Dissecting the neural circuit for color vision.

摘要

Color perception is a robust and salient feature of mammalian visual systems. This dissertation addresses the question---what neural processing steps are performed to generate percepts of color? Towards answering this question, the neural circuit underlying blue-yellow color vision was explored in two rodent species, the gerbil and rat. These animals share evolutionarily conserved neural circuits for processing the output of two classes of spectrally distinct cone photoreceptor; however, each species has distinct advantages for different experimental approaches. The anatomy and physiology of the blue-yellow circuit was examined at all levels, from photoreceptor activation through the circuits in the central nervous system to the ultimate endpoint of visual behavior. This was accomplished by a combination of retinal imaging, gross potential electrophysiology, and high resolution functional magnetic resonance imaging (fMRI) plus behavioral vision testing. The circuitry for color coding was dissected by testing hypotheses using pharmacological blockade of specific pathways and by selectively adding input to pathways using gene therapy. Textbook color theory attributes the perception of blueness to comparisons between S(blue)-cones and M(green)-cones made by small bistratified ganglion cells. Experiments presented here using high resolution fMRI and pharmacological blockade of the ON-pathway in rats demonstrate that the small bistratified cells are not the substrate for S-cone responses, but rather they provide inhibitory input that gates spurious responses to dark-light contrast in the blue-yellow circuit. The observed cortical activation by short-wavelength light under ON-pathway blockade indicates that S-cone driven responses are mediated by OFF responses via horizontal cell feedback from S-cones. This was further examined by adding new sensitivity to chromatic or achromatic pathways using retinal gene therapy and assaying the functional consequences by measuring the neuronal response in the visual system with fMRI and behavioral tests of color vision. The results reported in this dissertation provide evidence for a modern, unifying theory of color vision that accounts for the spectral signatures observed from psychophysics, provides a mechanism for the disambiguation of responses from color and dark-light contrast that are confounded in the retina, and explains the spectral and spatial opponency of the small-bistratified cell.