Event-related brain potentials to changes in the acoustic environment during sleep and sleepiness

Abstract

The experiments in this study were designed to examine the processing of auditory stimuli by young human adults during sleep and sleepiness. Event-related brain potentials (ERPs) were used to indicate the stimulus processing. The focus was on an ERP response termed mismatch negativity (MMN), which has been thought to represent a pre-attentive response to a change in the acoustic environment. Experiment I suggested that MMN occurs as a response to a pitch change (20%) during stage 2 sleep, but only when the pitch deviation also elicits a K-complex. Experiment II showed that the appearance of a K-complex to a pitch change is preceded by an increase in the susceptibility of the brain to the pitch change and to the standard tone that immediately precedes the pitch change. The MMN-like deflection appeared in association with the elicitation of the K-complex when the pitch deviation was small (10%) but not when it was large (100%). Study III showed a P3b-type wave to a large pitch change (100%) during tonic, but not phasic rapid eye movement (REM) sleep. No MMN appeared during REM sleep. Experiment IV demonstrated that the MMN recorded under optimal stimulus conditions was attenuated even before actual sleep, during sleepiness. The results suggest that i) traditionally categorised sleep stages contain microstates and substages which systematically differ in terms of the processing of auditory events and ii) the highly automatic processing of an auditory stimulus change indicated by MMN declines already during sleepiness, even though the elicitation of full-amplitude MMN may be possible during some microstates of sleep.