UC Davis

Theories of hippocampal function have consistently focused on this remarkable regions’ important role in spatial navigation and memory. Recent work in cognitive and systems neuroscience have suggested that the hippocampus might support planning, imagination, and navigation by forming “cognitive maps” that capture the abstract structure of physical spaces, tasks, and situations. However, a critical aspect of planning and navigation is that both involve simulating, and acting upon, a sequence of actions to reach a goal. Despite several decades of work investigating hippocampal function, the precise mechanism and neural basis of planning during navigation is still unclear. This dissertation investigated how goals impact activity patterns in the hippocampus during planning and navigation. In Part 1, we examined hippocampal activity patterns in humans, using a goal-directed navigation task, to examine how goal information is incorporated in the construction and execution of navigational plans. Interestingly, we found that hippocampal activity patterns were more similar when participants planned routes that led to the same goal. During navigation, we found that rather than simply representing the current location in space, or the immediate future (as predicted by many theories of hippocampal function), the hippocampus reactivated a key decision point along a route. Building on these findings, in Part 2, a biologically inspired neural network of the hippocampus was used to investigate two plausible theories of the mechanisms underlying planning in humans; chaining and chunking. To simulate chaining during planning, the network was trained to retrieve pairs of overlapping associations in sequence. Using this framework, the model performed quite poorly at moments of high overlap between other sequences. This was in contrast to a model that utilized chunking, which was provided with only the most relevant information for retrieving a navigational memory. Taken together, this dissertation extends prior work in planning and navigation by highlighting the importance of external inputs into the hippocampal circuit to provide both structure and goal-relevant information, which are essential components of human memory.