Each retinal cell type shows a characteristic set of physiological properties and connections with other cell types within the retinal layers. In an astounding feat of neural efficiency, all of this complex circuitry is packaged in a thin, precisely laminated sheet of tissue. Without doubt, understanding the functional architecture of the retinal circuitry must begin by first characterizing these cell types. Most of these cell types have not yet been studied in detail, but their existence is no longer disputed. Additional networks of interneurons allow lateral interactions to modify these parallel pathways: 2 horizontal cell types, at the level of the photoreceptor–bipolar cell synapse, and 20–40 amacrine cell types at the level of the bipolar –ganglion cell synapse ( 2– 4). The bipolar cells in turn contact 20–25 distinct ganglion cell types, which give rise to an equal number of parallel pathways to the visual brain. ![]() Rods and two or three types of cone photoreceptors relay signals to at least 10 types of bipolar interneurons. But only within the last decade has it become clear that the retina contains a diversity of neural cell types comparable, in fact, to that of the cerebral cortex. Since the anatomical renderings of Cajal ( 1), the basic framework of retinal circuitry has been known. And like other parts of the brain, the retina is a beautiful and complex piece of neural machinery, although it has taken nearly 100 years for the degree and nature of its complexity to be fully appreciated. The vertebrate retina is that part of the central nervous system where multiple parallel representations of the visual world first emerge. The solution to this color puzzle no doubt lies in the great diversity of cell types in the primate retina that still await discovery and analysis. For example, receptive field mapping argues for segregation of Land M-cone signals to the midget cell center and surround, but horizontal cell interneurons, believed to generate the inhibitory surround, lack opponency and cannot contribute selective L- or M-cone input to the midget cell surround. Red-green spectral opponency has long been linked to the midget ganglion cells, but an underlying mechanism remains unclear. Surprisingly, this cone opponency appears to arise by dual excitatory cone bipolar cell inputs: an ON bipolar cell that contacts only S-cones and an OFF bipolar cell that contacts L- and M-cones. Intracellular recording and staining has shown that blueON/yellow-OFF opponent responses arise from a distinctive bistratified ganglion cell type. The circuitry for spectral opponency is now being investigated using an in vitro preparation of the macaque monkey retina. In the classical model this antagonism is thought to arise from the convergence of cone type-specific excitatory and inhibitory inputs to retinal ganglion cells. ![]() Within the retina these signals combine in an antagonistic way to form red–green and blue –yellow spectral opponent pathways. (color opponent/cone photoreceptors/ganglion cells/horizontal cells/bipolar cells)ĭepartment of Biological Structure, University of Washington, Box 357420, Seattle, WA 98195-7420ĪBSTRACT Human color vision starts with the signals from three cone photoreceptor types, maximally sensitive to long (L-cone), middle (M-cone), and short (S-cone) wavelengths. Circuitry for color coding in the primate retina
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