Brain’s GPS cells map time and distance, not just location

When rats run on a treadmill, grid cells act like clocks and odometers

KEEPING TIME  A grid cell fires off signals at particular times as a rat runs on a treadmill. The cell behaved similarly as the rat ran at slow (blue), moderate (brown) and fast (green) speeds. 

B. Kraus et al/Neuron 2015

Specialized cells that make up the brain’s GPS system have an expanding job description. In addition to mapping locations, these cells can keep track of distance and time, too, scientists report in the Nov. 4 Neuron.

Those specialized cells, called grid cells, were thought to have a very specific job, says neuroscientist Loren Frank of the University of California, San Francisco. But, he says, the new study says, “not so fast, everybody.”

These cells’ ability to detect time and distance is unexpected. “And I think it’s important,” Frank says. The growing to-do list of grid cells shows that the brain’s navigational system is surprisingly flexible.

The discovery of grid cells, found in a part of the brain called the entorhinal cortex, was recognized with the Nobel Prize last year (SN Online: 10/6/14). These brain cells fire off regular signals as animals move around in space, partially forming an internal map of the environment. Neuroscientist Howard Eichenbaum of Boston University and colleagues wondered what those cells do when an animal stays put. By training rats to run on a treadmill, the researchers had a way to study grid cells as time and distance marched forward, but location remained the same.

Unlike recently discovered “speed cells” (SN: 8/8/15, p. 8), these grid cells don’t change their firing rates to correspond to changes in the rats’ swiftness, the researchers found. Instead, these cells stay tuned to distance or time, or both.

Most of these grid cells fired off bursts of messages at particular distances or times, electrodes implanted into the rats’ brains revealed. During a 16-second run, for instance, a time-detecting grid cell might become active at second 5, and then again at second 10. Similarly, a distance-marking grid cell might fire every time the rat ran 200 centimeters. Those responses stayed the same even when the scientists varied the speed of the treadmill. About 40 percent of grid cells detected both time and distance.

This recurring rhythmicity for both time and distance echoes the way that grid cells map locations. “It’s not clear why they have this kind of cycle, but it’s really the same thing the grid cells would do in space, where they fire as the animal passes through a series of locations,” Eichenbaum says.

When a rat is on a treadmill, visual landmarks and locations in space no longer matter, and the grid cells switch gears accordingly, Eichenbaum says. “The important part to the animals is how far they ran and how long it took, and that seems to be what the cells are tracking.”

The results show how adaptable the brain is, Frank says. “The major, important point is that the brain reconfigures itself,” he says. “Even these things that look like they’d be crystalline and rigid can be reconfigured or reappropriated for other things when those other things matter.”

The cells that make up the navigational system are located in brain areas known to be important for memory. At its heart, navigation is a memory, Eichenbaum says. Cells that identify location, time and distance “provide a framework — scaffolding onto which memories are placed,” he says. 

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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