Final Comments

The current experiment points to differential contributions of the cerebellum and basal ganglia in the performance of synchronized tapping. Lesions in the cerebellum appear to perturb the internal timing mechanism, manifest as an increase in the noise of this system. In terms of overall variability, as well as the estimates of clock and motor noise, medicated PD patients performed comparably to the control participants.

Nonetheless, the PD patients differed from the controls on two measures, the error correction parameter a and mean asynchrony. Together, these findings suggest that these patients have difficulty in adjusting their movements based on sensory information. In contrast, no differences were observed between the cerebellar patients and controls on the measures of error correction. This null finding is rather surprising given the frequently suggested role of the cerebellum in the comparison of expected and actual sensory information for rapid error correction (Flament and Ebner, 1996; Kawato et al., 1987; but see Smith et al., 2000). Our results indicate that the cerebellar contribution is more of a feed-forward signal, indicating when the next response should be emitted. Online modulations of these timing signals may come from extracerebellar structures.

The role of the basal ganglia in error correction has been suggested in a very broad sense (Lawrence, 2000). One important distinction is between online adjustments that are used to ensure that the current movement is executed accurately and trial-by-trial information that is used to develop stable internal models for the production of future movements. Neither of these has been extensively tested. Smith et al. (2000) provide evidence of the role of the basal ganglia in online error correction of reaching movements, demonstrating that patients with Huntington's disease (HD) and asymptomatic HD gene carriers are impaired in correcting for external and self-generated perturbations of reaching movements. It remains to be seen how best to characterize error correction processes in synchronized tapping. While the adjustments appear to occur over time spans that are comparable to online error correction, the discrete nature of the taps and pacing signals may create conditions more akin to trial-by-trial error correction.

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