MAGNETIC STIMULATION OVER THE CEREBELLUM IN HUMANS

Magnetic stimulation performed with a double-cone coil placed over app ropriate positions on the back of the head reduced the size of electro myographic responses evoked by magnetic cortical stimulation in the fi rst dorsal interosseous muscle when it preceded the cortical stimulus by 5, 6, and 7 msec. No suppression of responses to electrical cortica l stimulation occurred. Greater suppression was evoked by stronger cer ebellar stimuli; lesser suppression was elicited by stronger: cortical stimuli. These physiological findings correspond to those obtained wi th electrical cerebellar stimulation. The most effective position for magnetic stimulation over the back of the head was slightly rostral to the foramen magnum level on the ipsilateral side of the muscle studie d. This indicates that the conditioning stimulus activates certain str uctures at the back of the head on the ipsilateral side of the muscle, consistent with the cerebellum, because the part of the cerebellum re gulating limb muscles is positioned about there on the ipsilateral sid e. In 2 patients with only cerebellar dysfunction, this suppression ef fect was not elicited, which also supports that the suppression is cau sed by activity in cerebellar structures. We conclude that magnetic st imulation over the cerebellum with a double-cone coil elicits the same suppressive effect: on the motor cortex as electrical stimulation, bu t with less discomfort; moreover, we believe that this effect is produ ced by activation of certain cerebellar structures.