MULTINEURONAL SIGNALING BY THE RETINA

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This research project aims for a detailed understanding of how the eye signals visual information to the brain. Functional investigations of the retina have primarily determined the properties of single neurons and their synaptic connections, while comparatively little is known about how a given cell's activity correlates in time with that of others. Yet the visual centers in the brain draw on the pattern of simultaneous firing in a large population of retinal ganglion cells in order to detect important features of our visual environment. Thus, the ultimate goal of the proposed work is to determine precisely how the retina encodes a visual scene by the patterns of electrical activity in the ganglion cell layer. The work focuses on the representation of visual information in a population of neurons, rather than individual cells. In these experiments, the isolated retina of a tiger salamander is placed on a glass surface with many embedded metal microelectrodes, covering an area of about 0.2mm(2). The retinal ganglion cells lie in close proximity to the electrode array, such that each site records extracellular action potentials generated by a few nearby neurons. Thus, one can monitor simultaneously the electrical signals from up to 100 ganglion cells. The retina is stimulated with sensory input by projecting an image from a computer monitor onto the photoreceptor layer. These methods will be used in the following specific projects: (1) A survey of the functional properties of ganglion cells in the retina. This study will determine the numerical proportions among cells of different response types, whether they are arranged in a spatially organized pattern, and how their distribution relates to the type of information they can extract from the visual stimulus. (2) A study of correlations between the firing patterns of different ganglion cells. These neurons are often strongly correlated in their activity, and participate in modes of concerted firing that can only be observed by multielectrode recording. Do these distributed firing patterns convey visual information? (3) An investigation of how features of the visual stimulus might be estimated from the recorded ganglion cell spike trains. The goal is to construct an algorithm that can interpret the retinal output to provide a reconstruction of the visual input. This will uncover the structure of the retinal code and suggest how circuits at subsequent visual centers might perform similar tasks of feature detection. These studies will reveal important aspects of collective function in the circuits of the retina. The results will provide a better understanding of how we see, and how the brain represents and processes information. Ultimately, this 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.