The neuropsychology and functional anatomy of timing
This thesis explores the neural correlates of motor and perceptual timing. Motor timing involves the production of a timed movement (e.g. dancing), whereas perceptual timing requires a perceptual judgement (e.g. deciding which of two events lasted longer). A body of research has investigated this type of timing, concentrating on millisecond- and seconds-range intervals. Patients with Parkinson's disease (PD) and cerebellar pathology exhibit motor and perceptual timing deficits, which has led to the suggestion that both the basal ganglia and cerebellum are involved in this type of temporal processing. The research presented here uses a variety of techniques (functional imaging, transcranial magnetic stimulation (rTMS) and clinical studies on patients) to investigate the contribution of different neural structures to temporal processing. Using positron emission tomography (PET), the basal ganglia and cerebellum were both found to be active during millisecond- and seconds-range timing. However, only the basal ganglia were active when non-temporal aspects of the task were controlled for. At the cortical level, rTMS was used to show that the right dorsolateral prefrontal cortex was essential to the reproduction of seconds-range intervals, possibly due to a role in memory processes. In a further study, the motor and perceptual timing performance of patients with PD was modulated by dopaminergic medication, with medication improving performance. Patients with cerebellar disease displayed increased variability in timing tasks that included a significant motor component, but did not show impaired accuracy. A second PET study, comparing patients with PD and healthy controls, showed that the basal ganglia were active during motor timing for the control group. Compared to their medicated state, the patients showed decreased coupling between the basal ganglia and dorsolateral prefrontal cortex when 'off' medication. These studies support the notion that the basal ganglia, and not the cerebellum, play a fundamental role in motor and perceptual timing.