Geerts, Jesse Pepijn; (2021) Hippocampal predictive maps of an uncertain world. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Humans and other animals can solve a wide variety of decision-making problems with remarkable flexibility. This flexibility is thought to derive from an internal model of the world, or ‘cognitive map’, used to predict the future and plan actions accordingly. A recent theoretical proposal suggests that the hippocampus houses a representation of long-run state expectancies. These “successor representations” (SRs) occupy a middle ground between model-free and model-based reinforcement learning strategies. However, it is not clear whether SRs can explain hippocampal contributions to spatial and model-based behaviour, nor how a putative hippocampal SR might interface with striatal learning mechanisms. More generally, it is not clear how the predictive map should encode uncertainty, and how an uncertainty-augmented predictive map modifies our experimental predictions for animal behaviour. In the first part of this thesis, I investigated whether viewing the hippocampus as an SR can explain experiments contrasting hippocampal and dorsolateral striatal contributions to behaviour in spatial and non-spatial tasks. To do this, I modelled the hippocampus as an SR and DLS as model-free reinforcement learning, combining their outputs via their relative reliability as a proxy for uncertainty. Current SR models do not formally address uncertainty. Therefore I extended the learning of SRs by temporal differences to include managing uncertainty in new observations versus existing knowledge. I generalise this approach to a multi-task setting using a Bayesian nonparametric switching Kalman Filter, allowing the model to learn and maintain multiple task-specific SR maps and infer which one to use at any moment based on the observations. I show that this Bayesian SR model captures animal behaviour in tasks which require contextual memory and generalisation. In conclusion, I consider how the hippocampal contribution to behaviour can be considered as a predictive map when adapted to take account of uncertainty and combined with other behavioural controllers.

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