140 — Integration of grid maps in merged environments

Wernle et al (10.1038/s41593-017-0036-6)

Read on 07 January 2018
#medial-entorhinal-cortex  #MEC  #grid-cells  #hexagons  #neuroscience  #path-integration  #place-cells  #hippocampus  #rat 

One question that has always fascinated me about spatial representation in the brain is how we reconcile disparate locations that wind up being the same place. In other words, if you start at point A and walk down a hallway to point B, turn right, walk down another hallway to point C, turn right… and are back where you started, you need to perform a “recalibration” of your mental map of the world; point A and point C are actually the same.

In this paper, the researchers discover the ways in which two mental spaces become one.

First, they place a rat in a rectangular box, creatively named $A$. “Let that rat explore the new and exciting place,” they proclaim! Then, they remove this rat from $A$ and place it in another similar box named $B$. “Again, let it be so, that the rat does explore and find it to be good,” they say! And verily does the rat do so.

But then they (¡and this is the clever part!) open one of the walls of $A$… and $B$ is revealed; the two are actually the same “room”, separated by a removable wall betwixt.

They they look at how the grid cells’ responses change once the rat realizes its been punked. Before the big reveal, the rats’ representation of the rooms were distinct; the hexagonal grid representations even differed in orientation and scale for some rats.

Removing the wall changed how the grid cells near the wall fired, but when the rat was near the external walls, the response did not change as much. This suggests that the rats used the nearest wall as an “anchor” for their grid, and when the central wall was removed, the rats immediately recomputed a grid to overlay in the new space which reeconciled their existing understanding of the $A$ and $B$ spaces.

In other words, when the rats were introduced to the new space, their brains compuled a new grid field almost immediately in order to reconcile the rotation and spatial distribution of the old maps.

This fusion process seems like an expensive undertaking for the brain, and it also would seem to me to be irreversible: I imagine that combining two maps into one is simple, but re-separating them is not so simple. I often have memories of my first introductions to spaces stored in “another place” than that same space once I get used to it: For example, thinking about my apartment when I first toured the building feels different than the rooms in which I now live, even though they are, of course, the same space. Perhaps there’s only a motivation for the brain to expend this energy when the combination of the spaces is relevant or useful for everyday use, goal-seeking, or survival.