Processing of auditory stimuli during tonic and phasic periods of REM sleep as revealed by event-related brain potentials.

The brain has been reported to be more preoccupied with dreams during phasic than during tonic REM sleep. Whether these periods also differ in terms of the processing of external stimuli was examined. Event-related brain potentials (ERPs) to a frequent standard tone of 1000 Hz (P = 97%) and infrequent deviant tones of 1100 and 2000 Hz (P = 1.5% for each) were recorded (n = 13) during wakefulness and nocturnal sleep. An ERP wave (called REM-P3) resembling a waking P3 wave was larger for the 2000 Hz deviant during tonic than during phasic REM sleep. Also the P210 wave was larger during tonic than during phasic REM sleep. A reliable mismatch negativity component appeared only in wakefulness. In summary, these results support the hypothesis that the brain is more 'open' for changes in an auditory input during tonic than phasic REM sleep.