DESCRIPTION (Investigator's Abstract): The long term goal of this research is to determine how the brainstem and cerebellum collectively act to generate accurate saccades. The dorsal-posterior vermis of the cerebellum (lobules V-VII) and the fastigial nucleus are postulated to maintain saccade accuracy despite pathological or experimental insults to the ocular motor system that would otherwise degrade saccadic performance. The objective of the current proposal is to investigate the neuronal mechanisms in the cerebellum that act to maintain saccade accuracy. Monkeys will be exposed to two paradigms known to cause adaptive modification of saccade size. In the first, monkeys monocularly view the world using an eye whose extraocular muscles have been surgically weakened. Initially, saccades in the operated eye fall short of the target, but over time increase in size to become orthometric. Subsequent monocular vision using the unoperated eye restores saccades to the unadapted state. In the second paradigm, a target is presented in one position but is moved while the saccade is in mid-flight, thereby causing an error. Again, saccade size is modified so that saccades eventually land on target. The principal aim of this research is to find the cerebellar signals associated with the adaptation. Neurons in the vermis and in the fastigial nucleus will be recorded extracellularly both before and after saccadic adaptation. Pooled neuronal responses obtained under each condition will be compared to test for correlates of the change in saccade size. Neurons will also be recorded during adaptation to test for signals related to saccade error, which is presumed to be the stimulus that causes adaptation of saccade size. Later (second aim), recordings will be extended to the interpositus nucleus to examine whether the neuronal signals related to adaptation are confined to the vermis and fastigial nucleus, which are traditionally though to mediate adaptation. The final aim of this proposal is to determine whether the two adaptation paradigms described above share a common neural mechanism. This will be done by examining whether the adaptation in both cases is susceptible to the same experimental variables, and by examining single unit recordings.