Target Set Size, Time Sharing and Vigilance Performance

Abstract

The empirical literature describing the effect of number of signals (target set size) and number of signal sources to be monitored simultaneously (time sharing) on visual vigilance performance is reviewed. It is concluded that greater target sets and time sharing generally result in lower vigilance performance but that these effects are probably mediated through different information processing mechanisms. Moreover, the classical vigilance decrement was oftentimes found not to occur under time sharing conditions. A possible explanation of this phenomenon in information processing terms is offered. In order to test these generalizations a visual vigilance experiment was set up in which subjects had to monitor one or two dials for the occurrence of one or two kinds of critical signals. Results show that target set size had the expected deteriorating effect on hit rate and reaction time. However, time sharing only affected reaction time (longer) and did not interact with number of signals or time on task with respect to hit rate, sensitivity, false alarm rate or R:S ratio. The latter measures did not change over time. However, reaction times showed a signficant improvement (shorter) over time under time sharing conditions. In the discussion of these results issues like power of the design, learning effects, and signal conspicuity are addressed. Recommendations are given for future research.