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“Free lunch” reshapes the brain’s map of space

Each time you get a reward, your brain's internal spatial map warps just a bit in a way that makes it easier for you to get back to wherever you got it.

Be advised that every time you gnaw on a chewy chunk of cheese or sip down a stream of stimulant-spiked sparkling soft drink, your brain's internal map of space warps just a bit, in a way that increases your ability to find your way back -- and, thus, your likelihood of returning -- to the spot where you got that little dose of delight. That's what a new study in Science shows.

Drugs of abuse may work that way, too, according to Stanford neuroscientist Lisa Giocomo, PhD, who led the study.

Scientists have learned that various nerve cells in a key structure in our brains act as compasses, speedometers, latitude and longitude coordinates, or boundary and landmark detectors. These cells are found in rodents, bats, monkeys and humans, suggesting such spatial-mapping circuitry is a universal mechanism of mammalian navigation.

It's only reasonable to assume, as scientists have up to now, that the brain's "internal GPS" acts pretty much the way the map on your smartphone does. But it turns out that's not quite right. From my news release about the study:

'Every time you check your Google map for a particular address or restaurant name or pair of grid coordinates, you get the same map regardless of why you're looking at it,' Giocomo said. 'The global positioning system that generates that map doesn't care what you're doing or where you're going, or whether you're happy, hungry or hungover. It's always going to give you the same information.'

Not so for our brain's internal map of space, according to the study.

"We've learned your internal map changes depending on your behavior, memories and state of mind," Giocomo told me. "We pull up different maps for the same space, depending on what we're actually trying to do in that space."

To show how this works, the researchers built two big open-top boxes, both exactly the same size and shape. On the floors of each box were randomly scattered bits of Cheerios.

"Rats' spatial mapping system is the same as ours," Giocomo told me. "For rodents, they're pretty smart. They like to move around. And they love Cheerios." So the scientists used rats for their experiments, depositing each of them sometimes in one box, sometimes in another.

The only important difference between the two boxes: In one stood a roughly 8-by-8-inch "reward zone" in a fixed location on the Cheerios-strewn floor. The test animals soon learned that if, in response to an auditory cue, they navigated to the reward zone tout suite, they'd get a guaranteed, good-sized dose of crunchy Cheerios. If they arrived too late, the reward zone was closed for business.

It's as if in the middle of a forest there sits a magical "free lunch" counter. It's only in operation some of the time -- but when it is, a forest-wide advertisement blares the news to hungry foragers. The foragers' (in this case, rats') responsiveness to that "ad" eventually warped their minds.

When a rat came within about a foot of the center of the reward zone, whether or not the free-lunch counter was open, its position-signifying brain cells fired faster, and those position-signifying cells that were firing were spaced closer together, indicating higher spatial resolution.

"[T]he rat's 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," Giocomo said. If the reward is a drug of abuse, she said, the improved accuracy at the center of this reward-based map could enable an addict's habit.

From my release:

[N]ext time you find yourself wending your way through some nondescript side street in a strange neighborhood in search of a parking spot, remind yourself to gobble a chunk of chocolate as you're getting out of your car. It might make it easier for you to remember where you parked.

Photo by Randy Fath

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