During procedures for parkinsonian tremor, neurons in the thalamic ventral nuclear group show periodic activity at tremor frequency (tremor-frequency activity). The tremor-frequency activity of some cells is significantly correlated with tremor. Cells in this region also display functional properties defined by activity related to somatosensory stimuli and to active movement. Cells with activity related to somatosensory stimulation were termed sensory cells while those with activity related to active movement were termed voluntary cells. Cells with activity related to both somatosensory stimulation and active movement were termed combined cells. Those with activity related to neither somatosensory stimulation nor active movement were termed no-response cells. Combined, voluntary and no-response cells were located in the region of thalamus where a lesion stops tremor and anterior to the region where sensory cells were found.

Spectral cross-correlation analysis demonstrated that many combined, voluntary and no-response cells had a peak of activity at tremor frequency which was significantly correlated with electromyogram (EMG). Analysis of the phase of thalamic activity relative to EMG activity indicated that voluntary and combined cell activity usually led EMG during tremor. These results suggest that thalamic cells unresponsive to somatosensory stimulation (voluntary and no-response cells) and those responsive to somatosensory stimulation (combined cells) are involved in the mechanism of parkinsonian tremor. The activity of sensory cells frequently lagged behind tremor while activity of combined cells often led tremor. This finding suggests that the activity of these two cell types, both responding to sensory input, is related to tremor by different mechanisms.