# Place cells in CA1 lack topographical organization of firing locations

## TL;DR

This study finds that place cells in the hippocampus do not have a topographical organization of firing locations, suggesting a fully distributed system for encoding spatial information.

## Contribution

The study provides high-resolution evidence that CA1 place cells lack anatomical clustering of firing properties, supporting a fully distributed spatial encoding model.

## Key findings

- No correlation was found between anatomical proximity and firing field similarity in CA1 place cells.
- Firing field correlations did not transfer between different environments during remapping.
- CA1 cells encoding local objects also showed no topographical organization.

## Abstract

Unlike neurons in sensory and motor cortices, hippocampal place cells do not exhibit large-scale topographic organization. Some studies have suggested, however, that amid a distributed organization, hippocampal neurons assemble into small clusters with correlated properties. Here, we used a miniaturized two-photon microscope to elucidate at high spatial resolution whether such clustering exists in CA1 place cells of freely moving mice. Recording from almost a thousand neurons per trial, we found no evidence for clustering, despite the increase in anatomical resolution and statistical power compared to previous approaches. A fully distributed organization is consistent with the ability of CA1 place cells to express large numbers of orthogonal position maps and the participation of such maps in high-capacity memory storage.

Topography is a well-described and well-known concept for cortical organization in primary sensory and motor cortices of mammalian brains. Similar isomorphic mapping of the environment is absent in most higher association areas such as the hippocampus; however, some studies have suggested that in hippocampal regions spatially tuned cells may be organized in small anatomical clusters with similar firing field locations. Here, we take advantage of the high spatial resolution provided by two-photon calcium imaging to probe the extent of local functional clustering of hippocampal place cells in freely moving mice. Almost one thousand highly packed CA1 neurons were recorded simultaneously per mouse while they foraged freely in open arenas. Among pairs of place cells, there was no relation between preferred firing position and anatomical proximity. Firing fields were no more similar for nearest neighbors than distant cell pairs. When place cells remapped between different arenas, there was no tendency for correlations in one environment to be transferred to the other. A similar lack of relationship between function and anatomical position was expressed in CA1 cells with firing fields tuned to local objects in the environment. The findings suggest that the encoding of location in CA1 place cells is likely fully nontopographical.