Missed signals and the vigilance decrement: an information processing approach

In visual detection tasks that require sustained attention, missed signals represent an ever-present danger due to the potential consequences of detection failure. Despite its relative importance, the cause of missed signals has not been clearly established. Most empirical data on vigilance performance has been concerned with the detection of signals. There are two explanations for why observers miss signals. One is that the observers do not perceive the information when it is presented. A second possible explanation is that the information is perceived, but the information is not translated into a correct response. Traditional vigilance paradigms have failed to show whether any information regarding missed signals is encoded into memory. In the present study, two vigilance tasks were used that required observers to make successive discriminations on verbal stimuli to at least an orthographic level to determine whether an event was a signal. The task event rates were either slow (15 events/minute) or fast (30 events/minute). To determine what information regarding missed signals was encoded into memory, observer frequency estimates, recognition, and recall of signal items from the vigilance task were evaluated. Results from the vigilance tasks support the general findings of the differential effect of background event rate. In the fast-event-rate task, response latency was constant, detection performance declined substantially over time, and false alarms decreased with time on task. In the slow-event-rate task, observers showed increasingly longer response times with time on task, but maintained a high correct detection rate. False alarms decreased with time on task. The data from the memory tests suggest that observers can recognize detected signals (hits) at a greater-than-chance proficiency. Observers did not show recognition for missed signals. Free recall was poor for any information in the vigilance task. Frequency of occurrence estimates were not accurate, and did not support any general conclusion regarding frequency processing. The results from both the vigilance and memory portions of the present study are explained by a limited capacity, control processing model of human information processing.