Diurnal Variation, Task Characteristics, and Vigilance Performance

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Abstract
Three experiments consider the effects of several task features along with the time of day when vigilance was tested. The first experiment found that detections on the Bakan task depended on event rate and signal frequency and that the adverse effects of lowering signal frequency were more marked in the afternoon than in the morning. The second experiment found that the typical sensitivity decrement on a brief, rapidly paced successive discrimination task occurred earlier when the stimuli were degraded—especially in the early evening as compared with the morning—and that a riskier criterion placement was adopted in the evening. The third experiment used an unpaced simultaneous discrimination task with equiprobable stimulus events. Efficiency was not stable but declined as time passed, while speed increased. These changes tended to be more apparent in the afternoon, and perceptual efficiency seemed genuinely lower than in the morning. The implications for interpretations of vigilance and time-or-day effects are discussed.