Control of saccadic eye movements and combined eye/head gaze shifts by the medio-posterior cerebellum.

The cerebellar areas involved in the control of saccades have recently been identified in the medio-posterior cerebellum (MPC). Unit activity recordings, experimental lesions and electrical microstimulation of this region in cats and monkeys have provided a considerable amount of data and allowed the development of new computational models. In this paper, we review these data and concepts about cerebellar function, discuss their importance and limitations and suggest future directions for research. The anatomical data indicate that the MPC has more than one site of action in the visuo-oculomotor system. In contrast, most models emphasize the role of cerebellar connections with immediate pre-oculomotor circuits in the reticular formation, and only one recent model also incorporates the ascending projections of the MPC to the superior colliculus. A major challenge for future studies, in continuation with this initial attempt, is to determine whether the various cerebellar output pathways correspond to distinct contributions to the control of saccadic eye movements. Also, a series of recent studies in the cat have indicated a more general role of the MPC in the control of orienting movements in space, calling for an increasing effort to the study of the MPC in the production of head-unrestrained saccadic gaze shifts.