Effects of sleep onset on the mismatch negativity (MMN) to frequency deviants using a rapid rate of presentation

This study examined the effects of sleep onset-the transition from a waking, conscious state to one of sleep and unconsciousness-on the mismatch negativity (MMN) following frequency deviants when a rapid rate of stimulus presentation is employed. The MMN is thought to reflect a brief-lasting sensory memory. Rapid rates of stimulus presentation should guard the sensory memory from fading. A 1,000 Hz standard stimulus was presented every 150 ms. At random, on 6.6% of the trials, the standard was changed to either a large 2,000 or a small 1,100 Hz deviant. During alert wakefulness (when subject ignored the stimuli and read a book), the large deviant elicited a larger deviant related negativity (DRN) than did the small deviant. This negativity may be a composite of both N1 and MMN activity while that following the small deviant is probably a 'true' MMN. The large deviant continued to elicit a DRN in relaxed wakefulness (eyes closed) and Stages 1 and 2 of sleep, although it was much reduced in amplitude. A significant MMN was recorded for the small deviant only in alert wakefulness. The failure to observe an MMN to small deviance and the attenuation of the DRN to large deviance at sleep onset therefore is probably not due to a decay of sensory memory. It is more likely that cortical encoding of both the standard and deviant is weakened during sleep onset because of prior thalamic inhibition of sensory input.