SINGLE-UNIT ANALYSIS OF THE HUMAN VENTRAL THALAMIC NUCLEAR GROUP - TREMOR-RELATED ACTIVITY IN FUNCTIONALLY IDENTIFIED CELLS

During procedures for parkinsonian tremor, neurons in the thalamic ven tral nuclear group show periodic activity at tremor frequency (tremor- frequency activity). The tremor-frequency activity of some cells is si gnificantly correlated with tremor. Cells in this region also display functional properties defined by activity related to somatosensory sti muli and to active movement. Cells with activity related to somatosens ory stimulation were termed sensory cells while those with activity re lated to active movement were termed voluntary cells. Cells with activ ity related to both somatosensory stimulation and active movement were termed combined cells. Those with activity related to neither somatos ensory stimulation nor active movement were termed no-response cells. Combined, voluntary and no-response cells were located in the region o f thalamus where a lesion stops tremor and anterior to the region wher e sensory cells were found. Spectral cross-correlation analysis demons trated that many combined, voluntary and no-response cells had a peak of activity at tremor frequency which was significantly correlated wit h electromyogram (EMG). Analysis of the phase of thalamic activity rel ative to EMG activity indicated that voluntary and combined cell activ ity usually led EMG during tremor. These results suggest that thalamic cells unresponsive to somatosensory stimulation (voluntary and no-res ponse cells) and those responsive to somatosensory stimulation (combin ed cells) are involved in the mechanism of parkinsonian tremor. The ac tivity of sensory cells frequently lagged behind tremor while activity of combined cells often led tremor. This finding suggests that the ac tivity of these two cell types, both responding to sensory input, is r elated to tremor by different mechanisms.