X marks the spot

Rewarding experiences alter the spatial maps in our brains

Illustration of circuits in the brain

In the Hansel and Gretel folktale, the children sacrificed their last pieces of bread to drop crumbs along the forest floor so they could find their way home, but became lost after birds ate the crumbs. 

They probably should have eaten the bread themselves. That’s because when we experience rewards, our brains warp in a way that increases our ability to get back to places where we experienced those rewards.

That finding, published March 29 in Science, builds on knowledge that our brains’ neural circuitry creates spatial maps as we navigate different environments.

The study shows that when rats learn they will receive a reward at certain places on those maps, their brain’s navigation circuitry redraws the maps to incorporate spots they want to revisit.

Lisa Giocomo, PhD, assistant professor of neurobiology at Stanford, researches the medial entorhinal cortex, an area of the brain where nerve cells act as compasses, speedometers, latitude and longitude coordinates, or boundary and landmark detectors.

The cells have been identified in rodents, bats, monkeys and humans, suggesting that such spatial-mapping circuitry is a universal mechanism in mammals.

Giocomo said that when we use GPS to find a location, we’ll get the same information every time, regardless of why we’re looking at it, how we feel or where we are. But a brain is more personalized than GPS.

“In this study, we’ve learned your internal map changes depending on your behavior, memories and state of mind,” said Giocomo, the study’s senior author. “We pull up different maps for the same space, depending on what we’re actually trying to do in that space.”

Study researchers observed rats as they foraged for Cheerios scattered on the floor of two big boxes. One of the boxes had a “reward zone” where they were guaranteed to find Cheerios during the short periods of time it was open. An auditory cue indicated open times.

As the rats roamed, nerve cells in the medial entorhinal cortex were monitored. Once they recognized the reward zone cue, their position-signaling cells fired faster and closer together as they neared it, researchers found.

“This tells us the rats’ brains are making a new map of space, in response to their experience of a reward, that reflects the importance of the place where they got it by providing a more accurate representation of its position,” said Giocomo.

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Bruce Goldman

Bruce Goldman is a science writer in the Office of Communications. Email him at goldmanb@stanford.edu.

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