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,