Constructing future behaviour in the hippocampal formation through composition and replay

Hippocampus is critical for memory, imagination, and constructive reasoning. However, recent models have suggested that its neuronal responses can be well explained by state-spaces that model the transitions between experiences. How do we reconcile these two views? Here we show that if state-spaces are constructed compositionally from existing primitives, hippocampal responses can be interpreted as compositional memories, binding these primitives together. Critically, this enables agents to behave optimally in novel environments with no new learning, inferring behaviour directly from the composition. This provides natural interpretations of generalisation and latent learning. Hippocampal replay can build and consolidate these compositional memories, but importantly, due to their compositional nature, it can construct states it has never experienced - effectively building memories of the future. This enables new predictions of optimal replays for novel environments, or after structural changes. Together, these findings provide a framework for reasoning about several seemingly disparate functions of hippocampus. ### Competing Interest Statement The authors have declared no competing interest.