A popular theory is that the
cerebellum functions as a timer for clocking motor events (e.g. initiation, termination). Consistent with this idea,
cerebellar patients have been reported to show greater deficits during
hand movements that repeatedly start and stop (i.e. discontinuous movements) as compared with continuous
hand movements (Spencer et al., 2003). Yet, this finding could potentially be explained by an alternate theory in which the
cerebellum acts as an internal model of limb mechanics. We tested whether a timing or internal model hypothesis best explains results from a circle-drawing task, where individuals trace a circle with the
hand at a desired tempo. We first attempted to replicate
prior results showing greater impairment for discontinuous versus continuous circling movements, and then asked whether we could improve patient performance by reducing demands in each domain. First, we slowed the movement down to reduce the need to predict and compensate for limb dynamics. Second, we supplied external timing information to reduce the need for an internal event timer. Results showed that we did not replicate the previous findings--
cerebellar patients were
impaired in both discontinuous and continuous movements. Slowing the movement improved
cerebellar performance to near control values. The addition of an external
visual timing signal paradoxically worsened timing deficits rather than mitigating them. One interpretation of these combined results is that the
cerebellum is indeed functioning as an internal model and is needed to make appropriate predictions for movement initiation and termination.
