The hippocampus as a cognitive map

Spatial goal coding in the hippocampal formation

An organism’s survival can depend on its ability to recall and navigate to spatial locations associated with rewards, such as food or a home. Accumulating research has revealed that computations of reward and its prediction occur on multiple levels across a complex set of interacting brain regions, including those that support memory and navigation. However, how the brain coordinates the encoding, recall and use of reward information to guide navigation remains incompletely understood. In this Review, we propose that the brain’s classical navigation centres — the hippocampus and the entorhinal cortex — are ideally suited to coordinate this larger network by representing both physical and mental space as a series of states. These states may be linked to reward via neuromodulatory inputs to the hippocampus–entorhinal cortex system. Hippocampal outputs can then broadcast sequences of states to the rest of the brain to store reward associations or to facilitate decision-making, potentially engaging additional value signals downstream. This proposal is supported by recent advances in both experimental and theoretical neuroscience. By discussing the neural systems traditionally tied to navigation and reward at their intersection, we aim to offer an integrated framework for understanding navigation to reward as a fundamental feature of many cognitive processes. The capacity to navigate to a previously encountered or predicted reward, such as food or safety, is crucial for survival. Here, Sosa and Giocomo examine the neural mechanisms that encode reward location and propose key roles for the hippocampus and the entorhinal cortex in storing and retrieving reward-related information in the brain.

Navigating for reward.

Astrocytic calcium dynamics has been implicated in the encoding of sensory information1-5, and modulation of calcium in astrocytes has been shown to affect behaviour6-10. However, longitudinal investigation of the real-time calcium activity of astrocytes in the hippocampus of awake mice is lacking. Here we used two-photon microscopy to chronically image CA1 astrocytes as mice ran in familiar or new virtual environments to obtain water rewards. We found that astrocytes exhibit persistent ramping activity towards the reward location in a familiar environment, but not in a new one. Shifting the reward location within a familiar environment also resulted in diminished ramping. After additional training, as the mice became familiar with the new context or new reward location, the ramping was re-established. Using linear decoders, we could predict the location of the mouse in a familiar environment from astrocyte activity alone. We could not do the same in a new environment, suggesting that the spatial modulation of astrocytic activity is experience dependent. Our results indicate that astrocytes can encode the expected reward location in spatial contexts, thereby extending their known computational abilities and their role in cognitive functions. 

Hippocampal astrocytes encode reward location

Rapid synaptic plasticity contributes to a learned conjunctive code of position and choice-related information in the hippocampus

https://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/252467/1/j.celrep.2020.107864.pdf

Distinct Mechanisms of Over-Representation of Landmarks and Rewards in the Hippocampus.

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Distinct Mechanisms of Over-Representation of Landmarks and Rewards in the Hippocampus.