The cerebellum, predictive control and motor coordination.

I argue that the cerebellum has at least two related roles, both sub-served by its operation as a 'forward model' of the motor system. First, it provides an internal state estimate or sensory prediction that is used for online control of movements; second, these predictive state estimates are used to coordinate actions by different effectors in the normal coordination of eye and hand, reach and grasp, etc. Preliminary electrophysiological data from cerebellar cortical neurons in the monkey supports the hypothesis that a proportion of cells code for the sensory consequences of movement. In a contrast between normal visually guided movement of a cursor and mirror reversed movement, approximately half the sample of 47 directionally sensitive cells were found to code for the movement of the cursor controlled by the monkey's limb, and not the limb movement itself. Functional imaging of the human cerebellum further supports the hypothesis that the cerebellum is involved in motor coordination. Subjects were tested performing ocular tracking, manual tracking without eye movement, or combined eye and hand tracking. Activation of cerebellar areas related to movement of eyes or hand alone was significantly enhanced when the subjects performed coordinated eye and hand tracking of a visual target.