Encoding of movement time by populations of cerebellar Purkinje cells

One of the earliest computational principles attributed to the cerebellum w as the measurement of time(1). This idea was originally suggested on anatom ical grounds, and was taken up again to explain some of the deficits in cer ebellar patients(2,3). The contribution of the cerebellum to eye movements, in contrast, has traditionally been discussed in the context of motor lear ning(4-7). This view has received support from the loss of saccade adaptati on, one of the key examples of motor learning, following lesions of the pos terior cerebellar vermis(8-11). However, the relationship between the prope rties of saccade-related vermal Purkinje cells and the behavioural deficits that follow lesions is unclear. Here we report results from single-unit re cording experiments on monkeys that reconcile the seemingly unrelated conce pts of timing and motor learning. We report that, unlike individual Purkinj e cells, the population response of larger groups of Purkinje cells gives a precise temporal signature of saccade onset and offset. Thus a vermal popu lation response may help to determine saccade duration. Modifying the time course of the population response by changing the weights of the contributi ng individual Purkinje cells, discharging at different times relative to th e saccade, would directly translate into changes in saccade amplitude.