Coordinated Computations for Naturalistic Memory Encoding and Retrieval in Hippocampal Neural Subspaces

Our naturalistic experiences are organized into memories through multiple processes, including novelty encoding, memory formation, and retrieval. However, the neural mechanisms coordinating these processes remain elusive. Using fMRI data acquired during movie viewing and subsequent narrative recall, we examined hippocampal neural subspaces associated with distinct memory processes and characterized their relationships. We quantified novelty in character co-occurrences and the valence of relationships and estimated event memorability. Within the hippocampus, the novelty subspaces encoding each type exhibited partial overlap, and these overlapping novelty subspaces aligned with the subspace involved in memorability. Notably, following event boundaries, hippocampal states within these subspaces aligned inversely along a shared coding axis, predicting subsequent recall performance. This novelty-memorability alignment was selectively observed during encoding but not during retrieval. Our findings offer novel insights into how the hippocampus dynamically coordinates computations underlying memory encoding and retrieval at the population level to transform ongoing experiences into enduring memories.