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 ove r the lateral basiocciput of normal subjects has up to the present tim e been attributed to an activation of cerebellar structures. This hypo thesis was tested in one patient with complete agenesis and two patien ts with extensive infarction of one cerebellar hemisphere. Unexpectedl y, stimulation over the intact and absent or damaged cerebellar hemisp heres reduced the susceptibility of the contralateral and, to a lesser degree, the ipsilateral motor cortex to cortical magnetic test stimul i given 9 msec after the stimulus over the cerebellum. The anatomic st ructure, activated by stimulation over the lateral occiput, remains un clear but activation of brain-stem structures, rather than the cerebel lum has been postulated. Increased thresholds for the excitation of th e hand-associated motor cortex contralateral to the cerebellar lesion correlated with slow and clumsy finger movements ipsilateral to the ce rebellar lesion, which suggests facilitatory influences of the cerebel lum on the contralateral corticospinal system under normal conditions.