Because movements involve changes in muscle
length over time, motor control and timing are
inextricably related. Most movements involve the coordinated
activation of agonist muscles to initiate motion and antagonist
muscles as a brake. These activations require accurate timing on
the order of tens of ms. Indeed, pathologies that disrupt the
timing between agonist and antagonist actions lead to dysmetric or
inaccurate movements. Lesions of the cerebellum, for
example, tend to delay the activation of antagonist muscles, which
causes movements to be hypermetric or to overshoot (e.g., Hore et
al. 1991). Cerebellar patients often display oscillating-
like tremors during movements as they make a series of
overshoots and corrections.
Numerous studies focusing on timing have made
use of repetitive movements as their readout. In
particular, Keele, Ivry, and others have used such movements as
rhythmic tapping of the finger to pursue the hypothesis that
the cerebellum is a general-purpose timer in the tens-to-
hundreds-of- ms range (e.g., Ivry & Keele 1989). In the
prototypical experiment, subjects are first asked to tap their
finger in time with a metronome (say at 400-ms intervals). After a
brief training period, the subject continues tapping without the
metronome. The main dependent measure is variability in
the intertap intervals. This and similar paradigms have been
used as screens to find brain regions for which damage disturbs the
timing of the taps. These and related findings are discussed in
more detail below in the section on the cerebellum.
..... 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.
