Sleep stage dependant changes of the high-frequency part of the somatosensory evoked potentials at the thalamus and cortex

Objectives: It is known that the high-frequency oscillations (above 400 Hz) of the somatosensory evoked potentials (SEPs) diminish during sleep while the N20 persists (Neurology 38 (1988) 64; Electroenceph clin Neurophysiol 7 0 (1988) 126; Electroenceph clin Neurophysiol 100 (1996) 189). We investiga ted possible differential effects of sleep on the 600 Hz SEPs at the thalam us and cortex. Methods: SEPs from 10 subjects were recorded using 64 channels following el ectric stimulation at the wrist during awake state and sleep stages II, TV and REM. Dipole source analysis was applied to separate brain-stem, thalami c and cortical activity in the low-frequency (20-450 Hz) and the high-frequ ency (450-750 Hz) part of the signal. Results: The low-frequency SEPs showed a non-significant increase of the la tency of the N20 and a bifid change of the waveform in 3 subjects. The high -frequency SEPs showed a significant decrease of their amplitude at the lev el of the thalamus and cortex but not at the brain-stem. This decrease in a mplitude at the thalamus and cortex were significantly correlated. There wa s no effect on the latency of the signal. In addition, at the cortex, diffe rential effects on early and late parts of the 600 Hz oscillations were fou nd by time-frequency analysis using a wavelet transformation. Conclusions: Sleep dependent decrease of the high-frequency SEPs were first observed at the thalamus pointing to the known function of the reticular t halamic nucleus regulating arousal. The results presented here provide furt her evidence for a thalamic origin of the 600 Hz oscillations. In addition, on the basis of the differential effects on early (up to the N20 peak) and late (between 20 and 25 ms) parts of the signal, at least one intracortica l generator of these oscillations is proposed. In general, the high-frequen cy SEPs (600 Hz oscillations) are supposed to reflect activity of a somatos ensory arousal system. (C) 2000 Elsevier Science Ireland Ltd. All rights re served.