STRUCTURED HIPPOCAMPAL REPRESENTATIONS OF NATURALISTIC VISUAL EPISODES IN FREELY VIEWING MARMOSETS

Episodic memories arise from continuous visual experiences that unfold across space and time. Although visual cortical neurons encode many features of these experiences, how such information is transformed into episodic memory representations remains unresolved. In non-human primates, converging evidence suggests that the hippocampus is strongly driven by visual input. It receives dense projections from visual areas including inferior temporal (IT) and V4 cortices, participates in scene recognition and memory-related gaze behavior, and contains neurons selective for specific visual views. Yet it is unknown whether hippocampal ensembles encode extended visual episodes composed of sequential scenes. To address this question, we developed a free-viewing movie paradigm in common marmosets, simultaneously recording eye position (Eyelink 1000) and neural activity in hippocampal CA3 using Neuropixels probes. Gaze behavior during movie viewing was highly reliable: strongly correlated across repeated presentations of the same movie and substantially less correlated across different movies. These differences reflected movie content, as gaze consistently tracked salient features such as faces and moving agents. Hippocampal neurons (average of 100 simultaneously recorded units per session) showed robust modulation by movie presentation (~40% of isolated units) and selective responses to specific temporal segments. A linear support vector machine trained on ensemble activity reliably classified movie identity (80% accuracy; chance: 25%; p < 0.001). These findings demonstrate that the primate hippocampus encodes dynamic visual experiences as structured spatiotemporal episodes, providing a potential neural mechanism for the formation and retrieval of episodic memories.