Roles of the cerebellum in pursuit-vestibular interactions.

This mini-review focuses on cerebellar roles in on-line control of smooth-pursuit eye movements during vestibular stimulation in primates. The smooth-pursuit system is necessary to track smoothly moving targets and must interact with the vestibular system during movement of the head and/or whole body to maintain the precision of eye movements in space (i.e. gaze movements). This interaction requires calculation of gaze velocity commands that match the eye velocity in space to the actual target velocity. Two cerebellar regions, the floccular lobe that consists of the flocculus and ventral paraflocculus, and the dorsal vermis, are known to be involved in smooth-pursuit. However, potential differences in their involvement are incompletely understood. To understand their roles, in particular whether the output of these regions codes gaze velocity or eye velocity, simple-spike activity of Purkinje (P-) cells was examined during smooth-pursuit and pursuit-vestibular interaction tasks in various directions in head-restrained monkeys. The results showed differences in discharge characteristics of vertical and horizontal P-cells within the floccular lobe and between the floccular lobe and dorsal vermis. These differences and other available evidence suggest that the dorsal vermis is involved more in the control of gaze movement whereas the floccular lobe primarily controls eye movement (in the orbit) as a component of the oculomotor neural integrator. Smooth-pursuit without vestibular stimulation cannot dissociate eye movement from gaze movement. To understand the cerebellar role in various aspects of smooth tracking of targets moving in the three dimensional space, more information is needed particularly on how the above mentioned two regions along with the dorsal paraflocclus and underlying deep cerebellar nuclei are involved in vergence tracking, how the cerebellum is involved in prediction and perception of target motion, and whether complex-spike discharge is involved in a fast adaptive process that may be used for prediction in smooth ocular tracking.