Effects of transcranial magnetic stimulation of the cerebellum on performance of consecutive rapid movements in patients with idiopathic sporadic cerebellar ataxia and healthy subjects

It is known that cerebellum influences the action of the motor system. The cerebellum may exert a facilitatory influence in the motor cortex, and should be involved in temporal computations in movement performance. The inability to compute time differences would affect time-related tasks. However, the cerebellar role to control precise movement performance is contradictory. Furthermore, facilitatory effect of cerebellum might be decreased in cerebellar degeneration. The aim of the study was to investigate the performance of the rapid movements in patients with “pure” cerebellar ataxia. Movement performance was compared in 13 patients and 8 healthy subjects. Movements were performed from the initial to the target position, with the movement length of 40 deg in the elbow flexion. Motor threshold (MT) of the motor cortex was determined and TMS was then applied right of the inion, in two conditions: With the strength of 5% below the MT, at the moment of command to start the flexion movement, and with the same strength 20 ms before the movement start. Two additional conditions were also tested: TM stimulus was applied with the strength of 30% above the MT at the moment of movement start, and at 20 ms before movement start. Patients perform significantly longer movements then healthy subjects, and accuracy of the movements deteriorates. Length of the movements was also prolonged in the conditions where stronger stimulus was applied. Accuracy of the movements in patients was not different when the stimulus was applied in the phase of the movement preparation, compared in moment 0 and -20 ms. Same was true for healthy subjects in both conditions - stimulus below and above MT. Results that differences in disturbance timing and strength have no clear effects might speak for preprogrammed characteristics of rapid movements in both healthy subjects and ataxia patients, where sensory input is not of primary importance. Deficits in cerebellar ataxia for the movement length may be attributable to improper processing of motor command and/or sensory information.