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The population perspective : how primate retinal ganglion cells collectively encode visual space


Vision begins in the retina, where populations retinal ganglion cells (RGCs) collectively encode all visual information reaching the brain. RGC encoding is based on the spatial, temporal, and chromatic properties of the receptive field (RF) of each cell. Within each RGC type, RFs are arranged in a regularly spaced lattice that spans the visual field. While much is known about the functional properties and circuitry of individual RFs, the organization of the population has remained elusive. This dissertation employs a recently introduced methodology to make simultaneous, large scale recordings from isolated patches of primate retina. These recordings allow many features of the population level organization of RGCs to be appreciated for the first time. The output of the primate retina is numerically dominated by four RGC types: ON- and OFF-parasol cells and ON- and OFF-midget cells, which respectively form the major inputs to the magnocellular and parvocellular pathways. The spatial arrangement of these cell types is studied in several important regards. First, Chapter 2 demonstrates that the average RF overlap is nearly identical in all four types. This echoes the result of a previous study that found nearly the same degree of overlap in several RGC types in the rabbit retina. However, such homogeneity is surprising given the very different anatomical organization of parasol and midget cells and their diverse roles in visual encoding. Chapter 3 explores the fine shape of individual RFs. Previous studies have found irregularly shaped RFs, and concluded that this "noise" in retinal organization was likely detrimental to visual coverage. However, appreciating the arrangement of a full population revealed that individual RF shapes interlock like puzzle pieces to provide more uniform coverage of visual space. This finding bears an interesting relationship to RGC anatomical organization, and might point to novel developmental mechanisms. Finally, the organization of midget cells is studied at its elementary resolution: individual cone inputs to the RF. Chapter 4 reveals that midget RFs can sample the same cones as neighboring cells, a finding with several implications for the anatomical and functional organization of the midget cell pathway

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