INHIBITION OF VOLUNTARY ACTIVITY BY THALAMIC-STIMULATION IN HUMANS - RELEVANCE FOR THE CONTROL OF TREMOR

The motor effects of stimuli delivered through four-channel, quadripol ar macroelectrodes chronically implanted in the ventrolateral thalamus were studied in 20 awake cooperating human subjects. Single stimuli c ould inhibit voluntary contraction of the contralateral first dorsal i nterosseous muscle (FDI) for up to 200 ms. The inhibition was often fo llowed by a rebound facilitation or by oscillatory activity. This inhi bition appeared to arise from the ventrolateral thalamus and could not be obtained in other patients by stimulation of the periventricular g rey matter (PVG), the globus pallidus internus (GPI), or the subthalam ic nucleus (STN). The neural elements activated by the stimulus had a short chronaxie and a short refractory period, implying that they were large-diameter axons. Similar effects were obtained from each of the four electrodes in the row, suggesting that this fiber system lay para llel rather than perpendicular to the implanted macroelectrode. The in hibition resulting from a single stimulus was diminished by a prior st imulus or train of stimuli. A continuous train of stimuli produced inh ibition for only the first 200 ms. We propose that the thalamic stimul us activates a neural network which includes thalamic relay cells and neurons of the thalamic reticular nucleus and that the inhibition of t halamic relay cells habituates with repeated stimuli. It has been sugg ested that parkinsonian rest tremor results from synchronization of th e oscillatory activity of this network. If this is the case, continuou s thalamic stimulation might disrupt this oscillation by diminishing t he inhibitory phase.