition to amplitude changes, signals can be made more conspicuous by increasing their duration. Signals of brief duration are less likely to be detected than those which remain visible for longer periods of time (Adams, 1956; Baker, 1963b; Warm, Loeb, & Alluisi, 1970). c. Stimulus Uncertainty. As noted earlier, critical signals in vigilance experiments typically occur in an infrequent and aperiodic manner. Thus, observers are often confronted with a considerable amount of temporal uncertainty-uncertainty as to when a critical signal will appear. Such uncertainty markedly affects the observer’s performance. One means of experimentally manipulating the observer’s temporal uncertainty is through variations in the density or the number of critical signals. The more frequently such signals occur in a fixed time period, the greater the a priori signal probability and the less the observer’s average uncertainty as to when critical signals will occur. The accuracy of signal detections varies directly as a function of signal density (Jerison & Pickett, 1963; Warm & Jerison, 1984). Increments in signal density also enhance the speed of signal detections. This effect is illustrated by the work of Alluisi and his associates (Smith, Warm, & Alluisi, 1966; Warm & Alluisi, 1971) who used an informationtheory analysis to measure the density-determined temporal uncertainty in the appearance of critical signals. These investigators found that response times to detections could be represented as a linear increasing function of the temporal uncertainty due to density. This finding is particularly interesting because it suggests that an important quantitative generalization concerning speed of performance in other perceptual tasks may also apply t o vigilance tasks. That generalization is Hick’s law, which states that the rate

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of information processing is a linear increasing function of stimulus uncertainty (see Dember & Warm, 1979). The observer’s temporal uncertainty in a vigilance experiment can also be manipulated through variations in the intervals of time between signals or the intersignal intervals. These intervals can be made highly irregular or quite regular and therefore easily predictable. Although variations in signal regularity do not produce effects as strong as those associated with signal density, they do have an impact upon performance efficiency. For the most part, the more regular the intersignal intervals, the greater the number of signal detections and the greater the speed with which detections occur (Adams & Boulter, 1964; Lisper & Tornros, 1974; Warm, Epps, & Ferguson, 1974).

In some vigilance studies spatial uncertainty is introduced into the situation by varying the probability that signals will appear in different portions of a monitored display. This form of uncertainty also has an interesting effect upon performance. Under conditions of spatial uncertainty, observers come to bias their attention within the display and detect the greatest number of signals in those portions in which the likelihood of signal appearance is highest (Adams & Boulter, 1964; MiloSeviC, 1974; Nicely & Miller, 1957). d. Event Rate. A major feature of vigilance experiments is that they frequently employ dynamic displays in which critical signals are embedded within a matrix of recurring neutral background events. For example, in the Clock Test described earlier, the small movements of the black pointer constituted a set of neutral events in which the large double jumps occasionally appeared. Although the background events may be neutral in the sense that they require no overt response, they are far from neutral in their influence upon the quality of sustained attention. The frequency of neutral events, or the background event rate, is a very important determinant of performance efficiency. Both the speed and accuracy of signal detections vary inversely with event rate, and the vigilance decrement tends to be less pronounced in the context of a slow as compared to a fast event rate (Guralnick, 1973; Jerison & Pickett, 1964; Johnston, Howell, & Goldstein, 1966; Parasuraman & Davies, 1976; Wiener, 1977). The effects of event rate are paradoxical. As Warm and Jerison (1984) have indicated, they imply that the more one is required to look or to listen for critical signals, the less likely one is to detect such signals. Upon reflection, one might conclude that this paradox is easily resolved. If critical signal density is held constant (as is usually the case), increments in event rate reduce the conditional probability that an event is a critical signal. Thus, event rate may simply be another example of signal uncertainty. This does not appear to be the case, however. The quality of sustained attention varies

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inversely with event rate even when signal density is adjusted so that the conditional probability of critical signals is equated within event rates (Jerison, 1965; Loeb & Binford, 1968; Taub & Osborne, 1968, Parasuraman, 1979).

It is important to note that background event rate not only influences performance in its own right, it also modifies the effects associated with other stimulus parameters. For example, Metzger, Warm, and Senter (1974) have shown that the effects associated with variations of signal amplitude are magnified in the context of a fast as compared to a slow event rate, and Moore and Gross (1973) have reported that the effects of signal regularity are altered by event rate. In their study, the beneficial effects of regular signals appeared late in the watch under a slow event rate and early in the watch under a fast event rate. Recently, Bowers and his associates (Bowers, 1983; Bowers, Warm, & Dember, 1982; Wolk, Bowers, Vandenboom, Dember, & Warm, 1983) have reported that the speed with which observers can detect probe stimuli while attending to a vigilance task varies inversely with the event rate employed in that task. In terms of resource limitations in information processing (cf. Kahneman, 1973; Wickens, Kramer, Vanasse, & Donchin, 1983), it appears that vigilance tasks are more capacity demanding when the event rate is fast than when it is slow. Taken together, findings such as these support a suspicion articulated by Jane Mackworth several years ago (1968, 1969) that background event rate is probably the prepotent psychophysical factor in vigilance experiments (see also Dember & Warm, 1979; Warm & Jerison, 1984). 2. THE COMPLEXITY ISSUE A common feature of the experiments discussed thus far is the relative simplicity of the perceptual discriminations that they required. For the most part, observers were asked to detect discrete changes in the illumination level, sound level, duration, or movement of stimuli. Since performance generally suffers a decline over time when such simple stimuli are involved, one might think that the ability to sustain attention to a task at hand would be even more fragile under more complex conditions. This suspicion is borne out under some circumstances. In an early experiment, Jerison (1963) found that the decrement could be observed from the very first signal in a vigil when observers were asked to monitor three vigilance displays simultaneously. More recently, Fisk and Schneider (198 1) approached the problem of complexity in terms of automatic and controlled processing theory (cf. Schneider & Shiffrin, 1977). According to this view, automatic processes are fast, fairly effortless skilled behaviors while “controlled” processes refer to relatively slow, effortful, capacity-limited activities. In a carefully conducted study in which automatic processing was

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developed over several hundred trials, Fisk and Schneider (1981) found that the vigilance decrement was restricted to controlled processing tasks; automatic tasks were performed in a stable manner throughout the vigilance session. In contrast to these studies are several experiments by Adams and his associates which varied complexity by having observers monitor 3 to 6 stimulus sources concurrently under conditions in which any one source could present a signal at any moment in time (Adams, & Humes, 1963; Adams, Humes, & Sieveking, 1963; Montague, Webber, & Adams, 1965). The vigilance decrement was absent or minimal in Adams’ experiments, even though the vigil lasted for several hours! More recently, Adams’ findings have been confirmed by Warm and his collaborators who varied complexity by increasing the cognitive demand placed upon observers in a vigilance task (Lysaght, Warm, Dember, & Loeb, 1984; Warm, Dember, & Lanzetta, 1984). The problem of stimulus complexity was identified as an important topic in vigilance research 25 years ago (Jerison, 1959). It has been dormant for several years, but as Warm and Jerison (1984) have noted, it has emerged once again as a perplexing issue. Depending upon the approach employed, it is possible to amplify or eliminate the vigilance decrement through modifications of complexity. Clearly, a resolution of these disparities will be necessary in order to develop a complete functional equation for the psychophysics of vigilance. C. Theories of Vigilance Several theoretical models have been offered to account for the data just described as well as for other elements of vigilance performance. Taken together, one of the most impressive characteristics of these models is their diversity. Theories of vigilance feature bottom-up views in which performance is considered to be data driven, and top-down views in which the monitor’s actions are considered as being controlled by higher-order processes involving hypothesis formation, decision making, and expectation. We shall discuss a number of the more influential positions along with some recent theoretical developments. More complete treatments of the theories of vigilance can be found in Broadbent (197 l), Craig and Colquhoun (1975), Davies and Parasuraman (1982), J. F. Mackworth (1970), Loeb and Alluisi (1977, 1984), and Warm (1977). 1 . BOTTOM-UP VIEWS

Two of the most influential theories focus upon the neurophysiological state of the observer as a determinant of the quality of sustained attention. One of these is the arousal or activation position. It derives from the idea

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that behavior can vary along a continuum from deep sleep to high excitement (Malmo, 1959, 1975) and from Hebb’s (1955) suggestion that sensory input both conveys information about the environment and provides a diffuse amount of stimulation which the brain needs for alert behavior. According to the arousal position, the alerting function of sensory input depends upon a varied perceptual environment; when variability falls below a critical level, wakefulness is reduced. As applied to vigilance (cf. Frankmann & Adams, 1962), the arousal theory maintains that the monotonous aspects of the vigilance task reduce the level of varied stimulation needed to maintain continued alertness and thereby lead to a decline in the efficiency of signal detection. Considerable evidence is available to link vigilance performance with neurophysiological indices of arousal. For example, the vigilance decrement has been found to be accompanied by declines in electrical skin conductance (MiloSeviC, 1975; Verschoor & Van Wieringen, 1970) and blood and urinary adrenaline levels (Frankenhaeuser, 1971; O’Hanlon & Beatty, 1976), and the deterioration of sustained attention has been accompanied by forms of electrocortical activity characteristic of lowered states of alertness (see Davies & Parasuraman, 1982; Parasuraman, 1983, 1984, for reviews of these studies). Moreover, enhancement or impairment of vigilance performance has been associated with stimulant and depressant drugs, respectively (Colquhoun & Edwards, 1975; Loeb, Hawkes, Evans, & Alluisi, 1965), and with sources of environmental stress which raise and lower arousal, such as mild and moderate heat (Hancock, 1984; Poulton, 1977). In addition, the quality of sustained attention shows a time-of-day effect which covaries with that of body temperature-both follow an inverted-U function which rises from a perigee at the end of sleep (0800 hours) to an apogee late in the day (1700/2100 hours), and then declines again (Craig & Colquhoun, 1975; Colquhoun, 1977). Variations in diurnal rhythm are considered to reflect changes in a general state of arousal or reactivity (Davies & Parasuraman, 1982). Given data such as these, it would be tempting to conclude that the arousal position provides a rather good account of vigilant behavior. However, a conclusion of this sort is unwarranted. While arousal seems to be clearly implicated as a factor in the maintenance of sustained attention, there are reasons to believe that the arousal position is either incomplete or unable to accommodate several facets of vigilance performance, or both. In a careful examination of the matter, Parasuraman (1983, 1984) has pointed out that performance efficiency during a vigil is not always related to psychophysiological activity, and declines in arousal are not necessarily task related-arousal often declines in any prolonged and monotonous testing situation, even when no task is being performed at all. In addition, there is some debate as to whether certain physiological changes associated with

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the vigilance decrement are more closely related to information-processing activities than to a general state of arousal (Beatty, 1982; Parasuraman, 1983; Wilkinson & Seales, 1978), and the notion of arousal as a unitary concept is open to question. Several investigators have recently argued for a multistate arousal notion (Gale, 1977; Loeb & Alluisi, 1977, 1984; Pribam & McGuinness, 1975). Other problems for the activation position arise when we examine its implications on a psychophysical level. At this level of analysis, the model can be criticized for a lack of precision in formulation. The amount of external stimulation that will be related to measurable aspects of performance is not specified precisely and psychophysical tests of the model are often inconsistent. In discussing this issue, Dember and Warm (1979) point out that making the vigilance environment more variegated through extraneous stimulation enhances performance in some cases and degrades it in others. In addition, they note that the vigilance decrement has appeared under conditions in which there was a rich source of stimulation, and that the decrement is sometimes more pronounced under extremely stimulating than under less stimulating circumstances. One could, of course, invoke the Yerkes-Dodson law and argue that performance is adversely affected by too little and too much arousal. However, the point on the arousal function where a given experimental manipulation will fall often cannot be specified until after the experiment has been completed. Thus, we are left in the somewhat frustrating position of having a mechanism, arousal, which certainly seems to play a part in sustained attention, but is difficult to pin down in a rigorous experimental manner. An alternative neurophysiological approach to the psychology of vigilance is the habituation theory offered by J. F. Mackworth (1968, 1969). Habituation refers to the waning of neural responsiveness due to repeated stimulation (Groves & Thompson, 1970). An active process of inhibition, it is differentiated from adaptation or fatigue by the phenomenon of dishabituation, the sudden reappearance of responsiveness following qualitative or quantitative changes in the pattern of stimulation (Sharpless & Jasper, 1956). Generally, the degree of habituation in a given situation varies directly with the frequency of stimulus presentation. Mackworth suggests that the repetitive stimulation of the continuous background events of the vigilance task serves to habituate the nonspecific a block, an arousal response which usually accompanies incoming stimuli, and cortical evoked responses to these stimuli. With the development of habituation, the observer’s ability to discriminate critical signals is degraded, attending to the task becomes increasingly more difficult, and the observer’s performance deteriorates over time. Mackworth notes that the model fits well with a number of empirical

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facts regarding the decrement function. For example, the rate of habituation is usually a negatively accelerated function of the number of stimulus presentations per unit time, and the decrement often displays a similar course over time. In addition to accounting for the decrement, the habituation model forges a link between the decrement and the effects of event rate by tying them to the same explanatory mechanism. Due to the rate of stimulus repetition, habituation accumulates more rapidly at fast than at slow event rates, resulting in poorer performance at fast event rates. The generality of the model is enhanced further by the fact that it can accommodate the data on pharmacological agents and environmental stress described above in regard to the arousal theory. Although the habituation model offers a simple and elegant account of vigilant behavior, efforts to provide experimental support for the model have met with mixed success. Positive neurophysiological evidence comes from studies of individual differences which indicate that subjects who demonstrate rapid habituation of the galvanic skin response show a greater vigilance decrement than those who are slow to habituate (Crider & Augenbraun, 1975; Siddle, 1972). On the other hand, an extensive review by Gale (1977) has indicated that for almost every neurophysiological study supporting the model, there is one which contradicts it. For example, the a index can increase or decrease in time, evoked potentials can be relatively smaller with poor performance or relatively larger, and subjects who are good watchkeepers can also be those who show lowered arousal as measured by skin conductance or EEG abundance. Experimental tests using psychophysical measures have failed to support the habituation model. In one such test, Krulewitz, Warm, and Wohl(l975) increased or decreased event rates suddenly during a vigil. According to the habituation position, any change in stimulus conditions should result in dishabituation and improved performance. Shifts in event rate represent drastic changes in stimulation. Therefore, the habituation position leads to the expectation that such shifts should enhance the quality of sustained attention regardless of the direction of the shift. Contrary to expectation, observers who experienced a slow-to-fast shift performed more poorly than nonshifted controls at the fast event rate. Moreover, while the performance of subjects who experienced the fast-to-slow shift eventually exceeded that of nonshifted controls at the slow event rate, it did so only after the shifted group had experienced the new event rate for 20 minutes. This result does not fit with the known time course of dishabituation, which is considerably more rapid than 20 minutes (Thompson & Spencer, 1966). Still another psychophysical test of the habituation model focuses upon J. Mackworth’s (1968) point that the development of habituation can be arrested when the habituating stimulus occurs in an irregular manner. Ac-

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cordingly, one might expect the overall level of performance to be enhanced and the effects of event rate to be attenuated if critical signals are embedded in a temporally irregular matrix of neutral events instead of the temporally regular matrix normally used in vigilance experiments. When such a procedure has been tried, however, irregular event schedules have suppressed rather than augmented the overall level of signal detections, and these schedules have failed to attenuate the effects of event rate (Jerison, 1965; Richter, Senter, & Warm, 1981). 2. TOP-DOWN VIEWS

By virtue of their participation in the vigilance task, observers have many opportunities to acquire information about the signals t o be detected and the conditions under which they will appear. It seems reasonable to argue that such information can be of significant value to the observer during the course of the watch. One higher-order model which emphasizes stored information is the expectancy hypothesis (Baker, 1963a; Deese, 1955). According to this view, observers in vigilance experiments act as temporal averaging instruments and continuously form expectations about the approximate time course of critical signal appearances on the basis of samples of signal input. Readiness to detect a signal is assumed to be positively related to the observer’s level of expectancy. The expectancy hypothesis maintains that the vigilance decrement results from a progressive deterioration in the observer’s ability t o make accurate predictions as to when critical signals will appear. The model also explains the effects associated with the temporal uncertainty of signals along similar lines. According t o this position, accurate expectancies are developed more easily when signals appear more often (high density) and on a regular time schedule. Strong support for the expectancy hypothesis comes from studies by Colquhoun and Baddeley (1964, 1967) and McFarland and Halcomb (1970) which demonstrate that observers who experience a high probability of signal appearances during the training phase of a vigilance session perform better in a subsequent test phase than those who initially experience low signal probability during training. The expectancy hypothesis also draws support from the fact that knowledge of results (KR) enhances the speed and accuracy of signal detections in vigilance experiments (cf. Davies & Parasuraman, 1982; Dember & Warm, 1979; Warm & Jerison, 1984). This effect would be anticipated from an expectancy view, since KR can aid observers to generate more veridical expectations. In spite of these favorable findings, the expectancy hypothesis does not provide a completely satisfactory framework for explaining vigilant behavior. The accurate estimation of time plays a crucial role in this model. Yet, as Warm (1977) has noted, human observers are not always precise esti-

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mators of time, and the ability to estimate time intervals accurately has been found to be unrelated to the efficiency of vigilance performance (McGrath & O’Hanlon, 1967). Further, the complex relations between signal regularity and event rate described earlier (see previous section) suggest that more than just temporal averaging and extrapolation is involved, and data are available which challenge the role of KR as a factor in expectancy formation. In the case of vigilance, the contribution of feedback may be more along motivational than instructional lines. This statement is based upon experiments which show that KR need not be accurate to be effective. False KR or KR that is given on a random basis is often as effective as true KR in enhancing performance efficiency (Loeb & Schmidt, 1963; Warm et al., 1974), and self-evaluations of performance made while engaged in a vigilance task can enhance the quality of sustained attention even though the accuracy of such evaluations is quite poor (Warm et al., 1972). For reasons such as these, Davies and Tune (1969) have suggested that the role played by expectancy in sustained attention is a limited one. Observers may resort to temporal expectancies only when the task is difficult and other cues are not available. This line of reasoning fits well with the results of a study by Krulewitz and Warm (1977) which demonstrated that the residual effects of training with different levels of signal probability on subsequent performance during testing may show up only under the difficult conditions of a fast event rate, but not under the easier conditions of a slow event rate. Related to the expectancy hypothesis is a recent suggestion by Craig (1976, 1978) which accounts for the vigilance decrement in terms of probability matching. This position takes note of the fact that the temporal decline in the number of signal detections (hits) in vigilance experiments is often accompanied by a drop in the number of errors of commission or false alarms. Thus, the total number of detection responses (hits and false alarms) declines over time. Craig suggests that observers may start out a vigil by responding at a rate greater than the signal rate. However, as time on watch progresses, the observers develop a more adequate appreciation of the low conditional probability of critical signals characteristic of vigilance tasks and adjust their detection responses accordingly. Therefore, the vigilance decrement does not reflect a decline in the perceptibility of signals, but rather a response adjustment toward probability matching. Craig offers a novel way to interpret the vigilance decrement, one which brings it in line with what takes place in several other psychophysical tasks in which binary detection responses are involved (the signal was or was not present). This view is constrained, however, by the fact that it only applies to the frequency of signal detections; it does not account for temporal changes in the speed of response when detections are asymptotic and sub-

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jects are essentially responding to all critical signals. Moreover, as Loeb (1 978) has suggested, the probability matching position is also constrained by the fact that it is limited to single-session data. This position does not account for multisession data which also reveal systematic changes in the frequencies of hits and false alarms. Across several sessions, the overall frequency of false alarms declines as it does in a single-session experiment, but the overall frequency of hits increases. According to Loeb, such changes over sessions probably reflect learning the differential characteristics of signals and nonsignals rather than probability matching. The covariation of hits and false alarms within sessions has led to still another approach toward explaining vigilance performance. This approach involves the theory of signal detection (TSD), a major theory in the general study of psychophysics which characterizes the observer as both a sensor and a decision maker. This is accomplished through two orthogonal measures of performance: d ’, an index of perceptual sensitivity, and 0, an index of caution in responding (Green, & Swets, 1974). The drop in total detection responses with time on task led Egan, Greenberg, and Schulman (1961) to suggest that the vigilance decrement might reflect a shift to a more conservative response criterion rather than a decline in alertness or sensitivity during a vigil. This idea was supported in experiments by Broadbent and Gregory (1963, 1965) which indicated that the sensitivity index, d ’ , remained undiminished during a vigil while the response criterion, p, increased over time. Similar results have been reported in an impressive array of subsequent experiments (cf. Davies & Parasuraman, 1982; Swets, 1977; Warm & Jerison, 1984). The finding that subjects become more conservative over time during a vigil does not in itself constitute an explanation of the decrement. It is merely a more elegant and specific description of temporal changes in performance. In order to explain the decrement within the framework of a TSD analysis, Williges (1969) adopted a decision theory approach featuring the notion of the ideal observer-an optimal detector that yields the best possible performance under specified conditions. Williges pointed out that if the rewards for correct responses and the penalties for incorrect responses are symmetrical, the optimum value of the response criterion (p) is given by the ratio of nonsignal to signal events. According to the ideal observer hypothesis, subjects embark upon a vigil with the response criterion set at some intermediate level (- 1). As the vigil continues, the criterion is adjusted toward the optimum value. Since the ratio of nonsignals to signals is usually quite high in vigilance experiments, the optimum value will be > 1, and the observer will become more conservative as time wears on. Conversely, if the ratio of nonsignals to signals were low (signals outnumber

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nonsignals), the value of the optimum criterion would be < 1, and the ideal observer hypothesis would predict a decline in p over time. Consistent with predictions from this model, Williges (1969, 1973) found that observers do become more conservative over time when nonsignals exceed signals, and in the unusual case where signals exceed nonsignals, observers become less conservative as time on watch progresses. On the basis of these findings, he suggested that instead of dwindling performance efficiency, the vigilance decrement might imply a tendency toward more optimal decision behavior. Williges’ approach is ingenious. Nevertheless, it should be considered with caution in the light of some recent work by Vickers and his associates (Vickers & Leary, 1983; Vickers, Leary, & Barnes, 1977). These investigators used a procedure in which critical signal frequency was lowered progressively over watchkeeping periods. Such a procedure systematically inflates the ratio of nonsignal to signal events and, in terms of the ideal observer hypothesis, should produce increased 0 values over time. To the contrary, observers became considerably less conservative as the vigil progressed. Clearly, they do not always resort to an ideal observer strategy, even when the relative frequencies of nonsignals and signals suggest that they should. In addition to these findings, there are two other reasons for considering the ideal observer hypothesis with caution. First, evidence is available to indicate that under some circumstances, perceptual sensitivity (d I ) does indeed decline as time on watch progresses. Thus, the vigilance decrement is not completely a responses bias effect. We shall discuss this evidence in the section to follow. Second, the indiscriminate application of TSD measures to vigilance has been criticized. Among other things, these criticisms have centered upon the violation of TSD assumptions, upon the nonindependence of d ’ and 0, and upon unrealistic values of 0 (Jerison, 1967; Long & Waag, 1981; Taylor, 1967). While these criticisms do not vitiate the application of TSD to vigilance (cf. Craig, 1979; Davies & Parasuraman, 1982; Swets, 1977), they do indicate a need for prudence in its use. The final model we shall consider also features a decision theory approach toward understanding vigilant behavior. It is the elicited observing rate hypothesis proposed by Jerison (1970). This position assumes that during a vigilance task, observers constantly make sequential decisions about whether to emit observing responses toward the source of stimulation to be monitored. Observing responses are treated very broadly in the theory as “unitary attentive acts” which involve some form of message selection by the central nervous system. According to the elicited observing rate hypothesis, detection failures occur when the observer fails to emit an observing response or does so in an imperfect manner-observing is blurred

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or misdirected. The theory also asserts that the effort involved in observing has a definite cost and that decisions to observe or not to observe are based upon their utility, that is, the overall cost of observing relative to the reward involved in detecting a critical signal. In terms of this position, progressive increments in the cost of observing resulting from factors such as fatigue, inhibition, or poor motivation produce a deterioration in the frequency and quality of observing responses over time and thereby bring about the vigilance decrement. In addition to accounting for the decrement, Jerison’s hypothesis offers a way to explain the effects of many of the psychophysical factors which influence sustained attention. With respect to event rate, for example, it asserts that increasing event rate speeds up the pace of observing and adds to task demand. The increased cost of observing leads to a decline in the observer’s willingness to attend t o the task and, therefore, to poor performance efficiency. Along similar lines, factors which reduce the demands of observing would be expected to enhance performance efficiency. Very notable signals and the opportunity to generate veridical expectations about when and where critical signals will appear could be considered to increase the utility of emitted observing responses. Hence, Jerison’s model could also incorporate the effects of signal conspicuity and the temporal and spatial uncertainty of signals described earlier. The elicited observing rate hypothesis is an appealing position with broad explanatory value. A position of this sort, which emphasizes inappropriate observing as the source of detection failures in vigilance, is supported by experiments which demonstrate that the quality of sustained attention is improved when the opportunities to make overt responses incompatible with observing a display to be monitored are minimized. Thus, Warm, Wait, and Loeb (1976) have reported improvement in visual vigilance through the use of head restraint, and Hatfield and Loeb (1968) have shown that techniques which minimize the role of eyeblinks and eye movements also enhance visual monitoring. Still, however, the elicited observing rate hypothesis does not provide a completely satisfactory account of vigilant behavior. Data are available which point to a “looking but not seeing” effect in which signals are missed even when visually fixated (see Warm & Jerison, 1984), and as we have described, vigilance effects are found with auditory tasks in which subjects are closely “coupled” t o the source of stimulation through headphones or a surrounding sound field. In such tasks, simple orienting responses are unlikely to be sources of inappropriate observing. Clearly, “unitary attentive acts” contain covert as well as overt elements, and a more detailed description of the character of “internal” observing responses and the factors which modify their utilities is necessary. Jerison (1970) was aware of

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these limitations and suggested that the character of the observing response may be found in physiological measures. These might include indices such as heart rate variability and evoked cortical potentials (Davies & Parasuraman, 1982) or phasic pupillary responses (Beatty, 1982). It is possible, therefore, that the future may witness an integration of what we have called the bottom-up and top-down views of vigilance.

3. TASK TAXONOMY Our examination of the theories of vigilance reveals that sustained attention probably depends upon a number of complex mechanisms. Each of the six models that were considered can account for part of the data, but none can account for it all, and each position is open to important criticism. Evidently, contemporary understanding of the nature of sustained attention is far from complete, and an acceptable general model of vigilance is not available. These conclusions are echoed in other recent treatments of the theories of vigilance, which advise that a synthesis of current models is an important charge for the future (Craig & Colquhoun, 1975; Davies & Parasuraman, 1982; Loeb & Alluisi, 1984; Warm, 1977). Some recent work by Davies and Parasuraman (1982) on a taxonomic analysis of vigilance tasks provides further evidence of the need for a multistate approach to sustained attention. The essence of their taxonomy is a twofold classification system in which the dimensions are event rate and task type. Event rate is dichotomized as fast (24 or more events/minute) or slow ( < 24 events/minute). Task type involves “successive discrimination” tasks in which subjects must determine if a stimulus differs from a remembered standard (absolute judgment) and a “simultaneous discrimination” task in which all information needed to make a discrimination is present in the stimulus events themselves (differential discrimination). An exhaustive search of the literature and their own empirical research (Parasuraman, 1979) led Davies and Parasuraman to an important regularity in the nature of the vigilance decrement. The decrement reflects a deterioration of perceptual sensitivity (d ’) when a fast event rate is coupled with a successive discrimination task. Under all other combinations of event rate and task type, the vigilance decrement is nonperceptual-it reflects increased caution in responding (increased 0). Clearly, perceptual and decision processes have different weightings in determining the nature of vigilant behavior under different stimulus conditions. In addition to its theoretical implications, Davies and Parasuraman’s taxonomy is a major innovation in the psychophysics of vigilance. It links determinants of sustained attention to the information-processing demands inherent in vigilance tasks and implicates memory as a potentially critical factor in vigilant behavior. It may also foster a reexamination of the em-

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pirical equation for vigilance in terms of the degree to which the effects associated with different stimulus parameters depend upon the need to make successive and simultaneous discriminations.

111. SUSTAINED ATTENTION IN THE MENTALLY RETARDED The study of vigilant behavior in the mentally retarded often involves preadolescent children. Laboratory tasks designed for adults have been modified to accommodate younger observers, and some familiarity with these modified tasks is necessary to understand the research. Accordingly, our discussion of experiments in this section will feature a more detailed description of the stimuli to be monitored than before. Toward this end, we begin the section with an examination of the Continuous Performance Test (CPT), one of the most frequently used vigilance tests with children, particularly in the study of special populations. A description of the CPT is followed by an appraisal of the contemporary literature on developmental changes in watchkeeping performance and the relation between monitoring efficiency and IQ in intellectually normal individuals. These studies provide a useful framework for interpreting findings regarding sustained attention in the mentally retarded, the main topic of this section. A.

The Continuous Performance Test

The CPT was devised originally by Rosvold, Mirsky, Sarason, Bransome, and Beck (1956) to assess brief lapses of attention in brain-damaged individuals. The early version of the test required the visual detection of either the letter “X” or the letter sequence “X following A.” In both parts, critical signals appeared intermittently within a 3 l-letter series. Subjects were tested in two 10-minute periods, receiving the X task in the first period and the AX task in the second. The letters were shown at a rate of one per second, resulting in the presentation of 600 letters during each 10-minute period. Critical signals appeared 160 times in the X task and 120 times in the AX task. Subsequent modifications of the CPT have included an auditory version, the use of digits, colors, or geometric shapes, shorter letter sequences (e.g., 12), longer presentation rates (e.g., one letter per 1.5 seconds), lower proportions of critical signals to neutral events, briefer watchkeeping periods, and an alternate form in which stimulus presentation rate is adapted to the subject’s error rate on a trial-to-trial basis (Buchsbaum & Sostek, 1980). Finally, a potentially important modification has recently been devised in

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which the stimuli are degraded (Nuechterlein, 1983). This is achieved by blurring the figures so as to create a burden for initial stimulus encoding. The purpose of this procedure is to increase the difficulty of the task, which results in higher error rates, thereby permitting a more valid use of signal detection analysis for separating sensitivity from criteria1 components of performance. As compared with standard laboratory vigilance tasks, the CPT and its various versions are of shorter duration overall (ranging from 4 to 20 minutes), involve very high critical signal probabilities (ranging from 15 to 25%), incorporate extremely fast event rates (ranging from 35 to 60 events/ minute), and are sometimes presented in a manner that does not permit an unequivocal assessment of performance changes over time, since the two 10-minute periods confound type of task (X and AX) with watchkeeping periods.

B.

Developmental Changes in Vigilance Performance

1. EARLY CHILDHOOD

Only a few studies are available which probe the quality of sustained attention in young children. The initial experiment along these lines was performed by Locke (1970). He noted that information processing in such children depends heavily upon the input of auditory stimuli. Since sustained attention to these stimuli is necessary for learning-related activities, Locke felt it important to determine if the vigilance performance of youngsters could be measured adequately. In order to accomplish this, he devised a task in which 4-year-olds ( N = 80) were asked to detect the appearance of a brief (1 89 msec) 2000-Hz tone which occurred at irregular intervals ranging from 5 to 25 seconds. The children were instructed to press a button whenever they heard a “beep” during a short vigil which lasted for 5 minutes and 40 seconds. Although the time of watch was considerably less than that typically used with adults, a significant vigilance decrement was obtained, and the frequency of detections varied inversely with the length of the intersignal interval. The latter finding is also characteristic of adult performance (Baker, 1963a). Despite this reasonably successful initial attempt at measuring the maintenance of attention in preschool children, subsequent work did not appear until 10 years after the publication of Locke’s article. Moreover, in none of the later work has this original study ever been cited. Using the visual X version of the CPT, Levy (1980) investigated the monitoring performance of 230 children ranging in age from 3 to 7 years, with the goal of providing normative data concerning early development of the

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ability to sustain attention. Children who were able to complete the 4minute, 40-second task included 27% of the 3- to 3.5-year-olds, 55% of the 3.5- to 4-year-olds, 71% of the 4- to 4.5-year-olds, and 100% of the 4.5to 7-year-olds. Moreover, the number of missed signals (omissions) declined with age in a negatively accelerated fashion, leading the author to conclude that these results provide clear evidence for early development in the capacity for sustaining attention. Unfortunately, the age-related ability to simply discriminate the critical letter X from the neutral letters was totally confounded in this experiment, rendering Levy's interpretation of the decline in omission with age tenuous at best. Somewhat clearer evidence for developmental changes in the vigilant behavior of young children comes from a recent study by Simon (1982), who explored the possibility of using a vigilance procedure as a predictor of school readiness in kindergartners. The task in this study was one in which the youngsters had to report when a brief (.5 second) change occurred in the color of a star located on the fuselage of a cutout jet fighter. Although this task did not result in a decrement in correct detections during a 15minute vigil, there were age-related differences in the overall level of detections. The detection scores for children whose ages exceeded the mean of the sample (5.5 years) were greater than those for children whose ages were below the sample mean. Moreover, the detection scores were correlated significantly with scores on the Metropolitan Readiness Test (r = 38). 2. MIDDLE CHILDHOOD

THROUGH EARLY ADOLESCENCE

To date, the most notable investigation aimed specifically at examining age-related changes in the vigilance performance of school-age children has been conducted by Gale and Lynn (1972). Working with large samples of children 7-13 years old, they administered an auditory monitoring task which required the detection of a digit embedded in a series of letters. The children at each successive age level detected significantly more signals than did their younger counterparts, with the greatest improvement occurring between the ages of 8 and 9. Similar findings have been reported recently by Mohan (1 982). The studies by Gale and Lynn (1972) and Mohan (1982) were carried out with normal children. Their findings have been supported by several experiments whose primary objective was the investigation of attentional deficits in special populations. While the overall level of performance of the children in these populations has generally been poorer than that of their normal controls, the nonnormal children have shown similar age trends as the normals. For example, Anderson, Halcomb, Gordon, and Ozolins (1974) examined the vigilance performance of hyperactive boys in a task in which

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critical signals consisted of a red-green combination of flashing lights. Neutral events were red-red or green-green combinations. They found that 9to 12-year-olds made significantly more correct detections and fewer false alarms than 6- to 8-year-olds. Using the visual and auditory modes of the AX version of the CPT, Sykes, Douglas, and Morgenstern (1973) showed that correct detections were positively correlated with age for hyperactive children and normal controls, ages 5-1 1 years. Age-related improvements in the monitoring performance of hyperactive children have also been reported by Brown (1982). Other investigators have provided evidence for developmental improvement in the vigilant behavior of schizophrenic (Rutschmann, Cornblatt, & Erlenmeyer-Kimling, 1977) and behaviorally deviant children (Kupietz, 1976), and in children with reading difficulties (Kupietz & Richardson, 1978). All of these expriments made use of the CPT. A more complete analysis of vigilance performance in children with cognitive and emotional dysfunctions has been provided by Berch and Kanter (1984).

In summary, the studies described above have demonstrated that it is possible to measure the maintenance of attention in different populations of children across a broad spectrum of ages. Indeed, children as young as 4 years old have been shown to exhibit a decrement which appears very rapidly during a brief vigil. The experiments that we have considered also demonstrate rather clearly that the overall level of performance in vigilance tasks improves with increasing chronological age (CA) from early childhood through early adolescence. C.

Monitoring Efficiency and IQ Level in the Intellectually Normal

Vigilance researchers concerned with the role of individual differences in the monitoring performance of adults have long been interested in determining if the ability to sustain attention is correlated with intelligence, at least within the normal range of IQ scores. Using standardized individual or group intelligence tests, a few investigators have demonstrated a positive correlation between IQ and correct detections (Cahoon, 1970; Kappauf & Powe, 1959). In contrast, the results of the majority of experiments in which intelligence was the primary variable of interest suggest that there is no relationship between IQ and monitoring efficiency (Halcomb & Kirk, 1965; McGrath, 1960; Sipowicz & Baker, 1961; Ware, 1961). As pointed out by Davies and Parasuraman (1982) in their review, this conclusion is supported by ancillary findings from other experiments. The absence of a relation between vigilance performance and IQ has also been reported in studies with nonretarded children, 7 to 15 years old (An-

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derson, Siegel, Fisch, & Wirt, 1969; Gale & Lynn, 1972; Kupietz & Richardson, 1978; Mabel, 1969; Margolis, 1973; Ricks & Mirsky, 1974). The sole exception to this trend is a study by Herman, Kirchner, Streissguth, and Little (1980), who found a significant correlation (r = 3 3 ) between = 3 years, 10 months). correct detections and IQ in very young children However, this study warrants replication because of several experimental problems, including an extremely high false-alarm rate. Recently, Stankov (1983) has offered a novel approach to the relation between IQ and vigilance. He suggests that any correlation between these variables is mediated by the observer’s level of arousal. According to Stankov, the higher one’s general intellectual ability, the lower the arousal level toward the end of a watch (i.e., the smarter you are, the more easily you get bored with a simple task). This hypothesis leads to the prediction that if one were to calculate separate correlation coefficients for each watchkeeping period instead of one gross overall correlation, as is usually done, the relationship between IQ and correct detections should increase with time on task, in a negatively related direction. Drawing on the results of an unpublished dissertation, Stankov presents data to support his contention. The task involved the presentation of strings of audio or visual pulses arranged in groups of 3 to 7 and presented in a random fashion. The critical signal consisted of two successive groups containing the same number of pulses. In addition to participating in the vigil, the subjects (college students) were administered a culture-fair test of general intelligence (the Dominoes Test) along with other psychometric measures. Over the course of four, 15-minute watchkeeping periods, the correlation between IQ and correct detections increased from a value of - .02 for the first period to - .22 (statistically significant) for the last period. This is an intriguing finding that if successfully replicated could lead to a reevaluation of the view that IQ and monitoring efficiency are unrelated in individuals of normal intelligence.

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D.

Vigilance Studies with the Mentally Retarded

The first published study of monitoring performance in mentally retarded individuals was conducted by Ware, Baker, and Sipowicz (1962). Using a brief interruption of a continuous light as a critical signal, these investigators compared the performance of a group of institutionalized mentally retarded adolescents (@ = 58, = 17 years, 8 months) to that of nonretarded controls. Both groups showed similar decrements over a 3-hour vigil, and there were no differences in the overall levels of performance. Evidently, the performance of the mentally retarded matched that of the controls.

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VIGILANCE IN THE MENTALLY RETARDED

Quite a different picture was obtained in a subsequent study by Semmel (1965), who compared the performance of mildly (educably) retarded pre= 12 years, 4 months) with nonretarded controls adolescents (@ = 68, of the same age. Both groups were drawn from the same residential school. Using a task similar to that of Ware et al. (1962), Semmel found that the mentally retarded children detected fewer signals overall, and that they showed a steeper decrement than the controls over the first few periods of watch. This difference could not be attributed to inferior discriminative ability on the part of the mentally retarded group, since they performed as well as the controls under pretest and posttest alerted conditions. Semmel concluded that the steeper decrement exhibited by the mentally retarded reflected a more rapid decay of arousal. The disparity in the outcomes of these two experiments is considerable. One possibility is that it may be due to age differences among the observers. Ware and his co-workers tested adolescents while preadolescents served in Semmel’s investigation. To assess this possibility, Jones (1972) tested mentally retarded = 68) and nonretarded (@ = 110) preadolescents CA = 11 years, 6 months) and adolescents = 17 years, 3 months). Jones also challenged Semmel’s arousal explanation for the poorer vigilance performance of the mentally retarded. He took note of evidence from other areas which suggests that mentally retarded individuals may be more distractible than nonretarded individuals (cf. Denny, 1966; Heal & Johnson, 1970; Spitz, 1963). Consequently, instead of appealing to arousal, Jones suggested that the mentally retarded may be poor watchkeepers because they are susceptible to the intrusion of extraneous environmental stimuli and are therefore likely to miss the transient signals characteristic of vigilance tasks. Jones used a display devised by Smith et al. (1966) in which subjects monitored the alternate blinking of two small lamps, one above the other. A 2-second arrest of alternation (i.e., only one of the lamps continued to flash) was the critical signal for detection. The observers performed the task under two conditions of auditory stimulation: (1) continuous white noise, or (2) a background of variety-audio stimulation (music, drama, etc.). Jones reasoned that if the mentally retarded subjects were underaroused relative to nonretarded subjects, they would benefit more than the nonretarded from the added stimulation of the variety-audio condition. Conversely, if differences between nonretarded and mentally retarded observers were due to greater distractibility on the part of the retarded, such differences should be exacerbated by the variety-audio stimulation. Regardless of type of audio stimulation, the mentally retarded preadolescents detected significantly fewer signals than their nonretarded peers. Mentally retarded adolescents were inferior to nonretarded adolescents, but only

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Joel S. Warm and Daniel B. Berch

under the condition of variety-audio stimulation. Jones concluded that chronological age can be a critical factor affecting performance differences between mentally retarded and nonretarded persons on a vigilance task, at least under specific background conditions. Stated in another way, the degree of monitoring efficiency exhibited by mentally retarded individuals is at least in part a function of their mental age. An additional major finding was that performance was poorer overall under variety-audio stimulation than under white noise. This was taken as strong evidence against an arousal interpretation of performance differences between mentally retarded and nonretarded individuals and, concomitantly, as equally strong evidence in support of a distractibility explanation. Kirby, Nettelbeck, and Thomas (1979) have recently arrived at the same conclusion as Jones (1972) regarding the role of mental age in the vigilance performance of the mentally retarded. Using a visual task in which observers had to detect an increment in the brightness of a circular patch of red light, these investigators compared the monitoring efficiency of mentally retarded children (IQ = 68, = 13 years) with that of a chronological age control group (C&= 12 years, 9 months) and a mental age control group (CA = 7 years, 9 months). In relation to the chronological age controls, the mentally retarded children produced fewer correct detections overall and exhibited an earlier and more rapid decline in performance over time. By contrast, the overall frequency of detections for the mentally retarded was similar to that of the mental age controls, and both groups exhibited comparable decrements during the course of the vigil. On the basis of these findings, Kirby et al. concluded that the extent to which attention is maintained by mildly mentally retarded individuals in a monitoring task is primarily due to developmental factors. These authors also suggested, however, that retarded individuals may suffer from an intellectual deficit with regard to their discriminative ability in a vigilance situation. They based this conclusion on the finding of more false alarms for the mentally retarded group than the mental age controls. Finally, as the authors note, the results of this experiment become even more striking when one takes into account (1) the relatively high IQ range of the mentally retarded subjects(53 t o 81), and (2) that almost equal numbers of mental age control (10) and retarded (8) children (but no chronological age controls) were excluded from participation in the vigil due to their failure to pass a pretest criterion demonstrating an ability t o discriminate the critical signal intensity from that of a neutral event. The general findings that older mentally retarded persons do not differ significantly from nonretarded controls and that mentally retarded preadolescents perform more poorly than their chronological age controls, but do not differ significantly from their mental age controls, have also been reported by Kirby, Nettelbeck, and Bullock (1978) and by Johnson (1977), respectively.

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From the evidence presented thus far, it would seem that the quality of sustained attention in the mentally retarded generally increases with chronological age, just as it does in nonretarded individuals. While the rate of development seems to be slower in the mentally retarded, by the time they reach late adolescence they apparently achieve parity with their nonretarded peers, at least in situations requiring simple monitoring functions. The factors which account for early differences between mentally retarded and nonretarded children are not well understood. Semmel’s (1965) arousal explanation is rather naive by current standards because it fails t o distinguish between multistate levels of activation. Besides, as we have seen, it has not been supported in an experimental test (Jones, 1972). The early experiment by Jones (1972) found that a distraction explanation was promising, and a view of this sort is strengthened by two additional lines of evidence. One of these is Fuller’s (1975) report that mentally retarded preadolescents (m = 10.9 years) are more likely than nonretarded children = 10.6 years) to engage in what Jerison (1970) has termed of similar age “distracted” or “misdirected” observing activities while performing a vigilance task. These include rhythmic body movements, hand clapping, looking away from the display to be monitored, and restless twisting and turning. The second line of evidence comes from a visual reaction time study by Krupski (1977) which indicated that the slower reaction times on the part of mentally retarded in comparison to nonretarded individuals may be based, in part, on a greater degree of off-task glancing by the mentally retarded. Taken together, these two studies suggest that from a broader perspective, Jerison’s (1970) elicited observing response model may incorporate the distractibility hypothesis in accounting for the vigilance performance of mentally retarded preadolescents. Although the distractibility or elicited observing view of vigilance differences between mentally retarded and nonretarded individuals is appealing, there are data to challenge it. In an extensive study, Crosby (1972) compared the performance of mentally retarded subjects with that of mental age and chronological age controls on the visual X and AX versions of the CPT under conditions designed to distract the observers. The distracting stimuli were letters that differed from the CPT letters with which they were paired. The distractors were presented visually (alongside, but in a different color than the CPT stimuli), acoustically (via headphones), or in a combined audiovisual manner. As might be anticipated, the extra letters had a detrimental effect upon performance. However, contrary to expectations based upon the distractibility hypothesis, they did not have a differential effect upon the mentally retarded and nonretarded subjects. Crosby was careful to note that there was a wide range of individual differences in response to the various distractors. Some subjects were affected adversely by the least effective distractor (auditory), while the per-

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formance of others was bolstered by the most effective distractor (audiovisual). Accordingly, Crosby suggested that distractibility in retarded persons might be idiosyncratic and situation specific. Along these lines, Johnson (1977), also using the CPT, has shown that the manifestation of distractibility in mentally retarded preadolescents may depend upon the presence of identifiable brain damage. In this study, brain-damaged mentally retarded preadolescents were more adversely affected by distracting stimuli than their mental age controls, but the non-brain-damaged mentally retarded were not. In view of all of this, what can be said of the distractibility hypothesis? About all that can be said is that the major studies by Crosby (1972), Johnson (1977), and Jones (1972) differ in terms of several methodological features, and a final decision regarding the viability of this hypothesis must await further investigation. Still another approach toward explaining vigilance differences among mentally retarded and nonretarded individuals has been taken by Das (1970) and Das and Bower (1971). These investigators approached the problem in terms of J. F. Mackworth’s (1968, 1969) habituation model and suggested that the mentally retarded habituate more rapidly to stimuli in a vigilance experiment than do nonretarded persons. To test this idea, Das and Bower (1971) employed an auditory task in which children listened to the repetitive presentation of familiar words. The critical signal was the word “man.” Although mentally retarded children were less accurate than nonretarded children of comparable chronological age in detecting signals, the rate of habituation to signal words, as measured by the GSR, was similar for both groups. Thus, a differential rate of habituation does not seem to be a factor in the vigilance performance of nonretarded and retarded subjects. The bulk of the research on sustained attention in the mentally retarded has been concerned with assessing the efficiency of these individuals relative to that of their normally intelligent peers. There have been some studies along other lines, however. These experiments address the issue of training for vigilance in the mentally retarded. The chief vehicle for such training has been KR. The use of KR in vigilance studies with the mentally retarded was introduced by Ware et al. (1962), who found that KR for missed signals enhanced the detections of nonretarded and mentally retarded observers in a similar way. Griffin, Perryman, Landers, and Patterson (1 974) confirmed that the mentally retarded can profit from KR in a vigilance task and showed that KR in the form of information about correct detections can be useful. These investigators studied a sample of institutionalized mildly and moderately retarded subjects (@ = 47; range 33-73) ranging in chronological age from 14 to 22 years. The subjects monitored a three-section split screen. Flashes of light in the two side panels were neutral events, while a flash in

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the center panel was the critical signal for detection. Knowledge of results for correct detections was provided by the illumination of a pilot lamp located immediately above the display. Subjects who received KR had a significantly higher detection rate than those who did not. This same research group has continued to expand their study of feedback by exploring the effects of various specialized training methods. Perryman, Halcomb, and Landers (1981) have developed a multidimensional technique that has been quite effective with a group of mentally retarded = 18 years). The program features young adults (@ = 56, range 39-69, multiple practice sessions, KR for correct detections and false alarms, response prompting, verbal praise, and token and tangible reinforcers. Using the three-section split-screen display described above, these investigators found that the training procedure improved overall detections from a mean of 43% to one of 83%, and that it virtually eliminated the vigilance decrement. Unfortunately, a recent study by B. Locke et al. (1982), using the same task and training package as Perryman et al. (1981), has been unable to confirm that the training procedure eliminates the decrement. Evidently, techniques for enhancing vigilance performance in the mentally retarded through training are only in the initial stages of development. In addition to its implications for training, the study by B. Locke and co-workers addresses still another important issue in the psychology of vigilance with the mentally retarded. A postexperimental interview in that study revealed that a majority of the subjects considered the monitoring activity more boring, or otherwise unenjoyable, than interesting. As the authors note, these self-reports cast serious doubt on the notion that the type of monitoring activity one engages in during the vigilance task may be agreeable or even challenging to mentally retarded individuals (Kohn, 1977). IV.

SUMMARY AND CONCLUSIONS

Vigilance studies with the mentally retarded have been carried out primarily for one of three reasons: (1) to demonstrate that they may be relatively good monitors because the simple, repetitive nature of watchkeeping tasks could prove more appealing than boring to these individuals; (2) to determine if the mentally retarded suffer from an “attentional deficit”; and (3) to test various theoretical views of the greater inattentiveness that purportedly is characteristic of the retarded. With regard to the first reason, the evidence to date indicates that mildly mentally retarded individuals d o not outperform their nonretarded counterparts on laboratory vigilance tasks, nor do they even perform comparably to chronological age controls before reaching late adolescence. Moreover, self-report data clearly contra-

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dict the notion that the mentally retarded view monitoring situations as agreeable or challenging. Although the notion of “dull minds for dull jobs” can be easily laid to rest, the issue of an attentional deficit in the mentally retarded is more difficult to evaluate. In doing so, it is necessary to specify precisely what constitutes a deficit. According to Krupski (1980), the concept of an attentional deficit implies a trait-like characteristic involving qualitative and quantitative differences stemming from neurological damage. Similarly, Stanovich (1978) has pointed out that the nature of a deficit implies a permanent defect. Stanovich has also pointed out that methodological problems associated with the choice of appropriate comparison groups which control for all relevant extraneous variables make it necessary to use extreme caution in attributing deficits to mentally retarded populations. Along these lines, the poorer monitoring efficiency of mentally retarded preadolescents in comparison to their chronological age controls is qualified by the fact that these individuals do not differ from their mental age controls. Thus, from Stanovich’s view, differences between the mentally retarded and their chronological age controls should be interpreted as reflecting, at worst, a slower rate of development and not an attentional deficit in the sense described above. This conclusion is bolstered by findings showing that by late adolescence the mentally retarded perform as well as their chronological age controls, and by the fact that the vigilance performance of the mentally retarded can be improved through training. Krupski (1980) has suggested that for both theoretical and remedial purposes, an “interactionist” view would be a more appropriate way to approach the study of attentional problems in special populations than the classic deficit approach [see Keogh & Margolis (1976) for a similar discussion]. According to the interactionist approach, inattention is due to the interaction of child characteristics (e.g., IQ, diagnostic category) and task demands. Krupski argues that by adopting such an approach, research efforts would focus upon the identification of the specific conditions in which attentional difficulties would manifest themselves.’ An approach of this sort has potentially important implications for research on sustained attention in the mentally retarded. Even a cursory comparison of such research with the rich variety of investigations discussed in Section I1 will reveal that the experiments with the mentally retarded have been somewhat simplistic and superficial-they have neglected many of the important psychophysical parameters which affect vigilant behavior. Thus, ‘It should be noted that the ability x treatment interaction approach is not without its own difficulties. These involve methodological issues related to matters such as floor and ceiling effects (Baumeister, 1967) and task difficulty and reliability (Chapman & Chapman, 1973, 1974), as well as other problems (Baron & Treiman, 1980; Detterman, 1979).

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while contemporary research has identified some conditions in which mentally retarded observers differ from nonretarded observers, it may have missed subtle differences in other cases. The identities between mentally retarded and nonretarded monitors in these cases (i.e., those involving mentally retarded preadolescents and their mental age controls and mentally retarded adolescents and their chronological age controls) may be more apparent than real! One way to find out would be to test the resilience of these purported identities under more sophisticated experimental conditions. A research program designed to explore more carefully the circumstances under which the vigilant behavior of mentally retarded and nonretarded monitors may differ might involve comparisons between the mentally retarded and their chronological age and mental age controls in terms of (1) hemispheric specialization for vigilance, (2) the integration of redundant information from different sensory channels, (3) the formation of veridical temporal and spatial expectancies, (4) the ability to cope with the increased capacity demands of tasks featuring fast event rates, and ( 5 ) the ability to handle increasing sensory and/or cognitive complexity. In addition, the degree to which differences between mentally retarded and nonretarded observers depend upon the requirement for simultaneous or successive discriminations (Davies & Parasuraman, 1982) remains to be examined. Research along these lines could provide more convincing evidence for the existence of similarities between these populations of observers in vigilance. It could also delineate more precisely those conditions in which the monitoring performance of the mentally retarded differs from that of the nonretarded. Theoretical efforts to account for differences between mentally retarded and nonretarded persons in vigilance have focused primarily upon two positions: arousal and distraction. The data seem to favor a distractibility or elicited observing view, but this position is also open to several challenges. Once again, it seems that research into the vigilant behavior of the mentally retarded has been too parochial-it has not come to grips with broader elements in the study of vigilance. As we have seen, vigilance is a complex form of behavior, and it probably depends upon several mechanisms. The roles played by information-processing mechanisms such as expectancy formation, probability matching, and decision making (criteria1 adjustment) in the vigilance performance of the mentally retarded remain unexplored. This would seem to be a potentially fruitful line of investigation in view of studies in other areas indicating that difficulties encountered by mentally retarded invididuals often stem from inappropriate strategies in handling information (cf. Bray, 1979; Spitz, 1966, 1969). In terms of theory, however, it is worth noting that efforts to explain the performance of the men-

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tally retarded in a vigilance task are hampered by the absence of a completely acceptable theory of vigilance in nonretarded persons. Thus, an adequate account of the vigilance behavior of the mentally retarded, and other special populations as well, may need to await more general theoretical developments. ACKNOWLEDGMENTS This project was supported by Grant MCJ-000-912-18-0, awarded by the Bureau of Health Care Delivery and Assistance, Division of Maternal and Child Health, Health Resources and Services Administration, Public Health Service, DHHS, and Grant OSDD0502/02, awarded by Region V, ADD, OHDS, DHHS. We are indebted to Dr. Raja Parasuraman for helpful comments on an earlier version of the manuscript and to Mary Lynne Dittmar and Teri A. Role for technical assistance.

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