Experiments are proposed to establish the neural basis for velocity storage and its control by the nodulus and ventral uvula. Previously, regions of the vestibular nuclei (VN) where velocity storage is produced have been tentatively identified. Now cell types that are believed responsible for its generation win be studied in cynomolgus monkeys. Recent work has shown that vestibular-only (VO) and vestibular-plus-saccade (VPS) neurons in VN have activity related to velocity storage, and that electric stimulation at the site of VO neurons produces nystagmus and after-nystagmus with the dynamic characteristics of velocity storage. From this it is postulated that VO neurons participate in the production of velocity storage, and that VPS neurons may serve as an output of the velocity storage network to the oculomotor system. It is also proposed that the VO neurons are under control of the nodulus and uvula to produce vertical and torsional components of eye velocity that provide the spatial orientation of velocity storage. Experiments in this application will explore these hypotheses, first by identifying sites in VN where VO and VPS neurons are located using extracellular recording. These and other vestibular neurons win be tested during alertness and drowsiness to determine whether their activity is relatively insensitive to state changes in alertness, as is velocity storage itself. Midline medullary lesions will be made to abolish velocity storage, and effects of these lesions on the activity of VO and VPS neurons will be determined. Effects of nodulo-uvulectomy on the spatial orientation of velocity storage will be investigated. The postulate is that the spatial orientation of velocity storage is dependent on these structures and will disappear after they are lesioned. The nodulus and ventral uvula will be electrically stimulated to determine if the spatial orientation of velocity storage can be shifted by this manipulation. It will also be investigated electrophysiologically if the nodulus and uvula have projections to lateral and vertical canal-related VO neurons in MVN and SVN. In collaborative studies, the afferent and efferent connections and morphology of VO and VPS neurons will be determined. When these studies are completed, there should be a better understanding of the specific neurons that participate in generation of velocity storage and of the nature of the neural network that produces it. These results will help meet the long-term goals of the project, i.e., to determine the organization of the velocity storage integrator, to model its function at a cellular level, and to study the role of velocity storage in spatial orientation.
