Glance at a runner's wrist or smartphone, and you'll likely find a GPS-enabled app or gadget ticking off miles and minutes as she tries to break her personal record. Long before FitBit or MapMyRun, however, the brain evolved its own system for tracking where we go. Now, scientists have discovered a key component of this ancient navigational system in rats: a group of neurons called "speed cells" that alter their firing rates with the pace at which the rodents run. The findings may help explain how the brain maintains a constantly updated map of our surroundings.
In the 1970s, neuroscientist John O'Keefe, now at University College London, discovered neurons called place cells, which fire whenever a rat enters a specific location. Thirty-five years later, neuroscientists May-Britt and Edvard Moser, now at the Norwegian University of Science and Technology in Trondheim, Norway, discovered a separate group of neurons, called grid cells, which fire at regular intervals as rats traverse an open area, creating a hexagonal grid with coordinates similar to those in GPS. The Mosers and O'Keefe shared last year's Nobel Prize in Physiology and Medicine for their findings, which hint at how the brain constructs a mental map of an animal's environment.
Still mysterious, however, is how grid and place cells obtain the information that every GPS system requires: the angle and speed of an object's movement relative to a known starting point, says Edvard Moser, co-author of the new study along with May-Britt Moser, his spouse and collaborator. If the brain does indeed contain a dynamic, internal map of the world, "there has to be a speed signal" that tells the network how far an animal has moved in a given period of time, he says.
Previous studies have identified neurons that only fire up when an animal points its head in a certain direction—some for east, others for south, for example—but reports of neurons that respond to changes in an animal's speed are sparse and largely anecdotal, says Jeffrey Taube, a neuroscientist at Dartmouth College who was not involved in the work.
To search for such cells, the Mosers and their team delved into the medial entorhinal cortex (MEC), a slim arc of deep brain tissue where they had discovered the grid cells in 2005. They implanted the rats with electrodes that could record from thousands of MEC neurons, then put the rodents on a moveable treadmill, "like a Flintstone’s car," Moser says.
As the device moved along a 4-meter track, rats were forced to run at varying speeds programmed into the computer. In one experiment, the rodents increased their pace at a steady rate, whereas in another they only sprinted halfway. In a final experiment, the rodents were allowed to roam freely in an enclosure at their own pace. In all three tests, 13% to 15% of the recorded cells showed firing patterns that were significantly correlated with the animals' speed, the scientists report online today in Nature.
Neuroscientist Michael Hasselmo, of Boston University, says he isn't surprised by the findings as his own lab has recently found similar speed cells in the MEC, as well as several other different types of speed-responsive neurons. A paper describing those findings is under review, he says.
Particularly impressive in the Mosers' paper, Taube says, is that in some cases they were able to feed the activity of speed cells into a computer and accurately predict when the rat had slowed down or sped up. Although that fidelity was seen with only a handful of the cells, "nonetheless it's an interesting finding" Taube says.
How the speed cells behaved didn't depend on what the rats' environment looked like; for instance, placing them in a new box with different markings didn't change the firing pattern. That's a property they share with grid cells, Moser notes, and it makes sense, as the brain's GPS system can function even without sight, sound, or other external sensory inputs, he says. Close your eyes, for example, and you will still be able to find your way around your home or office, based on signals from within the body, such as feedback from muscles, and the brain's vestibular system, which maintains balance. Indeed, the cells "don't really care" what their surroundings look like, he says. "What matters is the distance and direction of movement."