TRANSCRANIAL MAGNETIC STIMULATION OVER THE POSTERIOR CEREBELLUM DURING VISUALLY GUIDED SACCADES IN MAN

Recent intensive neurophysiological experiments in the monkey have demonstrated the function of the cerebellum in the control of saccadic eye movements. Microstimulation studies on monkeys have determined that the cerebellar cortex, which is specifically involved in the control of saccades, is located in vermal lobules VIc and VII. The Purkinje-cell axons arising from this vermal area terminate almost exclusively in an ellipsoidal region which protrudes dorsocaudally from the fastigial nucleus. Saccade-related cells in the fastigial nucleus are located exclusively in the ellipsoidal region. Microstimulation of the vermis modulated the activity of saccade-related cells in the fastigial nucleus, and then produced dysmetric saccades. In this study, we investigated effects of cerebellar stimulation on saccade metrics in man using a transcranial magnetic stimulation (TMS) device. Focal TMS was applied over the posterior cerebellum at an area similar to 7 mm lateral and caudal to the inion during horizontal visually guided saccades in six normal subjects. The TMS device was triggered after the onset of saccades with a latency of 0, 20, 40 or 60 ms. We investigated the effect of TMS on the amplitude and velocity of saccadic eye movements. For visually guided saccades directed contralateral to the stimulation sine, TMS of the posterior cerebellum with the latency of 0 ms produced hypometric saccades followed by corrective saccades. Transcranial magnetic stimulation with latencies of 20, 40 and 60 ms had no effect on saccade metrics. On the other hand for ipsilateral saccades, TMS with latencies of 0, 20 and 40 ms produced hypermetric saccades followed by postsaccadic drift. Transcranial magnetic stimulation with the latency of 60 ms had no effect on saccade metrics. The peak velocity of ipsilateral saccades with TMS was significantly gl-eater than that of normal saccades with the same amplitude. These findings are consistent with previous neurophysiological observations in the monkey.