High-frequency (600 Hz) SEP activities originating in the subcortical and cortical human somatosensory system

Digitally high-pass filtered median nerve SEP show an oscillatory burst of low-amplitude high-frequency (600 Hz) wavelets superimposed on the N20 component which itself is generated by excitatory postsynaptic potentials of area 3b pyramidal cells. Prior studies using magnetoencephalography (MEG) localized one wavelet generator close to the primary somatosensory hand cortex. Since MEG recordings are biased towards tangentially oriented and superficial generators, a dipole source analysis of 32-channel electric SEP recordings was employed here to test for the possibility of deep and/or radially oriented burst generators: in 10 normal subjects low noise (16 000 averages) median nerve SEP were evaluated using dipole source analysis before and after applying a digital 475 Hz high-pass filter. Two main oscillatory 600 Hz burst sources were modeled; (i) a deep burst source close to the thalamus, most active in a time window between the brain-stem P14 and the cortical N20 sources, detectable in 7 of 10 subjects; most probably, this activity originates from deep axon segments of thalamocortical fibers; and (ii) a subsequent burst source timed around the N20 and located in the vicinity of the primary somatosensory hand cortex in all subjects, which was already known from MEG data. This superficial oscillatory source may be dominated by repetitive activity conducted in the terminal segments of the thalamocortical projection fibers initiated by the thalamic burst generator.