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Article Excerpt
Vigilance or sustained attention tasks typically require observers to monitor displays over extended periods, and to execute overt detection responses to the appearance of low probability critical signals. The signals are usually clearly perceivable when observers are alerted to them but are not compelling changes in the observers' operating environment. In addition, the signals are usually embedded in a context of recurrent non-signal (neutral) events, which, unlike signals, require no overt response from observers (Davies & Parasuraman, 1982; Warm, 1984; Warm & Jerison, 1984). Vigilance tasks and the processes that influence their performance are of interest because of the insights they provide into the factors that control attention (Broadbent, 1971). They are also of interests given the vital role that vigilance plays in automated human-machine systems in transportation, process and quality control, medicine, and, of special interest here, baggage inspection at airport security checkpoints (Hancock & Hart, 2002; Wickens & Hollands, 2000).
In a recent series of studies, Robertson and his colleagues (Manly, Robertson, Galloway, & Hawkins, 1999; Robertson et al., 1997) have argued that detection failures in vigilance tasks are due to gross inattention or 'mindlessness' on the part of observers. According to this perspective, when observers are confronted with the need to respond to infrequent signals separated by long intervals, as in the case of vigilance, a supervisory attentional system (Norman & Shallice, 1986; Shallice, 1988; Stuss, Shallice, Alexander, & Piction, 1995) loses its potency and observers cease to focus their awareness on the task concerned. Instead, they approach their assignment in a thoughtless, routinized manner, characterized by the withdrawal of effortful attention from the task. This approach reflects an endogenous modulation of attention rather than the decline in wakefulness and vigour accompanying lowered arousal (Dickman, 2002).
Based upon their conception, Robertson and his associates introduced a modification to the standard vigilance paradigm designed to more rapidly elicit this mindless state. Observers are required to respond overtly to non-signals and to withhold overt responses to signals. Presumably, detection failures in the modified vigil can be attributed to routinization, automaticity, and lapses of attentional focus, all features of 'mindlessness' (Laberge, 1995; Langer, 1989) generated by uniform, repetitive responding to more numerous non-signal events. The research of Robertson and colleagues supports the general role of mindlessness in vigilance based on their finding using the modified task that absent-minded individuals, defined by high scores on the Cognitive Failures Questionnaire (CFQ; Broadbent, Cooper, Fitzgerald, & Parkes, 1982), do more poorly on this task than those who have low scores on the CFQ.
However, the model proposed by Robertson and his associates has been challenged by a number of studies that indicate that, although tedious, vigilance tasks impose a substantial mental burden upon observers, as reflected in elevated scores on the NASA Task Load Index (NASA-TLX), an instrument which provides a reliable measure of the perceived mental workload incurred in performing a task (Hart & Staveland, 1988; Wickens & Hollands, 2000). The workload scores for vigilance are greater than those typically obtained in several other types of tasks, including time estimation, grammatical reasoning, and simple tracking, and the workload of sustained attention has been shown to be rooted in the information-processing demands of the vigilance task itself, rather than being a consequence of combating the boredom associated with the task (Hitchcock, Dember, Warm, Moroney, & See, 1999; Warm, Dember, & Hancock, 1996). Moreover, contrary to expectations derived from the mindlessness model, absentminded individuals (those with high CFQ scores) perform as well on a traditional vigilance task as non-absent-minded individuals (those with low scores on the CFQ) but rate the task as more mentally demanding on the NASA-TLX than do non-absent-minded individuals (Grubb et al., 1994). Such results are more consistent with the notion that performance failures in vigilance are a consequence of a depletion in information-processing resources rather than from a thoughtless approach to the task (Davies & Parasuraman, 1982; Matthews, Davies, Westerman, & Stammers, 2000; Warm & Dember, 1998).
In addition to imposing a substantial workload upon individuals, vigilance tasks can also be highly stressful, as reflected in observers' self-reports of negative mood shifts after participating in a vigil (Hancock & Warm, 1989; Helton, Dember, Warm, & Matthews, 1999; Szalma et al., 2004; Temple et al., 2000; Warm, 1993). Given current transactional models of stress, in which stress is viewed as arising from individuals' appraisal of their environment as taxing or exceeding their coping resources (Kemeny, 2003; Matthews, 2001), these results are also consistent with the view that detection failures in vigilance reflect limitations in effortful attention rather than mindlessness. Of particular importance to the Robertson model is a recent report by Grier et al. (2003) indicating that a high workload/high stress profile typifies observers' reactions to the Robertson group's modified vigilance task as well as to the more traditional vigilance tasks. In that study, workload was measured by the NASA-TLX while task-induced stress was measured via the Task engagement and distress scales of the Dundee Stress State Questionnaire (DSSQ; Matthews et al., 1999), a well-validated instrument for assessing transient states associated with mood, arousal, and fatigue (Matthews et al., 2002). The DSSQ is designed to reflect the affective, motivational, and cognitive aspects of stress via three factor-analytically derived scales, Task engagement, Distress, and Worry. Task engagement contrasts enthusiasm and interest with fatigue and apathy. The Distress factor reflects tension, feelings of unhappiness, and low confidence, while the Worry factor encompasses cognitive interference arising from task-related and personal concerns, low self-esteem, and self-focus of attention.
Although subjective reports of high workload and stress provide evidence against the role of mindlessness in the standard vigilance paradigm and also in the Robertson modification, they do so indirectly by measuring the cognitive and emotional consequences of the information-processing load imposed by the task. Consequently, although the subjective reports of observers in both the standard and modified vigilance tasks indicate that the observers found the tasks to be difficult, this...
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