MOTOR EFFECTS OF STIMULATING THE HUMAN CEREBELLAR THALAMUS

1. Observations were made on seven human subjects with electrodes impl anted in the cerebellar thalamus for the control of tremor. 2. Weak st imulation at 1-2 Hz resulted in sudden lapses of posture of actively c ontracting contralateral limb muscles. Stronger stimuli caused muscle twitches even in relaxed muscles. 3. The stronger stimuli produced sho rt latency facilitation of EMG activity in contralateral muscles; the pattern of muscle facilitation, the conduction velocity of the descend ing pathway and the estimated rise time of the underlying composite EP SP were consistent with direct activation of the corticospinal tract. 4. The lapses of posture produced by the weaker stimuli were associate d with inhibition of ongoing EMG for up to 150 ms. This was due to the interruption of tonic drive to motoneurons rather than to their inhib ition and was associated with depression of the motor evoked potential in relaxed muscles produced by weak anodal transcranial stimulation. The inhibition could be produced by 0.1 ms pulses, implying that a lar ge diameter fibre system was being activated. 5. A number of mechanism s could contribute to the inhibition, including inhibition from the re ticular nucleus of the thalamus.