Deciphering the impact of cerebellar and basal ganglia dysfunction in accuracy and variability of motor timing.

Studies in motor timing have shown that the basal ganglia and cerebellum play an important role in temporal processing. Timing studies in Cerebellar/ataxic Disorders (CD) and Parkinson's disease (PD) patients contrast the roles of the cerebellum and basal ganglia in motor timing. Here, we used a synchronization-continuation task to compare accuracy and variability of motor timing during repetitive tapping. We compared data collected for the present study - from patients with CD and healthy controls - to data from a previous study with patients with PD. We asked participants to tap at Inter-stimulus Intervals (ISIs) of 250, 500, 1000, and 2000 ms. Using Linear Mixed Models (LMMs), we explored how ISI, Task Phase, and Diagnosis interacted to determine the (i) the accuracy and (ii) the variability of tapping. In our analysis of accuracy, we found evidence that during the synchronization phase, at ISI=250 ms, CD patients lagged 'behind the beat'; whereas our previous work has suggested that medicated PD patients hasten 'ahead of the beat'. In our analysis of variability, we observed that at ISIs below 1000 ms, CD patients showed greater variability in motor timing than the healthy controls, while PD patients showed less variability than CD patients and healthy controls during the synchronization phase at the 1000 ms ISI. These results highlight the differential performance on explicit motor timing between patients with disorders of the cerebellum and basal ganglia. Our results illustrate a novel approach to discerning cognitive control of motor timing.