Shifting Gears in Hippocampus: Temporal Dissociation between Familiarity and Novelty Signatures in a Single Event

The hippocampus is known to be involved in encoding and retrieval of episodes. However, real-life experiences are expected to involve both encoding and retrieval, and it is unclear how the human hippocampus subserves both functions in the course of a single event. We presented participants with brief movie clips multiple times and examined the effect of familiarity on the hippocampal response at event onset versus event offset. Increased familiarity resulted in a decreased offset response, indicating that the offset response is a novelty-related signature. The magnitude of this offset response was correlated, across hippocampal voxels, with an independent measure of successful encoding, based on nonrepeated clips. This suggests that the attenuated offset response to familiar clips reflects reduced encoding. In addition, the posterior hippocampus exhibited an increased onset response to familiar events, switching from an online familiarity signal to an offline novelty signal during a single event. Moreover, participants with stronger memory exhibited increased reactivation of online activity during familiar events, in line with a retrieval signature. Our results reveal a spatiotemporal dissociation between novelty/encoding and familiarity/retrieval signatures, assumed to reflect different computational modes, in response to the same stimulus.