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Tuesday, January 26, 2010

The Grid in Your Head

According to a lovely new Nature paper combining fMRI imaging with animal experiments, the human brain encodes spatial information in the form of of a hexagonal grid - Evidence for grid cells in a human memory network.

If you've ever played Chinese checkers, you'll know what a hex grid is. It's already known that in rats, the entorhinal cortex of the brain contains "grid cells", each of which fires according to where in a certain place the rat is. The diagram above left shows how one example grid cell fires more often when the rat is in certain places in a 1m x 1m box.

Doeller et al wanted to test whether grid cells exist in humans, but being unable to just stick electrodes in people's heads, they made use of two useful facts about rat grid cells. First, the orientation of the grid is fixed in all the cells in each particular rat, although each cell prefers different locations, i.e. the "grids" are offset, but not rotated. Second, grid cells fire faster when the animal is walking or running in a direction which corresponds to "along the lines" of their brain's internal grid - especially when the movement is rapid.

So, if our brains do contain grid cells, our entorhinal cortex should be more active overall when we're moving along the lines of our grids, as opposed to across them. Bearing in mind that there are three axes, and that you could move either "forward" or "backward" along each one, that makes 6 directions, so the grid cell theory predicts that entorhinal cortex activity should correlate with direction of motion with "6-way directional symmetry", like this:

Doeller et al used fMRI to measure neural activity while 42 volunteers "walked" around a computer-generated landscape on a screen, and looked for areas where activity had the pattern above. Lo and behold, the entorhinal cortex did indeed show this pattern of activity in most volunteers. As a control, they looked for areas showing 4, 5, 7 or 8- fold directional symmetry, and didn't find any.

Doeller et al point out that they haven't directly proven the existence of grid cells in humans - in theory, these results could also indicate the presence of another type of cell which encodes direction with 6-way directional symmetry. But this is a great piece of research, and a nice example of using neuroimaging to test neurobiological theories, as opposed to just going hunting for blobs of activation without knowing what to look for, which I've criticized before.

ResearchBlogging.orgDoeller, C., Barry, C., & Burgess, N. (2010). Evidence for grid cells in a human memory network Nature DOI: 10.1038/nature08704

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