All information for vision reaches the brain through action potentials coursing along the optic nerve fibers. This research project aims to determine precisely how the visual scene is encoded in these neural messages. This is of fundamental importance, both for understanding the function of the retina, which generates these signals, and for the interpretation of visual processes in the brain, which rely on these signals. Ultimately, these insights may lead to improvements in detecting and treating visual deficiencies, and, in the far future, to the development of visual prostheses that can emulate the function of the retina. Whereas previous functional investigations of the retina have primarily focused on the properties of single neurons and their synaptic connections, the present approach relies on monitoring the patterns of firing simultaneously in a large population of retinal ganglion cells. The isolated retina of a tiger salamander or a rabbit is placed on a glass surface with many embedded metal microelectrodes, covering an area of about 0.2 mm2, The retinal ganglion cells lie in close proximity to the electrodes, and thus one can monitor simultaneously the extracellular action potentials from 50 or more ganglion cells. The retina is stimulated with sensory input by projecting an image from a computer monitor onto the photoreceptor layer. Comparison of the retinal response with the visual stimulus will assess the neural code used by the retina, with the following specific aims: (1) To investigate the retinal circuits that underlie synchronous firing among groups of retinal ganglion cells. (2) To assess what mechanisms give rise to long-range synchrony in ganglion cell firing, and how these interactions shape visual processing. (3) To determine the nature of the neural code employed by the retina, specifically under what conditions it is stochastic or deterministic. (4) To study how the retinal code adapts to the properties of the visual environment, specifically whether it adjusts to scene statistics other than the mean intensity, such as the intensity contrast.
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