Grieves, Jedidi-Ayoub, Mishchanchuk and Jeffery (2018) Place cells represent three-dimensional, volumetric space anisotropically

Place cells in the hippocampus represent places by increasing their firing rate when an animal visits specific regions of its environment, these regions of high firing are known as ‘place fields’. In flat mazes place fields are typically small and round. However, animals must often navigate complex three dimensional environments. Do place cells represent three-dimensional volumetric space and if so, how?In freely flying bats place fields have been observed to form spheres, suggesting that these animals can localise themselves equally well in all dimensions. In contrast, fields recorded in rats exploring vertical climbing walls form vertical columns, suggesting that they are less able to localise themselves vertically. Thus, we propose that rats have an anisotropic representation of space; they are less able to localise themselves vertically and their place fields will form elongated vertical columns in three-dimensions. To investigate this we wirelessly recorded the activity of place cells in a three-dimensional volumetric space; a cubic lattice composed of horizontal and vertical climbing bars.Over the course of 34 sessions (each 60 minutes long) we recorded a total of 428 place cells in this maze from 8 rats. Rats exhibited a strong bias in their style of locomotion; they navigated significantly more frequently and at a faster rate along the horizontal dimensions. Place fields had normal horizontal characteristics but many were elongated vertically. Decoding the position of animals in this maze using only the activity of place cells revealed lower spatial information in the vertical dimension. These results suggest that place cells represent three-dimensional volumetric space anisotropically.To determine if this is related to the rats’ bias in locomotion or the geometry of the maze we also recorded a further 225 place cells from 3 rats in the same lattice maze, rotated to stand on one vertex, so that all horizontal and vertical climbing bars were instead diagonally sloped. Results thus far suggest that in this environment no place fields form vertical columns, they are more likely to form spheres and those that are elongated do so along one diagonal axis of the lattice maze.Place cells recorded in freely flying bats have been shown to exhibit near spherical place fields, why might this differ in rats? Our results suggest that the place fields of rats are vertically elongated in a cubic lattice, but that this is dependent on the structure of the lattice and the locomotor constraints on the rat rather than allocentric dimension. Thus, in rats, the representation of volumetric space is shaped by the affordances of the environment for locomotion