Probable nature of higher-dimensional symmetries underlying mammalian grid-cell activity patterns

TL;DR

This paper explores how mammalian grid cells may utilize higher-dimensional lattice structures, such as hexagonal and face-centered cubic lattices, to optimize spatial resolution in neural representations of physical space.

Contribution

It predicts specific lattice arrangements for grid cells in higher dimensions, extending known two-dimensional patterns to three-dimensional and beyond, with testable experimental implications.

Findings

Hexagonal grid patterns optimize 2D spatial resolution.

FCC and HCP lattices are predicted for 3D spatial encoding.

Neural representations may involve nested lattice structures.

Abstract

Lattices abound in nature - from the crystal structure of minerals to the honey-comb organization of ommatidia in the compound eye of insects. Such regular arrangements provide solutions for optimally dense packings, efficient resource distribution and cryptographic schemes, highlighting the importance of lattice theory in mathematics and physics, biology and economics, and computer science and coding theory. Do lattices also play a role in how the brain represents information? To answer this question, we focus on higher-dimensional stimulus domains, with particular emphasis on neural representations of the physical space explored by an animal. Using information theory, we ask how to optimize the spatial resolution of neuronal lattice codes. We show that the hexagonal activity patterns of grid cells found in the hippocampal formation of mammals navigating on a flat surface lead to the highest spatial resolution in a two-dimensional world. For species that move freely in a three-dimensional environment, the firing fields should be arranged along a face-centered cubic (FCC) lattice or a equally dense non-lattice variant thereof known as a hexagonal close packing (HCP). This quantitative prediction could be tested experimentally in flying bats, arboreal monkeys, or cetaceans. More generally, our theoretical results suggest that the brain encodes higher-dimensional sensory or cognitive variables with populations of grid-cell-like neurons whose activity patterns exhibit lattice structures at multiple, nested scales.