Synaptic Mechanisms Underlying Direction Selectivity in Rabbit Retinal Ganglion Cells
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In the vertebrate visual system, direction-selective signals are first encountered within the direction-selective ganglion cells (DSGCs) in the retina. The On-Off DSGCs have been most intensively studied, and a number of models have been advanced to account for the physiological properties of these cells. In order to account for the directional asymmetry of the physiological responses, all models incorporate some type of spatial asymmetry in the arrangement of the synaptic circuitry. However, there is no general agreement as to the synaptic locus of this critical synaptic asymmetry. Some have suggested that direction selectivity is generated by synaptic interactions that occur prior to the DSGC, and that the excitatory or inhibitory inputs to the cell are already direction selective. Others have suggested that the inputs to the DSGCs are directionally isotropic, and that there is an asymmetry in the interaction of the excitation and inhibition within the dendrites of the DSGCs.