Probing awareness during sleep with the auditory odd-ball paradigm.

During the waking state, a late component of the auditory event-related potential, P300, is elicited when subjects detect a rare 'target' stimulus. It is usually not elicited when subjects either ignore or fail to detect the stimulus. The presence of P300 is therefore thought to reflect conscious processing of the stimulus. Since P300 has been shown to be an attention-dependent cognitive component in wakefulness, one might suppose that it would be absent during sleep-a time in which information processing of external stimuli is commonly thought to be inhibited. This review examines the presence or absence of P300 in studies employing auditory odd-ball paradigms in sleep. Research to date indicates that P300 can be recorded during the transition to sleep and then reappears in REM sleep. Stimuli that are rare and intrusive are more likely to elicit the classic parietal P300 in REM sleep. There is, however, little or no positivity at frontal sites. This is consistent with brain imaging studies that show frontal deactivation is characteristic of REM sleep. These findings indicate that while sleepers may be able to detect stimulus deviance in stage 1 and REM, the frontal contribution to consciousness may be lost. In non-REM sleep, a later positive wave at 450 ms does not vary according to experimental manipulation in the same way as the waking and REM P300s. This non-REM sleep-related positivity may therefore underlie mechanism distinct from the waking P300.