The hippocampus is an ancient brain structure, dating to the dawn of vertebrates a half billion years ago. Even if you’re not a neuroscientist, you’ve likely heard of it (it’s very well studied) and remembered it (weird name and all). The hippocampus is most famous for roles in memory and in the formation of internal maps of the environment. Interestingly and less known is this: the hippocampus prioritizes its signals based on interests or needs of the animal. That’s great but what about the challenge of continuously updating the map while the animal is searching for a reward? Think about that task. It requires a good map, a sense of time, a way to remember, and a way to judge the attractiveness of a reward. (For mice, this is food or water; for me, it’s currently horchata ice cream.) Can this ancient brain region do that?
A great recent Nature paper addressed this question by collecting high-resolution optical recordings from mice as they explored a virtual world. The virtual world allowed the experimenters to create and modify environments while providing rewards, which they could relocate at will. The final paragraph of the Introduction lays out their hypothesis and findings very nicely (albeit with some neuro jargon):
We reasoned that reward may anchor hippocampal activity across the entire environment, creating a map for experience in reference to remembered rewards in parallel to a map for space. We speculated that previously reported reward-specific cells may comprise a subset of a larger population encoding an entire sequence of events relative to reward. This hypothesis predicts that moving the reward within a constant environment should induce predictable remapping even at locations far from the reward, preserving behavioral timescale firing order between neurons relative to each reward location. In parallel, another subpopulation should preserve their firing relative to the spatial environment, allowing the hippocampus to flexibly anchor to both the spatial and reward reference frames for solving the task at hand. Here, we found that the hippocampus indeed learned a generalized representation of the task anchored to reward, while also maintaining a spatial map in dissociable population codes.
That paragraph is demanding but look at how it weaves previous knowledge into a specific hypothesis, then teases the results of the experiments and the new knowledge they acquired. I think this is a great way to think about the Introduction, and especially the final paragraph of that section. I also think that the hippocampus is amazing and I hope you agree.
A flexible hippocampal population code for experience relative to reward
In Nature Neuroscience, 11 June 2025
From the group of Lisa Giocomo at Stanford University School of Medicine.
Snippet by Stephen Matheson
Image credit: Figure 1 b-c from Sosa et al. cited above (CC BY). Full figure with legend here.