Stability of high-frequency (600 Hz) components in human somatosensory evoked potentials under variation of stimulus rate - evidence for a thalamic origin

The generators of spike-like high-frequency (600 Hz) wavelets superimposed on the primary cortical response (N20) in human median nerve somatosensory evoked potentials (SEP) have been localized anatomically both close to the primary somatosensory hand cortex and in deep axon segments of thalamo-cort ical projection neurons. Here, N20 and 600 Hz burst components were functio nally dissociated by varying the stimulus rate (1.5, 3, 6, 9 Hz). The N20 s ource amplitudes were significantly reduced at the higher stimulus rates. I n contrast, the source amplitudes of the 600 Hz oscillations remained stabl e across all stimulus rates. This reflects different source origins, confir ming a postsynaptic intracortical generation of the N20 component and provi des further evidence for a presynaptic origin of the 600 Hz activity like r epetitive neuronal population spikes conducted in deep and superficial segm ents of thalamo-cortical projection fibers. (C) 1999 Elsevier Science Irela nd Ltd. All rights reserved.