Smooth pursuit and saccadic eye movements need to be extremely accurate to provide clear retinal images of visual targets. Lesions of the cerebellum severely impair both the moment-to-moment accuracy of eye movements and the ability to adapt eye movements when damage to the brain or periphery makes them inaCCurate. Though the cerebellum is clearly indispensable to the accuracy and adaptation of eye movements, its specific role is unclear. The proposed research will provide significant new information about the role of cerebellum in smooth pursuit and saccadic eye movements by studying the two regions in the cerebellar nuclei most strongly implicated in their control, the caudal fastigial nucleus (CFN) and the ventrolateral interpositus nucleus (VPIN). We have three goals. First, we will characterize the CFN's role in pursuit. We will describe the responses of CFN neurons during a variety of pursuit conditions and we will measure pursuit after temporarily inactivating GFN neurons with the GABA agonist muscimol. These results will tell us if CFN activity could support pursuit initiation and maintenance and how this activity, in fact, affects pursuit. Second, we will investigate the VPIN's role in pursuit and saccadic eye movements. We will characterize the responses of VPIN neurons during these movements and describe how inactivating the VPIN with muscimol affects pursuit and saccades. Preliminary results show that neurons in the VPIN respond differently during saccades than do CFN neurons indicating that they perform a different function. Our data will allow us to directly compare neurons in these two regions to understand why two separate parts of the cerebellum are active during the same movements. ThIrd, we will determine if either the CFN or the VPIN is necessary for adapting saccade gain. We will measure adaptation rate before and after we inactivate one of these areas with muscimol. If this impairs adaptation we will record from neurons in the critical area(s) during adaptation to see how their response properties alter as gain changes. Previous work shows that some part of the cerebellar nuclei is necessary for saccade adaptation but does not identify the critical area. GFN and VPIN are the most likely areas because they are the only known saccade-related regions of the nuclei. By describing the role of CFN and the VPIN in pursuit, saccades, and saccade adaptation this work will help us understand what the cerebellum contributes to movements in general.