The grid code for ordered experience

Entorhinal cortical grid cells fire in a periodic pattern that tiles space, which is suggestive of a spatial coordinate system. However, irregularities in the grid pattern as well as responses of grid cells in contexts other than spatial navigation have presented a challenge to existing models of entorhinal function. In this Perspective, we propose that hippocampal input provides a key informative drive to the grid network in both spatial and non-spatial circumstances, particularly around salient events. We build on previous models in which neural activity propagates through the entorhinal-hippocampal network in time. This temporal contiguity in network activity points to temporal order as a necessary characteristic of representations generated by the hippocampal formation. We advocate that interactions in the entorhinal-hippocampal loop build a topological representation that is rooted in the temporal order of experience. In this way, the structure of grid cell firing supports a learned topology rather than a rigid coordinate frame that is bound to measurements of the physical world. Entorhinal cortical grid cells have been suggested to encode an internal map of the environment during spatial navigation. In this Perspective, Rueckemann, Sosa and colleagues propose that grid cells and hippocampal place cells cooperate to provide a topological representation of experience through temporal ordering of events, in both spatial and non-spatial contexts.

Automated summary

Researchers propose that hippocampal input drives the grid cell network in both spatial and non-spatial contexts, with interactions between the hippocampus and entorhinal cortex building a topological representation based on temporal ordering of events.