Functional Architecture and Visual Response Properties in the Mouse Superior ColliculusPublic Deposited
The mouse visual system has recently been shown to possess many of the properties observed in the visual systems of the classically studied carnivores and primates. An ever-expanding genetic toolkit has given researchers who study vision in mice many advantages unavailable in other species. In this thesis I study the mouse superior colliculus (SC), a midbrain structure involved in sensory-motor integration. The SC is central to the mouse’s vision, and visually-guided behaviors; a function that is gradually deferred to the visual cortex in carnivores and primates. The centrality of the SC in mice in fact gives us some advantages. It represents a compact and simplified visual system, receiving direct input from the sensory periphery (the retina), with nonetheless great influence on behavior. Here, and with invaluable help from many collaborators, I specifically explore three central questions about the organization and function of the most superficial lamina of the mouse SC, the stratum griseum superficiale (SGS), where neurons are responsive to visual stimuli. First, I demonstrate a functional organization of neurons in the SGS following the degree of their direction selectivity (DS). Specifically, I show that neurons that are selective for motion direction tend to be concentrated in the topmost lamina of the SGS, and become gradually scarcer in the deeper laminae. Second, in a knockout mouse line where the DS of direction selective ganglion cells (DSGCs) is reduced, I show a complimentary reduction in the DS of SGS neurons. This is the first direct demonstration that retinal DSGCs are the source of the DS observed in the SGS. Third, I provide a first description of visual saliency responses in the mouse SGS, showing a fundamental difference in the response of excitatory and inhibitory neurons. This sets the stage for a better understanding of the contribution of different functional cell types in the SGS to visual signal processing at the microcircuit level.
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