Independent short-term variability of spike-like (600 Hz) and postsynaptic (N20) cerebral SEP components

Human scalp-derived somatosensory evoked potentials (SEP) elicited by median nerve stimulation contain an early (20 ms latency) high-frequency (600 Hz) wavelet burst which is supposed to reflect non-invasively the timing of rapidly repeating population spikes in thalamocortical afferences and/or the receiving neocortical cell populations. This burst is superimposed onto the slower (< or = 100 Hz) primary cortical response (N20) representing intracortically generated postsynaptic events. The present study addressed the temporal dynamics and correlation of these response components in awake human subjects and found that at a 3 min time scale the burst response was significantly more variable than the concomitant N20, and that the burst and N20 varied independently of each other. Thus, wavelet burst and N20 represent parallel and partly independent steps in sensory processing at cortical input stages in awake human subjects. We propose that the N20 represents a stable somatosensory input whereas the more fluctuating high-frequency burst could index variable modes of processing, such as a floating focus of attention.