The research will examine the mechanisms that support spatial discrimination (specifically, orientation selectivity) in single striate cortical neurons and correlate physiological characteristics at a specific recording site with intracortical connectivity at that same site. The goal is to produce a comprehensive model of striate cortex that integrates structure and function. Single neurons in the retina and lateral geniculate nucleus dissect a visual scene with respect to location in space and spatial frequency (periodicity of repeating patterns). Striate cortical neurons add selectivity for boundary orientation and direction of motion; the physiological mechanisms that support such selectivity are as yet poorly understood. Two existing hypotheses suggest that selectivity is based primarily either on excitation or inhibition. This laboratory has developed a method that uses two grating stimuli simultaneously presented to separate excitatory and inhibitory influences. Orientation selectivity was found to be the result of three different processes combined in different degree across the total cell population. Recent progress in tracing connectivity in the striate cortex would predict that the relative importance of each process would vary systematically with the laminar location of the recorded cell. This experimental program has four primary objectives: (1) Correlation of the organization of orientation selectivity in a given cell with its laminar location, which will help to position the cell in the processing hierarchy in striate cortex; (2) Verification of the double grating method of separating excitation and inhibition by blocking inhibition pharmacologically to reveal the isolated excitatory input to the cell; (3) More detailed study of the inhibitory mechanisms resolved through double-grating tests, with the intent of testing one mechanism for possible identity with the cortical gain control described by others; (4) Tracing intracortical connectivity at specific recording sites through local injection of a retrogradely transported tracing agent (HRP), which will help to establish the microcircuitry responsible for known electrophysiological characteristics.
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