Human somatosensory evoked potentials (SEP) contain high-frequency (600 Hz) wavelet bursts possibly reflecting repetitive population spikes in thalamocortical axons and/or postsynaptic responses. To dissociate thalamic and cortical burst components the recovery of intrathalamic SEP (derived from electrodes implanted for movement disorder therapy in seven patients) was compared with scalp SEP in six age-matched Parkinsonian patients and six healthy younger subjects. Upon electric median nerve double-pulse stimulation conditioned scalp bursts were found attenuated in both groups, more for 10ms than 20ms interstimulus intervals; moreover, intraburst frequencies decreased from 690Hz to 590Hz. By contrast, intrathalamic burst amplitudes and frequencies (around 1 kHz) remained largely stable. These dissociations indicate functionally distinct generator mechanisms for scalp and intrathalamic high-frequency SEP bursts.
