Hippocampal place cells map terrain geometry independently of behavior

How does the brain map uneven terrain? While spatial neurons such as hippocampal place cells and entorhinal grid cells have been extensively studied in flat, horizontal environments, the natural world is hilly and irregular. To investigate how place cells represent irregular terrain, we recorded from the hippocampus of rats foraging across either flat or ridged terrain. Place cell activity reflected terrain shape, consistent with a surface-bound cognitive map rather than a volumetric one. Place fields were elongated parallel to terrain contours, which contrasted with the movement biases of the rats, and are inconsistent with a predictive coding model. Reflecting the importance of terrain information, a third of the place cells exhibited repeating fields on each ridge. A boundary vector cell model of place cell firing replicated these results. These findings demonstrate that the cognitive map is sensitive to topography, bringing our understanding of spatial cognition closer to the real world.