Cerebellar timing systems.

Coordinated movement requires the normal operation of a number of different brain structures. Taking a modular perspective, it is argued that these structures provide unique computations that in concert produce coordinated behavior. The coordination problems of patients with cerebellar lesions can be understood as a problem in controlling and regulating the temporal patterns of movement. The timing capabilities of the cerebellum are not limited to the motor domain, but are utilized in perceptual tasks that require the precise representation of temporal information. Patients with cerebellar lesions are impaired in judging the duration of a short auditory stimulus or the velocity of a moving visual stimulus. The timing hypothesis also provides a computational account of the role of the cerebellum in certain types of learning. In particular, the cerebellum is essential for situations in which the animal must learn the temporal relationship between successive events such as in eyeblink conditioning. Modeling and behavioral studies suggest that the cerebellar timing system is best characterized as providing a near-infinite set of interval type timers rather than as a single clock with pacemaker or oscillatory properties. Thus, the cerebellum will be invoked whenever a task requires its timing function, but the exact neural elements that will be activated vary from task to task. The multiple-timer hypothesis suggests an alternative account of neuroimaging results implicating the cerebellum in higher cognitive processes. The activation may reflect the automatic preparation of multiple responses rather than be associated with processes such as semantic analysis, error detection, attention shifting, or response selection.