Cerebellar TMS evokes a long latency motor response in the hand during a visually guided manual tracking task

Previous studies showed that transcranial magnetic stimulation (TMS) to the cerebellum evokes a long latency motor response in the soleus muscle during a postural task. The cerebellum is activated not only during postural tasks but also during motor tasks for which eye-hand coordination is required. The purpose of this study was to investigate whether TMS over the cerebellum evokes long latency motor responses in the hand during a visually guided manual tracking task. Eight healthy humans tracked an oscillatory moving target with the right index finger or pointed the finger at a stationary target, and TMS was delivered to the scalp over the cerebellum during the motor tasks. Trials with sham TMS were inserted between the trials with cerebellar TMS. The trajectory of finger movement fluctuated 92 ms after cerebellar TMS with a 24% probability during tracking of a moving target. The fluctuation was preceded by an electromyographic burst in the first dorsal interosseous muscle starting at 65 ms after TMS. The probability of fluctuation evoked by cerebellar TMS was significantly larger than that evoked by sham TMS during tracking of a moving target. This significant difference was absent in trials during which subjects pointed their index finger at a stationary target. These findings indicate that cerebellar TMS evokes a long latency motor response during a visually guided manual tracking task. The long latency motor response may be related to cerebellar activity associated with eye-hand coordination or to the detection of and correction for visuomotor errors.