Magnetic stimulation over the cerebellum in humans.

Magnetic stimulation performed with a double-cone coil placed over appropriate positions on the back of the head reduced the size of electromyographic responses evoked by magnetic cortical stimulation in the first dorsal interosseous muscle when it preceded the cortical stimulus by 5, 6, and 7 msec. No suppression of responses to electrical cortical stimulation occurred. Greater suppression was evoked by stronger cerebellar stimuli; lesser suppression was elicited by stronger cortical stimuli. These physiological findings correspond to those obtained with 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 studied. This indicates that the conditioning stimulus activates certain structures at the back of the head on the ipsilateral side of the muscle, consistent with the cerebellum, because the part of the cerebellum regulating limb muscles is positioned about there on the ipsilateral side. In 2 patients with only cerebellar dysfunction, this suppression effect was not elicited, which also supports that the suppression is caused by activity in cerebellar structures. We conclude that magnetic stimulation over the cerebellum with a double-cone coil elicits the same suppressive effect on the motor cortex as electrical stimulation, but with less discomfort; moreover, we believe that this effect is produced by activation of certain cerebellar structures.