The long-term goal of our research is to determine exactly what role the substantia nigra plays in the generation of normal eye movements. While previous research has focused on how neurons in the reticulata subdivision of this nucleus may influence the generation of saccades by the superior colliculus, very little effort has been focused on attempting to understand how both the reticulata and compacta subdivisions might participate in the movement control process in general. Over the past several years our laboratory has begun to approach this goal by examining the activity of reticulata neurons. While we have confirmed earlier findings that some reticulata neurons are suppressed when subjects plan and initiate movements for which they receive rewards, we have also found that reticulata neurons are unmodulated when subjects produce movements for which they are not rewarded. These data suggest that reticulata activity is more tightly coupled to sequences of movements that are reinforced, and hence adaptive for the subjects, than with movements in general. Existing studies of the compacta subdivision seem to indicate a similar functional role for that area, linking compacta rate to the adaptive value of behavioral responses. The goal of this proposal is to test the hypothesis that both pars compacta and pars reticulata carry signals related to both the adaptive significance of stimuli that guide movements and to the reinforced movements themselves, not simply signals locked to the precise stimuli the animal perceives or the precise movements the animal executes. To examine this question, and to identify precisely what role reticulata and compacta signals might play in oculomotor processes, we propose two sets of interlocking physiological experiments: one set directed at the pars reticulata and a second set directed at the pars compacta. The reticulata experiments proposed here are designed to determine: whether reticulata task-related modulations can be elicited by non-visual stimuli, whether saccade-related reticulata modulations can be associated with entire sequences of movements, whether reinforcers not associated with eye movements modulate reticulata neurons, and whether changes in the relevance of a stimulus to a behavioral task can alter the reticulata modulation associated with that event. The compacta experiments proposed here test the hypothesis that the two nigral subnuclei are both involved in the generation of adaptively valuable movements by directly comparing the responses of the compacta and reticulata neurons under identical conditions, while subjects perform eye movements for which they receive reinforcement. These experiments may ultimately tell us how this nucleus, which has been implicated in a number of human disorders, participates in the generation of movements by normal individuals.
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