How Rewards and Goals Redraw Your Brain’s Map of the World

“The observer plays a role in what is being observed.” It might be the only sentence you remember from your Psychology 101 class — and maybe that’s the point. When you enter a room — whether it’s your bedroom that you’re well-acquainted with, or an unfamiliar room — the living room during a dinner party at a house you’ve never been to — you’re not so much experiencing what’s there, but your brain’s interpretation, quickly being constructed in real time as you enter. In order to do this, it makes use of two different types of specialized cells: place cells and grid cells, that act as coordinates on a map.

Place cells, based in the hippocampus, confirm recognizable landmarks in a given location — when you pass by a couch for example. The hippocampus itself plays an active role in the formation of memories. Grid cells, located in a portion of the hippocampus called the entorhinal cortex, send a message when you look around the room, forming a hexagon shaped map in the brain. On one side may be the doorway to another room, a bit higher may be a window or a mounted picture frame, and each observation forms a layer in the brain’s spatial map.   

Until recently, grid cells were thought to use a precise system of coordinates when it came to mapping out space — a reason grid cells were described as the “brain’s inner GPS” when the researchers who made this discovery won the Nobel Prize. Neurobiologist Lisa Giocomo of Stanford University suspected that the coordinates might be more than just metrics related to our position — that perhaps the points were determined by the brain’s reward system and memory.

To test her hypothesis, Giocomo placed a group of rats in a maze, allowing them to search for treats hidden in random corners of the labyrinth. They repeated the experiment after conditioning the rats to associate specific portions of the maze with the larger portions of food. Scans of the entorhinal cortex showed that the rat brains were rewired by the training. When they were used to finding the treats in a particular spot, the neurons fired closer to that point in the hexagon, and reduced the brain’s navigational space around it. The grid itself rotated, as though the rat was in entirely new surroundings.    

“The goal of the behavioral task, or potentially the behavioral state of the animal, really is enough to drive a completely new map of the same spatial location,” says Giocomo. The grid cells fired more frequently when the rats approached the reward location, suggesting that the entorhinal cortex helped represent the reward location more accurately than anywhere else in the labyrinth — perhaps to make it easier to find rewards in the future. It’s the same reason why fishermen have their favorite fishing spots in the lake, and coordinate their brain’s understanding of the lake around it — figuring out the best way to approach it from each side or channel of the lake.  

Giocomo’s findings were further reinforced by a study conducted by a team led by neuroscientist Jozsef Csicsvari at the Institute of Science and Technology Austria. Csicsvari began by training the rats on a board without any treats, then given a board with holes and rewards placed into specific holes until they knew where to look — before they were again placed on a board with no rewards. The researchers found that after repeated exposure to rewards at fixed locations, the rat brains altered, with grid cells bouncing between their initial points and the new locations. They’re not so much representations of space — but of memories as well.

“An organism has a sense of what things matter or don’t, and that overlays its sense of where things are in the world,” says Loren Frank, a neuroscientist at the University of California, San Francisco who did not partake in either study. The GPS metaphor is therefore not entirely accurate, since the internal map is much more subjective than it was thought to be just four years ago.  For Giocomo, there’s a whole host of questions left to explore: how the brain’s mapping system fares in the face of punishments rather than rewards, and whether the rat’s motivation alters the grid cells — and what happens if he’s not hungry?

Rats, like most mammals, are foragers by nature. During the winter, food is scarce — but there are parts of the forest where things like nuts, berries, or mushrooms are more abundant — and so animals, humans included, map these locations according to what they’re looking for in order to survive. Over the summer, we may find a huge thicket of blackberries by surprise, and suddenly our map of the forest changes to fit it in.  

It’s also why you may one day discover a new cafe by surprise, not far from your home, that you never realized was there — as you were simply focused on the quickest way to get home from work. By setting a goal, even one as simple as finding the quickest way home to avoid traffic, you are essentially rewiring your brain — and seeing the world differently — in terms of how it relates to your new goal, surrounded by obstacles.

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