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Authors & Affiliations
Shonali Dhingra,Chinmay Purandare,Mayank Mehta
Abstract
Visual cortical neurons are known to encode the position and motion direction of specific stimuli retrospectively, without any locomotion or task demand. Hippocampus, though receives projections from these visual cortical areas, is hypothesized to require self-motion or cognitive task to generate allocentric spatial selectivity that is abstract and prospective. In an attempt to bridge these disparities, we measured hippocampal responses to a moving bar of light in a body-fixed rat, without imposing any task demand on the animal. For a revolving bar, about 70% of dorsal CA1 neurons showed stable activity modulation as a function of the bar’s angular position, independent of behavior and rewards. A third of tuned cells also encoded the direction of revolution. For a linearly moving bar, neurons encoded for distance of the bar from the animal, with a preference for the approaching motion. Collectively, we term these results as visually evoked vectorial selectivity (VEVS) in CA1 cells. Unlike most place cells, VEVS was found to be largely retrospective. Changes in the visual stimulus or its trajectory did not lead to remapping, but only caused gradual changes in these responses. Most VEVS tuned neurons behaved like place cells during spatial exploration and the selectivity in these two domains were found to be correlated. We thus hypothesize that VEVS responses could form basic hippocampal building blocks, through which responses from the visual cortex get relayed after some filtering, for further processing and context-associations. When combined with self-motion, reward, or multisensory stimuli, these can generate the diversity of abstract and prospective representations including space, time, and episodes.