AUDITORY EVENT-RELATED POTENTIALS TO DEVIANT STIMULI DURING DROWSINESS AND STAGE-2 SLEEP

Twelve subjects were tested using a 3-tone auditory oddball paradigm c onsisting of a standard 1000 Hz tone (P = 80%) and two deviants, namel y, a 1200 Hz tone and a 2000 Hz tone (both P = 10%). Testing took plac e in 3 conditions: (1) attend, in which the subject had to count one o f the deviant tones; (2) ignore, in which the subject read a book; and (3) sleep, in which the subject was encouraged to go to sleep during presentation of the tones. In the awake conditions stimulus deviance e licited mismatch negativity (MMN) and P-3. During drowsiness, no separ ate mismatch negativity (MMN) could be detected, but the 2000 Hz tone evoked a broad fronto-central early negative deflection, suggesting an overlap of N-1 and MMN. In the same condition, P210, N330 and P430 ap peared, all being sensitive to magnitude of deviance. During stage 2, the P210, N330 and P430 amplitudes increased, most notably to the larg e deviant. These data indicate that differential processing of auditor y inputs is maintained during drowsiness and stage 2 sleep, but do not support the notion that MMN or P-3 activity comparable to the waking state occurs to oddball stimuli during this stage. It is hypothesised that during light sleep, scanning of the environment is performed by a different system than in the awake state and that during drowsiness a gradual switch between these two systems takes place.