Encoding of movement time by populations of cerebellar Purkinje cells

One of the earliest computational principles attributed to the cerebellum was the measurement of time. This idea was originally suggested on anatomical grounds, and was taken up again to explain some of the deficits in cerebellar patients. The contribution of the cerebellum to eye movements, in contrast, has traditionally been discussed in the context of motor learning. This view has received support from the loss of saccade adaptation, one of the key examples of motor learning, following lesions of the posterior cerebellar vermis. However, the relationship between the properties of saccade-related vermal Purkinje cells and the behavioural deficits that follow lesions is unclear. Here we report results from single-unit recording experiments on monkeys that reconcile the seemingly unrelated concepts of timing and motor learning. We report that, unlike individual Purkinje cells, the population response of larger groups of Purkinje cells gives a precise temporal signature of saccade onset and offset. Thus a vermal population response may help to determine saccade duration. Modifying the time course of the population response by changing the weights of the contributing individual Purkinje cells, discharging at different times relative to the saccade, would directly translate into changes in saccade amplitude.