Evoked potentials as a tool for the investigation of human sleep

This review summarizes studies of evoked potentials (EPs) applied to the investigation of human sleep and of sleep disorders. The first part is devoted to studies dealing with the nature, mechanisms and extent of information processing during sleep. EP studies suggest that the brain's ability to detect salient stimuli persists during even the deepest sleep stages, while discrimination of the stimulus' intrinsic significance and/or semantic content may persist only in stage II and paradoxical sleep (PS). Deviance detection in non-rapid eye movement (NREM) sleep is reflected by amplitude recovery of K-complexes elicited by stimulations that differ from the background. The evoked K-complex appears to be formed by two functionally different modules. The first may be related to the discrimination of relevant information. The second appears more sensitive to stimulus salience than to its intrinsic significance. In PS, the EP signs of stimulus discrimination are similar to those observed during waking. Thus, if the possibilities of information processing are fairly similar during stage II and PS, their respective neural mechanisms are not the same, as judged by their electrophysiological counterparts. The second part of the paper reviews clinical application of EPs to the study of sleep/wake disorders. While early potentials are of little use for diagnosis of sleep disorders, the cognitive P300 may help to quantify cognitive slowing and pathological sleepiness. However, intersubject variability restrains the use of these techniques in individual patients. A promising approach is the utilization of late responses to the study of sleep inertia with the use of "forced awakening" recording paradigms.