Consistent with earlier reports, the cerebellar group exhibited increased variability of the intertap intervals for both the 500- and 900-msec conditions. In contrast, we failed to observe a significant increase in the PD group at either rate. The PD patients were medicated, and previous results have suggested that the impairment in this group is especially marked when tested off medication or during the early stages of the disease process (Ivry and Keele, 1989; Wing et al., 1984). Nonetheless, despite their medication, the PD patients did exhibit clinical evidence of PD at the time of testing.

Estimates of clock and motor implementation variability were obtained through decomposition of the overall variability. All three groups exhibited similar estimates of motor variability, consistent with earlier work on these patient populations (Duchek et al., 1994; Harrington et al., 1998a; Ivry and Keele, 1989; Ivry et al., 1988). Notably, the increased variability in the cerebellar group was attributed to the clock component. Thus, these results again point to a central role for the cerebellum in the generation of the central signals related to the production of consistently timed responses (Franz et al., 1996; Ivry and Keele, 1989).

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