<P> Stressful activities involve continuous application of extensive cognitive resources . If the vigilance decrement were the result of less brain activity rather than more, vigilance tasks could not be expected to be stressful . High levels of epinephrine and norepinephrine are correlated with continuous extensive mental workloads, making these compounds good chemical indicators of stress levels . Subjects performing vigilance tasks exhibit elevated levels of epinephrine and norepinephrine, consistent with high stress levels and indicative of a significant mental workload . Vigilance tasks may therefore be assumed to be stressful, hard mental work . </P> <P> Large individual differences in monitoring task performance have been reported in a number of vigilance studies . For a given task, however, the vigilance decrement between subjects is generally consistent over time, such that individuals exhibiting relatively higher levels of performance for a given task maintain that level of performance over time . For different tasks, however, individual performance differences are not consistent for any one individual may not correlate well from one task to another . An individual exhibiting no significant decrement while performing a counting monitoring task may exhibit a significant decrement during a clock test . Relative performance between subjects may also vary based on the nature of the task . For example, subjects whose task performance is well correlated for a successive task may exhibit a poor performance correlation for a simultaneous task . Conversely, subjects performing similar monitoring tasks, such as radar versus sonar target detection, can be expected to exhibit similar patterns of task performance . </P> <P> Levine et al. propose that individual differences in task performance may be influenced by task demands . For example, some tasks may require rapid comparisons or "perceptual speed", while others may require "flexibility of closure", such as detection of some predefined object within a cluttered scene . Linking task performance differences to task demands is consistent with the Vigilance Taxonomy proposed by Parasuraman and Davies described above, and also supports the hypothesis that vigilance requires mental work, rather than being a passive activity . </P> <P> Considerable research has been devoted to the reduction of the vigilance decrement . As noted above, the addition of non-target signals can improve task performance over time if the signals are similar to the target signals . Additionally, practice, performance feedback, amphetamines and rest are believed to moderate temporal performance decline without reducing sensitivity . </P>

According to research the greatest gains in vigilance occur