Propofol narcosis dissociates human intrathalamic and cortical high-frequency (> 400 hz) SEP components

Human somatosensory evoked potentials (SEP) contain a brief burst of high-frequency wavelets (>400 Hz) presumably reflecting rapidly repeated population spikes of as-yet undetermined origin. To study state-dependent response changes, SEP after electric median nerve stimulation were recorded in six Parkinson's disease patients perioperatively from intrathalamic electrode implants, and in five non-implanted patients from scalp electrodes, before and under propofol narcosis. In all intrathalamic recordings burst amplitude and intraburst frequency (approximately 950 Hz) proved to be almost stable under propofol administration. In strong contrast, the scalp burst (640 Hz) was significantly slowed (480 Hz) under propofol narcosis, and its amplitude reduced to 28% of the pre-propofol baseline. Low-frequency SEP components which underly the burst at thalamic (P16) and cortical level (N20) did not change significantly. This dissociation of bursts indicates neuronal generators showing different sensitivities to propofol narcosis, with a robust thalamic response and a state-dependent cortical contribution, possibly from pyramidal chattering cells and/or inhibitory interneurons.