The superficial layers of the superior colliculus (SC) contain two cell types that both respond to the movement of visual stimuli, but are morphologically and functionally distinct. A projection from the SC to the lateral posterior nucleus (LPN) originates from wide-field vertical (WFV) cells, while a projection from the SC to the dorsal lateral geniculate nucleus (dLGN) originates from narrow-field vertical (NFV) cells. WFV cells have been described as motion detectors based on their responses to small stimuli moving in any direction within a very large receptive field. In contrast, NFV cells may be specialized to code more detailed motion parameters based on their small receptive fields and strong direction selectivity. The parallel WFV and NFV pathways remain segregated in that the tectorecipient dLGN and LPN project differentially to the striate and extrastriate cortex. However, little is known regarding the interaction between the SC, the tectorecipient thalamus, and the cortex. We propose to analyze the circuits that connect these structures in mice by using novel combinations of optogenetics, in vitro whole cell recordings from neuronal populations identified by retrograde tracing techniques, as well as quantitative electron microscopic investigation of synaptic connections.
The Aim 1 experiments will use in vitro whole cell recording from identified cortical cell populations and optogenetic activation of terminals tht originate from the tectorecipient dLGN or LPN to determine which cell types are directly innervated and to characterize the electrophysiological properties of these synapses. Electron microscopy will quantify ultrastructural features of these synapses.
The Aim 2 experiments will use in vitro whole cell recordings from NFV and WFV cells and optogenetic activation of corticotectal terminals to determine whether these cells receive direct or indirect input from V1 or the lateral extrastriate cortex, and to characterize the electrophysiological properties of thes connections. Electron microscopy will quantify ultrastructural features of corticotectal synapses and the distribution inputs to NFV and WFV cells that do and do not contain gamma amino butyric acid (GABA). As comparisons of parallel geniculocortical pathways have led to insights regarding cortical processing streams, a comparison of WFV and NFV tecto-thalamo-cortical pathways will help us to understand how different aspects of visual motion are utilized by the visual system. In addition, our circuit analysis can help reveal whether corticotectal pathways are organized to enhance segregation, or synthesis, of motion signals
The results of these experiments will significantly enhance our understanding of the brain circuitry underlying visual motion perception, thus providing information relevant to the treatment of disorders in which visual motion processing is compromised, such as dyslexia and schizophrenia.
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