DISSOCIATED CHANGES OF FRONTAL AND PARIETAL SOMATOSENSORY-EVOKED POTENTIALS IN SLEEP

We studied the changes of frontal and parietal somatosensory evoked po tentials (SEPs) in the awake state versus different stages of sleep in 10 normal adult subjects. Frontal and parietal SEP components were af fected differentially as sleep stages progressed. in general, the ampl itudes of frontal components, notably P22, were increased in sleep, wh ereas the amplitudes of parietal components were decreased in sleep. A sensitive waveform change from the awake state to sleep was present i n the frontal response, where a subtle notched negativity, termed ''N4 0,'' was present only in the awake state and quickly dissipated in all stages of sleep, including stage 1. The amplitude changes from the aw ake state to stage 3/4 sleep were neither linear nor parallel among SE P components. The most discordant changes occurred in stage 3/4. The a mplitudes for the frontal N18-P22-N30 complex and parietal N20-P26-N32 complex increased from stage 2 to stage 3/4, while those for frontal N30-fP40 and parietal N32-pP40 decreased. In contrast to these diverge nt amplitude changes, the latencies of all components except P14 and f rontal N18 showed progressive prolongation from the awake state to slo w-wave sleep. The SEP waveforms and latencies in REM sleep approximate d those in the awake state, although amplitudes for frontal peaks stil l remained slightly higher and amplitudes for parietal peaks slightly lower. We postulate that interactions of excitatory and inhibitory phe nomena are responsible for the component-dependent and sleep-stage-dep endent amplitude enhancement or depression in sleep.