Changes of short latency somatosensory evoked potential in sleep.

We studied how the first negative waves (frontal 'N18' and parietal 'N20') of median somatosensory evoked potentials (SEP) change from waking to sleep in 9 healthy volunteers. Frontal and parietal responses in awake subjects showed multiple fast frequency potentials (FFP) over the ascending and descending phases of the slow negative waves. The main frontal FFP consisted of N16, P17, N18, P18, N19 and P20, with an additional small FFP, n15, over the ascending phase of N16. The parietal FFP included 3 major peaks (N15, P18 and N20) and 3 small FFP (n17, n19 and n20). Frontal FFP, except for n15, were markedly attenuated or totally disappeared in stage II sleep. Only a few FFP were identified in stage IV. The FFP returned in REM sleep, but amplitude was smaller than the waking state. Parietal FFP were also attenuated in NREM and recovered in REM sleep, but these changes were less prominent compared to those of frontal FFP. Latencies of frontal P20 and parietal N20 were prolonged in NREM sleep with greater prolongation of P20 than N20. These returned to waking values in REM sleep. These findings suggest that the frontal and parietal major negative peaks ('N18' and 'N20') consist of multiple physioanatomical substrates mediated through complex thalamocortical projection systems, and that the FFP are closely related to the sleep-wake mechanism possibly reflected by mutual interaction between cortex and the thalamic reticular system.