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D.A. Burkhardt, P.K. Fahey; Retinal Bipolar Cells in Land- and Aquatic-Phase Tiger Salamander: Quantitative Relations Between Contrast Encoding and Properties of Natural Images . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4151.
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Purpose: To investigate relationships between contrast encoding in bipolar cells and the distribution of contrasts in natural images. Methods: Intracellular recordings were obtained from 57 cone-driven bipolar cells in the light-adapted retina of the land-phase (adult) tiger salamander (Ambystoma tigrinum). Responses to flashes of negative and positive contrast for centered spots of optimum spatial dimensions were analyzed as a function of contrast magnitude. Results: On average, the contrast/response curves of depolarizing and hyperpolarizing bipolar cells in the land-phase tiger salamander are remarkably similar to those reported previously by us for the aquatic-phase tiger salamander. The predictive coding hypothesis of S.B. Laughlin was examined by comparing the average cumulative contrast/response curve of bipolar cells with the cumulative probability distribution of contrasts in natural images. Good correspondence was found at 20 cd/m2 but at lower levels of light adaptation, the contrast/response curves were much too shallow. A mismatch between data and hypothesis was found for cones, even when the retina was light-adapted to 20 cd/m2. For both land- and aquatic-based animals, the dynamic range of the contrast/response of bipolar cells varied greatly from cell to cell in a manner consistent with distributed coding of contrast information. Conclusions: 1) The primary retinal mechanisms mediating contrast coding in the outer retina remain functionally stable as the salamander evolves from the aquatic to the land phase. 2) Light adaptation and the transfer across the cone->bipolar synapse act to bring the response of bipolar cells into register with the distribution of contrasts in the natural environment. This provides a new view for the function of light adaptation and the cone-bipolar synapse. 3) The dynamic range of the contrast/response of bipolar cells varies appreciably and in an apparently continuous manner from cell to cell. As a result, the response of the bipolar cell population effectively brackets the distribution of contrasts found in natural images.
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