With functional MRI, we recently identified fronto-cerebellar activations in predicting time to reach a target and basal ganglia activation in velocity estimation, that is, small interval assessment. We now tested these functions in patients with Parkinson's disease (PD) and degenerative cerebellar ataxia. They watched a ball that repeatedly appeared, moved, and disappeared. Velocity, stop locations, and predicted target locations as well as time to reach a target were indicated. Compared with controls, PD patients showed impaired velocity estimation (momentary mode) whereas temporal prediction was selectively impaired in cerebellar ataxia patients. The latter highlights feed-forward processing within fronto-cerebellar circuitry. Impaired velocity estimation in PD fits the concept of a basal ganglia clock function.