Visual signaling from retina to superior colliculus

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Project Summary A major target of retinal output is the superior colliculus (SC). In fact, more retinal ganglion cell (RGC) types may project to the SC than any other retinal target including the lateral geniculate nucleus (LGN). Understanding retinal input to SC is important because SC plays a major role in a range of attentional and decision-making processes in both rodents and primates – two major model systems used in biomedical research supported by the National Institutes of Health. However, the functional diversity of retinal input to SC, and ultimately how it impacts SC signaling, remains poorly understood in mammals. This gap is particularly pronounced in primates. The overarching goal of this proposal is to determine the diversity of cell types and the visual signals they transmit from the retina to SC in rats and rhesus monkeys. The rationale for this proposal is that to understand the role of SC in visually guided behaviors, we must determine how retinal signals converge and are processed in SC. The first step toward achieving this goal is to determine which RGC types project to SC and what visual signals they carry. Performing these experiments in both rodents and macaques is critical not just for understanding which specific visual pathways are conserved (or diverge) from rodent to the primate brain, but also what evolutionary advantages such specializations endow to each species. In Aim 1 of this proposal, we will use and optimize viral methods for retrogradely infecting RGCs that project directly to SC in rats. We will determine the morphological diversity of these RGCs. We will also determine their receptive fields and other visual response properties, ex vivo, using large-scale multi-electrode arrays. The outcome will be a complete catalog of the morphological and functional types of RGCs that project to SC in the rat brain. In Aim 2, we will use the most effective viral approaches from Aim 1 to dissect the diversity of RGC types that project to SC in monkeys. As with rats, we will determine the morphological diversity of these RGCs in macaques and determine their receptive fields and other visual response properties using large-scale, high throughput electrophysiology. In Aim 3, we will determine the overlap of RGC projections to SC and LGN, separately for rats and primates. Retrograde viruses injected into SC and LGN will carry genes for different fluorescent proteins that will allow us to determine the types and functions of RGCs that project to one versus both brain areas. The overall outcome of this project will be a functional and morphological catalog of RGCs that project to SC in rats and primates, allowing for detailed cross-species comparison of this key visual circuit. This comparison is important given how much research is dedicated to the rodent visual system with the ultimate aim of understanding the human visual system. The data will be critical for designing next-stage studies that will measure and manipulate the functions of specific populations of SC-projecting RGCs in order to determine their contributions to visual processing and behavior and their potential impairments in ADHD and other attentional and visuomotor disorders.