Normal vision requires rapid eye movements called saccades to direct gaze to objects of interest. Although much is understood about the final stages of saccade generation and about the organization of the visual system, it is not known how the brain decides where to shift gaze. The long-term goal of this work, therefore, is to understand the neural circuits that select the target for a saccadic eye movement. Neural correlates of target selection and response specification will be investigated in macaque monkeys performing novel visuomotor tasks. The basic experimental condition requires monkeys to shift gaze to a target stimulus that is presented with multiple distractor stimuli. The activity of single neurons will be recorded in two structures: the frontal eye field, an area of the cerebral cortex involved in converting the product of visual processing into a command to move the eyes, and the thalamic nuclei that relay signals from subcortical oculomotor structures to the frontal area field. Neuronal discharges will be analyzed using several novel techniques to quantify the probability, latency and magnitude of activation to visual stimuli and before eye movements. Another analysis will measure the timecourse over which single neurons discriminate whether a stimulus is a target. The properties of frontal eye field cells will be contrasted with those of thalamic cells. Three physiological and two anatomical studies are proposed. The first physiological study will investigate the visual processing underlying target selection by presenting visual search stimuli distinguished by color, form or motion. The second study will investigate top-down influences on neural activity mediating target selection by presenting arrays of stimuli in which the target is difficult to locate or by introducing specific regularities in how stimuli are presented. The third experiment will probe the decision processes underlying target selection and eye movement specification by manipulating monkeys' ability to reprogram a planned saccade. This task provides data that will distinguish neurons carrying visual signals from neurons actively involved in selecting the target and programming the eye movement. The first anatomical study will investigate the functional architecture of frontal eye field, relating neural response properties observed in the physiological studies to their location in the cortex. The second anatomical study will investigate the functional architecture of the oculomotor thalamus, identifying the properties of cells found in different nuclei. The strength of this proposal lies in the simultaneous assessment of visuomotor behavior and the activity of neurons that are involved in production of the behavior. Data from these experiments will provide significant new insights into the functional organization of frontal eye field and associated thalamic nuclei. Such information is an essential step toward more effectively diagnosing and ultimately treating vision and gaze disorders.
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