This research project seeks to advance our understanding of the function of the retina in two fundamental aspects of vision: 1) Contrast Processing and 2) Light Adaptation. This will be done by analyzing the light-evoked responses of bipolar cells, horizontal cells, amacrine cells, and cone photoreceptor cells in the tiger salamander (Ambystoma tigrinum), an animal model that offers unique opportunities for intracellular recording. The primary goal of this proposal is to obtain a better understanding of the transformations that take place at the first and second synaptic stages of the retina. The experiments center on intracellular recordings from bipolar cells. Several of the experiments parallel human psychophysical work so the results could eventually lead to new models for clinical disorders in contrast perception and light adaptation. Four transformations take place between the photoreceptor and bipolar cell response: 1) The bipolar response to weak contrasts is amplified. 2) Some bipolar cells invert, while others conserve the polarity of the photoreceptor signal, thus splitting visual information into two channels for parallel processing. 3) There is diversity in the response properties of bipolar cells, indicating that distributed encoding is a central feature of bipolar cell function. 4) Bipolar cells show strong center-surround antagonism and thereby function as spatial filters of the retinal image. The present proposal is designed to obtain a deeper understanding of all four transformations. Specific experiments are proposed under the general headings of 1) Contrast Processing and Amplification and 2) Center-Surround Antagonism. These include: a) Experiments using sinusoidal modulation and steps of unusually low contrast to identify the linear and non-linear operating range of cone-driven bipolar cells, horizontal cells, and cone photoreceptors, b) Analyses of synaptic mechanisms by which bipolar cells shape the response of on-off amacrine cells, c) Studies of the role of feedforward vs. feedback pathways in mediating the surround of the bipolar cell receptive field and d) Studies of the mechanisms mediating large-field noise, surround antagonism, and spatial resolution in bipolar cells.
