Role of the cerebellum in movements: control of timing or movement transitions?

Patients with cerebellar damage are impaired on a range of timed tasks. However, recent research has indicated that the impairment on temporal production tasks is limited to discontinuous movements. The present experiments were designed to compare two accounts for the increased temporal variability observed in these patients when producing discontinuous movements. First, the impairment on discontinuous movements may be the result of the requirements associated with transitioning between movement onsets and offsets, requirements unique to discontinuous movement production. Second, the impairment may reflect a requirement to represent the temporal goal in timed, discontinuous movements. Patients with unilateral or bilateral cerebellar lesions and matched control subjects performed a key-pressing task. In one condition, the participants pressed and immediately released the key. The other conditions required the participants to press the key, and after either a 550-ms or 950-ms delay, release the key. Individuals with cerebellar damage were impaired on the two timed conditions. These results do not support the transition hypothesis. Rather, they are consistent with the hypothesis that the cerebellum is essential for tasks requiring precise event-like temporal control.