Reduction of corticospinal excitability by magnetic stimulation over the cerebellum in patients with large defects of one cerebellar hemisphere.

The reduction of motor cortex excitability by magnetic stimulation over the lateral basiocciput of normal subjects has up to the present time been attributed to an activation of cerebellar structures. This hypothesis was tested in one patient with complete agenesis and two patients with extensive infarction of one cerebellar hemisphere. Unexpectedly, stimulation over the intact and absent or damaged cerebellar hemispheres reduced the susceptibility of the contralateral and, to a lesser degree, the ipsilateral motor cortex to cortical magnetic test stimuli given 9 msec after the stimulus over the cerebellum. The anatomic structure, activated by stimulation over the lateral occiput, remains unclear but activation of brain-stem structures, rather than the cerebellum, has been postulated. Increased thresholds for the excitation of the hand-associated motor cortex contralateral to the cerebellar lesion correlated with slow and clumsy finger movements ipsilateral to the cerebellar lesion, which suggests facilitatory influences of the cerebellum on the contralateral corticospinal system under normal conditions.