Elise Laura Savier - US grants

PROJECT SUMMARY Distinct visual features are extracted in parallel along the visual pathways and combined at different levels. Two parallel pathways can be found early on, with projections from the retina targeting the dorsal lateral geniculate nucleus, which targets cortical areas, and the superior colliculus (SC), which projects to the pulvinar among other structures. In addition to receiving direct input from the retina, the SC also receives inputs from the primary visual cortex (V1). These two pathways are often referred to as the primary and the secondary pathway respectively and are inter-connected, notably at the level of the SC. Importantly, retinal and cortical inputs in the SC converge onto one morphologically and molecularly defined cell type: the wide-field vertical cells (WFV). WFV cells dendritic arborizations sample a large part of the visual field and span across the entire depth of the visual layers of the SC, suggesting that they integrate both local and long-range inputs. Their morphology and projection to the pulvinar are conserved across species. These observations place WFV cells as key integrators of visual information across pathways. To understand how WFV cells integrate visual information, the investigator will perform a careful characterization of their inputs, response properties and modulation by internal factors and V1 in two animal models: mice (Mus musculus) and tree shrews (Tupaia belangerii), an animal model which is highly visual and close to primates. First, WFV cells responses properties and the effect of locomotion will be characterized in awake mice using calcium imaging. WFV cells inputs will be identified by using retrograde viral tracing and further identified by histological methods. Secondly, cortical inputs will be isolated functionally by using a chemogenetic approach to determine how these inputs shape WFV cells visual responses. Finally, these findings in mice will be used as a reference as this work is expanded to tree shrews. Other studies carried in different animal models have shown a variability in the amount of feature selectivity that can be found in the SC and in the contribution of the primary visual pathway to visual responses in the SC. To reconcile these results, a comparative study will be conducted in both mice, where many genetic tools are available, and tree shrews, which display high visual acuity and visually-driven behavior. This project will yield compelling results regarding the integration of visual information across the visual system, allow a direct comparison of the same defined cell-type across two species, and reveal how internal states potentially shape visual responses in the SC.