Neurobiology Faculty Candidate Talk: Tristan Geiller: Organization and function of local hippocampal circuits for navigation

Cognitive functions are the result of the coordinated action of neuronal populations organized within, and across brain regions. The goal of my research is to uncover how these populations interact to form specialized brain circuits, and how imbalances in these circuits can result in pathological states. My work, to date, has primarily focused on the architecture and function of local circuits in the hippocampus, a region critically involved in memory and navigation. During spatial behaviors, individual hippocampal pyramidal cells, known as 'place cells', are selectively active at specific locations along the animal's trajectory. Populations of place cells collectively form a representation of the environment, providing a substrate thought to support spatial learning and navigation. While these patterns of activity have been extensively described, in particular in hippocampal area CA1, we still lack a mechanistic understanding of how the spatial selectivity of individual neurons emerge from local circuit structure, and how responses are coordinated to produce behaviorally-relevant representations at the population level. Here, I will present my recent work, as well as the novel experimental approaches used to examine the fine-scale organization of CA1 circuits in behaving mice. I will discuss how CA1 may intrinsically support the formation of cell assemblies, manifesting as strong interactions between excitatory neurons, which can be manipulated to enhance spatial learning performance. I will show that these dynamics are extensively regulated by various subpopulations of local inhibitory interneurons, whose distinct functions are now being delineated in the behaving animal. Building upon this work, my future research program will determine the mechanisms by which spatial representations of the hippocampus are read, utilized, and transformed in downstream regions involved in executive functions and long-term memory storage, to effectively support complex navigational behaviors