QUANTITATIVE STUDIES OF VISUAL PATHWAYS

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The overall aim of the work proposed here is to understand how central visual pathways analyze information about the chromatic and spatial properties of objects. Three major projects are proposed. Through a combination of physiological and psychophysical experiments these attempt to elucidate the connection between a particular aspect of vision and its underlying physiology. The first project will explore the possibility that the chromatic opponency in receptive fields of P-cells of macaque LGN arises not through selection of cone classes, but through the center receiving input from a single cone, with the surround drawing indiscriminately on inputs from L and M cones. The physiological measurements will use interference fringes of appropriate color to analyze the organization of cone inputs in the receptive fields of P-cells at various eccentricities. Psychophysical experiments to estimate the density with which chromatically opponent mechanisms sample the image will explore the corollary that a paucity of midget P-cells in the periphery accounts for our impaired color vision. The second project will use silent-substitution methods to examine how gain-controlling mechanisms in individual cone pathways act together with noise to control chromatic sensitivity in P-cells. Work undertaken during the previous period of support revealed paradoxical changes in the gain of P-cell responses following changes in background chromaticity, and showed that changes in noise interact with these in regulating detectability of stimuli. Because changes in background chromaticity also led to long- lasting changes in maintained discharge of P-cells, the project will also examine how these changes are expressed by cortical neurons. The third project will explore the idea that the behavior of neurons in the early stages of visual cortex accounts for the perceptual distinctiveness of visual textures. The experiments will examine how neurons (particularly complex cells) in V1 and V2 respond to visual textures of types employed in psychophysical studies. Work undertaken during the previous period of support suggested that a fuller understanding of the roles of cortical cells will require the use of visual stimuli that tap the nonlinear aspects of their behavior. The study of texture discrimination should get directly at this because perceptual discrimination of textures appears to require nonlinear mechanisms with properties like those found in complex cells.