Observers' perceptual sensitivity to critical target events can deteriorate when they must remain alert for prolonged periods. The dominant characterization of this sensitivity decrement has been the R. Parasuraman and D. R. Davies (1977) taxonomy of vigilance, which attributes the decline in perceptual ability to the type of discrimination (successive-absolute vs. simultaneous-comparative judgment) and the event rate (rate of stimulus presentation) required for task completion. According to the model, sensitivity decrements occur chiefly in tasks that couple successive discrimination with a high event rate. This meta-analysis of 42 vigilance studies attempted to refine the taxonomy by identifying other task characteristics related to the sensitivity decrement. The analysis confirmed that the magnitude of this sensitivity decrement is substantial and that it is a function of the type of discrimination and the event rate, but it also revealed that the current taxonomy should be revised to incorporate a new dimension: the sensory-cognitive distinction.Vigilance or sustained attention refers to the ability of observers to maintain their focus of awareness and remain alert to stimuli for prolonged periods of time (Davies & Parasuraman, 1982;. The systematic study of vigilance had its origins in signal detection problems during World War II. After only about 30 min. on watch, airborne British radar observers began to miss the blips of light on their displays that indicated the presence of enemy submarines in the sea below. The Royal Air Force commissioned Norman H. Mackworth (1948Mackworth ( ,1950Mackworth ( / 1961 to study the problem in his laboratory. His investigations were to mark the beginning of over four decades of research aimed at discovering the factors that might account for what has come to be termed the vigilance decrement, or the decline in performance efficiency that occurs over time. Vigilance DecrementThe vigilance decrement has been replicated consistently since N. H. Mackworth's initial research, and it continues to be the most ubiquitous finding in vigilance experiments (Davies & Parasuraman, 1982;. The decline in performance is typically complete 20 to 35 min. into the session, and at least half of the final loss occurs during the initial 15 min. of the watch (Teichner, 1974). The decrement can, however, appear even earlier under particularly demanding circumstances. In these cases, Jerison (1963) andNuechterlein, Parasuraman, andJiang (1983) discovered that the decrement in performance may begin within the first few minutes of the session. As Dember and Warm (1979) pointed out, the most salient aspect of the decrement function is that it seems to stem simply from the necessity of looking or listening for a relatively infrequent signal for a prolonged period of time.

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Robertson, Manly, Andrade, Baddeley, and Yiend (1997) proposed that the decline in performance efficiency over time in vigilance tasks (the vigilance decrement) is characterized by "mindlessness" or a withdrawal of attentional effort from the monitoring assignment. We assessed that proposal using measures of perceived mental workload (NASA-TLX) and stress (Dundee Stress State Questionnaire). Two types of vigilance task were employed: a traditional version, wherein observers made button-press responses to signify detection of rarely occurring critical signals, and a modified version, developed by Robertson et al. to promote mindlessness via routinization, wherein button-press responses acknowledged frequently occurring neutral stimulus events and response withholding signified critical signal detection. The vigilance decrement was observed in both tasks, and both tasks generated equally elevated levels of workload and stress, the latter including cognitions relating to performance adequacy. Vigilance performance seems better characterized by effortful attention (mindfulness) than by mindlessness. Actual or potential applications of this research include procedures to reduce the information-processing demand imposed by vigilance tasks and the stress associated with such tasks.

In 2 experiments, a 12-min computerized vigilance task was demonstrated to reproduce the vigilance decrement, high workload (NASA-TLX), and stressful character (Dundee Stress State Questionnaire) of vigilance tasks lasting 30 min or more. In Experiment 1, the abbreviated task was also shown to duplicate the signal salience effect, a major finding associated with long-duration vigilance tasks. Moreover, Experiment 2 showed that performance on the abbreviated task can be enhanced by caffeine - a drug that benefits long-duration tasks. This enhancement effect was limited to performance, however, suggesting that caffeine influences factors that control signal detection but not those that control task-induced stress. The results parallel those obtained with long-duration tasks and support a resource-depletion model of the vigilance decrement. The abbreviated task might be useful in situations in which long-duration tasks are precluded (e.g., performance assessment batteries, neuropsychological testing, and brain imaging).

Testing hypotheses derived from neuropsychological models of mood, as well as the association of mood states and personality characteristics with global-local visual processing, were examined. Fifty-nine men completed measures associated with depression and positive mood, and were administered a brief perceptual judgment task that assessed global-local visual processing biases. Additionally, 19 of these 59 subjects were administered measures of anxiety and optimism-pessimism and completed an expanded judgment task. Affective and personality variables were then correlated with judgment task performances. Consistent with predictions, positive mood and optimism were directly associated with a global bias and inversely related to a local bias. A converse pattern of findings was obtained with depression and trait anxiety. Implications for research concerning other aspects of visual processing are discussed.