6.2 Cerebellum
Although the cerebellum is generally viewed as primarily a motor structure, it has also been proposed to be a general-purpose interval timer in the range of tens to hundreds of ms. “General purpose” in this sense encompasses both sensory and motor timing. One advantage of such a theory is that the synaptic organization and physiology of the cerebellum are known. Much is known about the relationships between the cerebellum and forms of motor learning such as eyelid conditioning and adaptation of the vestibulo- ocular reflex (Raymond et al. 1996; Boyden et al. 2004, in this volume).
Support for the role of the cerebellum in timing is based on both motor and sensory timing experiments. Ivry and others presented a variety of evidence demonstrating cerebellar involvement in timing tasks. The fundamental observation was made in experiments in which the task required human subjects to make rhythmic taps with their finger.
The timing hypotheses of cerebellar function attempt to explain the various tasks for which the cerebellum is engaged or is necessary in terms of the need to gauge the explicit timing between events in the hundreds- of- ms range. Despite the intent that these theories build on a computational base, supporting data remain mostly task based. Most data involve demonstrations that the cerebellum is activated during, or is required for, tasks that we view as examples of timing.
This viewis also consistent with recent findings that apparent timing deficits are specific to discontinuous timing tasks relative to continuous ones. Spencer et al. (2003) tested cerebellar patients on two similar timing tasks.Two groups of subjects were required to draw circles at regular intervals. The “discontinuous” group was required to keep a beat by pausing at the top of each circle. The “continuous” group was instructed to keep a beat by drawing circles using a steady continuous motion. Cerebellar damage affected discontinuous drawing and not continuous. The authors interpret these findings as evidence that the cerebellum is required for tasks where timing is explicitly represented, as in the discontinuous task. In this view, the cerebellum is not required by the continuous task because timing can be implicit—that is, timing can be produced by maintaining a constant angular velocity. Alternatively, such findings can be seen as examples of the contributions of feed-forward prediction in the starting and stopping of movements.
..... the failure of a neurological disorder — such as cerebellar injury — to affect the scalar property is taken to indicate that the affected structures are not essential for proper interval timing. Instead, the cerebellum might contain an internal model of the motor–effector system, so cerebellar damage could increase variability in motor and perceptual timing.
Traditionally, because interval timing depends on the intact striatum but not on the intact cerebellum, the cerebellum has been charged with millisecond timing and the basal ganglia with interval timing. Despite this simplistic dissociation, two recent findings have shed new light on the involvement of the basal ganglia and cerebellum in motor control and interval timing.
The study of patients with neurological damage has revealed the importance of several brain structures in time processing. Early studies highlighted the cerebellum as a key component of the time processing network. Ivry and Keele (1989) demonstrated that patients with cerebellar lesions showed poor motor timing and time discrimination when comparing short intervals less than 1 s), while Mangels, Ivry, and Shimizu (1998) found that patients with cerebellar lesions cannot discriminate longer intervals (4 s). These results suggest that the cerebellum has a fundamental role to play in both sub- and supra-second time perception. In recent years, the evidence from lesion studies has been greatly extended by imaging studies using fMRI and PET. Cerebellar activity has been reported in temporal discrimination tasks using intervals of various durations (Mathiak, Hertrich,Grodd, & Ackermann, 2004; Jueptner et al., 1995; Rao, Mayer, &Harrington, 2001) and also in time production tasks (Penhune, Zatorre,&Evans, 1998; Tracy, Faro, Mohamed, Pinsk,&Pinus, 2000).