COMPONENTS OF COORDINATION: THE CEREBELLUM AS AN INTERNAL CLOCK (MOTOR CONTROL)

This dissertation investigated the effects of different types of neurological deficits on timing functions. Parkinson, cerebellar, cortical, and peripheral neuropathy patients were tested on two separate measures of timing functions. The performance of the patient groups was compared to two control groups: college age subjects and healthy, elderly subjects. The first task involved the production of timed intervals in which the subjects attempted to maintain a simple rhythm. A two-process model of timing developed by Wing and Kristofferson (1973) was used to decompose the overall variability score into separable estimates of timekeeper variability and implementation variability. The second task measured the subjects' perceptual ability to discriminate between comparable temporal intervals.

The primacy of the cerebellum in timing functions was demonstrated by the finding that these were the only patients who showed a deficit in both the production and perception of timing tasks. The cerebellar group was found to have increased variability in an internal timekeeping process when performing rhythmic tapping and they were less accurate than the other groups in making perceptual discriminations regarding small differences in duration. The deficit appears to be specific to the perception of time since the cerebellar patients were unaffected in a control task measuring the perception of loudness. The cortical and a small number of basal ganglia (Parkinson) patients also showed a timing deficit, but this deficit was only apparent in the tapping task.

These results are accounted for by postulating that the timing of an interval can only commence once the central command for the preceding response has been issued. Thus, deficits in the basal ganglia or cortex can affect the integrity of the timing process by delaying the preceding response even if the contributions of these systems are unrelated to timing control. The cerebellum, in contrast is critical for timing functions since lesions in this structure will impair the ability to either perceive or produce timed intervals. Furthermore, case study analyses of cerebellar patients indicate that the lateral region of the cerebellum is the most likely candidate for the location of an internal clock.