The hippocampal formation is a group of brain regions, including the hippocampus and some other structures closely connected to it. This set of brain regions is known to support various important brain functions, including the consolidation of memories, experience-based reasoning, decision-making and spatial navigation (i.e., the ability to map and navigate specific environments), according to Medical Xpress.

Most past neuroscience studies have focused on how the hippocampus supports episodic memory (i.e., the memory of personal experiences or life events) and spatial navigation. Yet recent findings suggest that this brain region also contributes to what is known as compositional memory, which entails 'breaking down' memories into fundamental building blocks and reassembling these blocks to form new memories or predict future outcomes.

Researchers at University of Oxford, University College London and other institutes recently carried out a study aimed at shedding further light on how the hippocampus supports this breaking down and re-purposing of memories, using a combination of computer simulations and brain activity recordings. Their findings, published in Nature Neuroscience, suggest that the hippocampus helps to shape future behavior via two connected processes, known as memory composition and hippocampal replay.

"The hippocampus is critical for memory, imagination and constructive reasoning," wrote Jacob J. W. Bakermans, Joseph Warren and their colleagues in their paper. "Recent models have suggested that its neuronal responses can be well explained by state spaces that model the transitions between experiences. We use simulations and hippocampal recordings to reconcile these views."

Previous studies found that the consolidation of memories could be supported by a process known as hippocampal replay. In other words, they suggested that neurons in the hippocampus 'replay' memories of past experiences, especially during sleep or while resting, to facilitate learning and help us remember things for longer periods of time.

As part of their recent work, Bakermans, Warren and their colleagues explored the possibility that hippocampal state spaces (i.e., mathematical models of how experiences are represented in the hippocampus) are compositional in nature. The brain could thus selectively use and re-arrange specific parts of our memories of experiences, known as primitives, to imagine or predict possible scenarios.

"We show that if state spaces are constructed compositionally from existing building blocks, or primitives, hippocampal responses can be interpreted as compositional memories, binding these primitives together," wrote the researchers. "Critically, this enables agents to behave optimally in new environments with no new learning, inferring behavior directly from the composition."

In their paper, the researchers also suggest that the replay of memories by neurons in the hippocampus contributes to the consolidation of compositional memories. Instead of being a passive memory process, as suggested by some previous works, hippocampal replay could actively support the formation of flexible memories that can be later broken apart and used to inform future behavior.

"We predict a role for hippocampal replay in building and consolidating these compositional memories," wrote the researchers. "We test these predictions in two datasets by showing that replay events from newly discovered landmarks induce and strengthen new remote firing fields. When the landmark is moved, replay builds a new firing field at the same vector to the new location. Together, these findings provide a framework for reasoning about compositional memories and demonstrate that such memories are formed in hippocampal replay."

This recent study by Bakermans, Warren and their colleagues offers a new perspective on how the hippocampus allows humans to use their past experiences to make predictions or imagine new scenarios. In the future, it could inspire more research aimed at testing the idea proposed by the researchers, potentially helping to paint a clearer picture of the hippocampus that coherently unifies its different functions.