For a decade, scientists have been puzzled by how the “internal maps” of the human brain are anchored to the external world. A new study published in the journal Nature attempted to solve this puzzle.

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The orange lines in this illustration depict forces acting on the internal map to produce a final grid geometry that is asymmetric to the environment.
Image credit: Tor Stensola/Kavli Institute for Systems Neuroscience

The scientists behind the new study – from the Norwegian University of Science and Technology in Trondheim, Norway – compared the brain’s internal map to how we might use a map and compass to relate longitude lines to the terrain around us while hiking.

Nobel Laureates Edvard Moser and May-Britt Moser had previously discovered “grid cells” in the brain that are the main reference in our brain’s spatial navigation system. These cells allow internal grid maps to be “pulled up” as appropriate to guide us in the right direction. The new study helps to explain how these maps are anchored to our surroundings.

The Norwegian team recorded the activity of grid cells in rats as the animals hunted for cookie crumbs around a 1.5 m2 box.

“We recorded the activity of hundreds of grid cells,” says Tor Stensola, a researcher at the Kavli Institute for Systems Neuroscience in Norway.

“Looking at the information from more than 800 grid cells, we noticed that grid patterns typically were oriented in the same few directions,” Stensola explains. “This was true for all the animals studied, which suggested the grid map aligns to its surroundings in a systematic way. Grid cells all seemed to be anchored to one of the local walls, but always with a specific offset of a few degrees. So we decided to investigate this.”

As a way of visualizing this, the researchers explain that if a map represented by the activity of grid cells were to represent the box, it would need to look and be aligned the same every time the rat was in the box.

Analyzing data from recordings of the grid cells’ activity in the rats’ brains, the team found that these recorded maps were consistent and that each grid pattern was linked to one of the walls of the box. However, the axes of the grids were not perfectly aligned – they were askew. The angle was always approximately 7.5 degrees off.

Stensola did some calculations and realized there might be a good reason for that particular angle:

At 0- and 15-degree angles, the map would be symmetric. In other words, if it were perfectly aligned with the wall, that would mean it would mirror either the cardinal axis or the diagonal of the square box respectively, making it likely that places would get mixed up. The 7.5 degrees angle of rotation is the one that minimizes symmetry with axes in the environment, thereby minimizing these kinds of potential errors.”

More asymmetry was found in the grids. If the map was symmetrical then the pattern would be perfectly hexagonal – instead, the researchers found that the points formed an ellipse. This distortion of the points was found to correspond “almost perfectly” to the direction the ellipse was pointing and the direction the grid was anchored toward.

Again, the researchers provide a visual metaphor to explain this skewing and how it displaces parts of an object differently according to location. Think of pushing a deck of cards along a table, they say. The bottom card stays in the same location and the top card will be moved the most, while the cards in the middle move a distance that reflects its position in the deck.

With this in mind, the researchers hypothesized that when an animal encounters a new environment, its brain forms a map of the environment where one of the axes is aligned perfectly with a nearby wall. Gradually, movement within the space distorts and skews the map 7.5 degrees away from the anchoring wall, which creates “a stable and robust map with low chance of error.”

In bigger boxes, which were 2.2 m2, the researchers found that the rats’ maps had the same asymmetries, but they now broke in two and became separate local maps for the same box, anchored to opposite walls.

“It is always a great feeling when we find the solution to a mystery that has puzzled us for a decade,” Prof. Edvard Moser, director of the Kavli Institute for Systems Neuroscience says. He believes that the findings are important and may connect different fields of science:

The findings give us more clues as to how our internal maps interact with the surroundings. Now we’ll have to figure out in detail how the information about the orientation of walls and boundaries in the surroundings reach the grid maps, and how this information is processed. Perhaps border cells will prove to hold the answer to this, we do not know this for sure yet.”