One of the most important problems in understanding how the brain can analyze space is how it can construct an accurate representation of the world around it from the shifting images generated by a moving eye. Patients with parietal lesions have difficulty with accurately perceiving the space around them and generating spatially accurate behavior, especially when their eyes move. A particular part of the parietal cortex, the lateral intraparietal area (LIP) is thought to be important in the mechanisms underlying spatial localization. Studies in LIP of the monkey have revealed a phenomenon, the process of perisaccadic receptive field shifts, by which the brain could possibly solve the problem of how to achieve spatially accurate behavior despite a moving eye. Most visually responsive neurons in the brain are defined by their receptive fields, the area of the retina from which visual stimuli evoke neural activity. In LIP, visual receptive fields are not fixed to the retina, nor do they statically or explicitly describe an area of space in supraretinal coordinates. Instead, preliminary work has demonstrated that receptive fields are dynamic: around the time of a saccade they shift on the retina. LIP neurons respond to stimuli briefly flashed outside their classic receptive fields when a saccade brings the spatial location of the vanished stimuli into their receptive fields. By freeing vector processing from retinal constraints, receptive field shifts provide a mechanism by which the brain could achieve spatially accurate visual analysis. This proposal seeks to understand the phenomenology and underlying mechanisms of receptive field shifts in LIP of the awake, behaving monkey, in order to understand the mechanisms that could underlie accurate spatial perception and action. It has 4 specific aims: (1) to quantify the phenomenon of receptive field shifts, (2) to analyze the long-term aspects of accurate spatial memory in the parietal cortex; (3) to determine the metric of space used by the parietal cortex; and (4) to use intracortical application of GABA agonists and antagonists to test specific models of how the phenomenon of shifting receptive fields might be accomplished by the parietal cortex. By elucidating the metrics of the spatial representation in LIP, the topology of perisaccadic receptive field shifts in LIP, the demonstration of a long-term, intertrial memory in parietal cortex, and testing current models of the genesis of receptive field shifts, these experiments should enable a better, more coherent understanding of how the parietal cortex represents visual space. This should, in turn, enable us to understand why patients with parietal deficits behave as they do, and how to design possible strategies for their rehabilitation and treatment.
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