Grid cells exist in the entorhinal cortex (EC) and are known to fire for spatial navigation on a hexagrid structure. Grid cells along with the other cells in the EC are implicated as a key component that allows for generalization and conceptual maps in the human brain. Recently I was reading “A Framework for Intelligence and Cortical Function Based on Grid Cells in the Neocortex” by Jeff Hawkins and Numenta. In that paper, their whole premise is built off the idea that main human level intelligence is in the neocortex and that grid like form a key part of that intelligence. I was somewhat skeptical, so I went through all their references to understand it more clearly.
There are 4 main papers that they reference and I read them all.
“Evidence for grid cells in a human memory network” by Doeller et al.
They wanted to see if they could find grid-like firing using fMRI signal, which reflects changes in metabolic activity across large portions of the brain resulting in very coarse readings of neuronal activity. They designed an experiment using a virtual environment in a way that they could measure 3 factors from the fMRI data. 1) The angular orientation relative to the environment, 2) The running direction, and 3) The running speed. They believe they did find grid-like cell activity in multiple neocortex regions such as the visual cortex and retrosplenial cortex. They note “Because we ca only measure effects of direction and speed (not location) in fMRI signal, our findings could reflect the presence of grid cells, or movement-related responses from head direction, or ‘conjuctive’ directional grid cell…”
“Direct recordings of grid-like neuronal activity in human spatial navigation” by Jacobs et al.
In their paper they recorded individual neurons from electrodes that were surgically implanted into 14 patients who were being treated for epilepsy. They had recordings for 893 individual cells in the hippocampus, amygdala, parahippocampal gyrus, EC, and cingulate cortex. They had those patients perform navigation tasks in a virtual environment cut up into a 28 x 28 array. They came up with a gridness score to classify if the neurons had grid-like codes.
Their results found that of all the recorded cells, 14% of cells in EC, 12% of cells in the cingulate cortex, and 8% of the hippocampus cells had a strong gridness score. Although they did use a virtual environment, they calculated that the grid-like cells had a firing rate of 1-6 meters. They also noted that although the data is statistically robust, the firing maps where much “noisier” compared to grid cell data collected from rodents. They outlined 2 interesting areas for further research: 1) Are the human grid-like cells effected by other non location factors such as eye-location. 2) How information coding works between non-navigation behaviors, grid-cells, head-direction cells, and eye movement.
“A novel somatosensory spatial navigation system outside the hippocampal formation” by Long et al.
In this paper, the scientists measures 2025 cells from 8 implanted rats that were able to freely move. They did 287 recording sessions of their neuron firing activity. They found that cells known primarily to be in the entorhinal cortex such as head direction cells and border vector cells, were found in the somatosensory cortex. Of all their cell recordings, they found 3.55% fired like grid cells. They found that these grid cells were a little nosier and less prevalent than the other cell types they found. They hypothesis that this may be due to other factors such as proprioception. They warn: “We cannot completely eliminate the possibility that the somatosensory spatial positioning system may be at least partially dependent on the hippocampal-entorhinal system”. Meaning that the data they found may be just passed along data coming from the EC system.
“Organizing conceptual knowledge in humans with a gridlike code” by Constantinescu et al.
I have read this paper several times before because it is referenced often as potential proof of the EC being used to organize conceptual knowledge. They designed an experiment for humans to navigate through an abstract conceptual presentation consisting of “bird space”, arrangements of different lengths of bird necks and legs. They then measure the participants over several sessions spanning weeks using fMRI. In the fMRI signal, they found hexagonally symmetric activity in the vmPFC, gridlike code! They also warn: “Although the coarse nature of the fMRI signal urges caution in making conclusions at the level of neuronal codes, we have reported an unusually precise hexagonal modulation of the fMRI signal during nonspatial cognition.”
After reading those papers, I am a lot more confident that there is grid like cell functionality in the neocortex and so is probably directly contributing to human level intelligence. In general, it is hard to rely on fMRI data, but some of the referenced papers do measure cells directly in humans and rats. If grid code is in other areas of the brain is true, I’m surprised there is not a lot more research here because the hippocampal-entorhinal system is THE main implicated system for human generalization and abstract concept representations. Timothy Behrens is one of the authors of “Organizing conceptual knowledge in humans with a gridlike code“. He is also one of the main researchers studying grid cells and cognitive maps. So I’m surprised I haven’t read more from him about grid cell activity outside of the EC system. This is exciting stuff and it really does feel like we are making tangible progress to understanding human level intelligence.