Impaired vigilance and performance decrements accompany sleepiness and an increase in response to sleep deprivation. Inattention, cognitive slowing, and lapses represent manifestations of sleepiness. Experimenter-paced, monotonous, timekeeping tasks with low target presentation rates generally fall into the category of "vigilance tests." Tests with higher throughput and greater intrinsic stimulation are classed as "performance tests." Vigilance tests do not require much skill, are less sensitive to educational level, and have little in the way of learning curves compared to performance tests. Landmark studies of sleep deprivation pioneered the use of vigilance and performance testing to assess sleepiness (39,40). These studies are collectively known as the Walter Reed experiments because they were conducted at Walter Reed Army Hospital. Results indicated that time-on-task, response slowing, and response lapsing were essential factors in sustained attention tasks [for an excellent review, see Dinges and Kribbs (41)]. Several vigilance and performance tests are in standard use in sleepiness research (42-44). Perhaps, the most popular is the psychomotor vigilance test (PVT) developed by David Dinges et al. at the University of Pennsylvania (45,46). This hand-held device measures response speed and accuracy in a signal detection algorithm. The PVT has been cross-validated with the SSS and MSLT and data are reported from normal controls, sleep-deprived individuals, and sleep disorder patient groups. Another test system that has been used in several studies is the Oxford sleep resistance (OSLER) test (47,48). The OSLER test consists of four 40-minute-long trials during which there are multiple signal presentations. The subject is instructed to respond to each signal with a simple button press. Trials are ended after 40 minutes or after a failure to respond (which is thought to indicate sleep onset).
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