The long range goal of this research is to understand the roles played by the superior colliculus (SC) in visual function. This requires a full accounting of the functional properties of the retinal ganglion cells innervating the SC. In the cat, almost all of these cells belong to the W-cell class. W-cells comprise 50 percent of cat ganglion cells and have obvious equivalents in primates and other mammals. In addition to dominating the retinocollicular pathway, W-cells provide most or all of the retinal input to a host of other visual nuclei that subserve visuomotor reflexes. Despite the ubiquity and importance of W-cells, study of their structure and function has been severely limited by technical factors. This project will overcome these limitations by means of a novel in vitro approach, thereby shedding new light on the organization of the retinocollicular pathway. W-cells are extremely heterogeneous in morphology and function. Rather than representing a true class, W-cells apparently constitute a loose grouping of many distinct ganglion-cell types, each as different from the others as it is from the X-and Y-cell types. Work from this laboratory in past and current project periods indicates that individual W-cell subtypes may exhibit distinctive patterns of collicular projection. The retinocollicular pathway is thus far more complex that has been generally appreciated. Functionally distinct W-cell channels may be processed independently by distinct collicular microcircuits, or may undergo a highly orderly integrative recombination by collicular neurons. Clearly, a prerequisite for understanding the functional meaning of this parallel retinocollicular organization is a detailed understanding of individual W-cell types. The proposed project begins this process through an in depth analysis of a single W-cell type -- the zeta cell -- which is a dominant contributor to the retinocollicular pathway. These W-cells will be fluorescently tagged, either by retrograde transport of tracers from the colliculus or by uptake of a vital dye. The retina will then be maintained in vitro and electrodes advanced toward tagged zeta cells under visual control. The visual response properties of these cells will be studied by extracellular and intracellular recording and their morphology revealed in detail by intracellular dye injection. This approach provides the first practical method for making direct correlations between the morphology, physiology and central projections of single ganglion cells. The method will be used to analyze the stimulus selectivities of these cells, to explore the intraretinal circuitry that produces these response properties, and to characterize the anatomical mosaic through which these cells sample the visual scene. The study will expand our understanding of diversity among the output cells of the retina and the parallel channels of visual information they emit to the SC and other brain centers. It will also establish a powerful new experimental paradigm with extremely broad applicability to the study of retinal ganglion cells.
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