The Neural Mechanism for Visual Adaptation

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In vertebrates, rod vision is the ability to see under low-light conditions with lower visual quality; cone vision functions at higher light levels and responsible for all high resolution and color vision. In order to transition from rod to cone vision, the retina must undergo adaptation to adjust the responding properties of the neural circuits. However, little is known about the neural mechanisms for switching from rod to cone vision. This project aims to identify a key neural process fundamental to the unique mechanism in visual adaptation. The objectives of the study are to address fundamental questions concerning how the actions of the network neurons releasing specific neurotransmitter - glycine, to modify the response properties and light sensitivity of the retinal circuits. A series of experiments are designed to systematically investigate this novel process involved in fine-tuning of cone vision in dim or twilight conditions. The project will also allow the researcher to continue the important mission of integrating research, education and training of future scientists in cutting-edge techniques for understanding the functional interactions of neural circuits and their behavioral manifestations. Finally, this project will support outreach to local high school, and community college students as well as the lifelong learning program for seniors in southern Florida. The goal of this project is to define a unique mechanism, utilizing network feedback of glycine, that modifies center-surround receptive field organization and subserves cone vision. Cone photoreceptors are responsible for high-resolution vision, but are less sensitive to light compared to rods. Indeed, the sensitivity of cone vision is suppressed at dusk or twilight. Psychophysical studies indicate that flickering light stimulation can improve spatial-temporal sensitivity (cone features), thereby improving cone vision in twilight conditions. Although it is argued that this process is related to adaptation- dependent changes in the retinal neural circuitry, the details of the adaptive mechanisms are largely unknown. Previous work by the principal investigator has shown that a group of network neurons, utilizing long-range feedback of glycine, can mediate rod-cone interactions that serve for improving cone visual sensitivity in twilight conditions. This project is to address the central hypothesis that the function of glycine feedback regulates the center-surround receptive field organization that is the key element for spatial and temporal tuning of cone vision. The primary objectives are to address fundamental questions concerning how the actions of glycine feedback network modify (i) the lateral inhibition and (ii) synaptic gains and contrast sensitivity in the distal retina. Multiple advanced techniques, including electrophysiology, fluorescence imaging and photic stimulation will be used in the study. The results from the study will extend the current understanding of glycine functioning solely as an inhibitory neurotransmitter and shed light on its excitatory actions on retinal circuits. As the retina is a part of the CNS and shares many neural mechanisms with the brain, the results from this proposal will reorient concepts about the function of glycine. In addition, the studies will provide a unique opportunity for undergraduate and graduate students to participate and conduct research in a high-demand area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.