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Walk, skip or run — people travel at many speeds, but how do our minds tell the difference?
Researchers in Norway have detected a subset of neurons in rats that serve as the brain’s speedometer — they signal how quickly the animal is moving. These “speed cells” are constantly updating the brain’s internal map, helping animals navigate their surroundings.
Scientists already knew how the brain governed location, direction, distance and boundaries. Speed cells are the last major piece of the mental map puzzle, said Jim Knierim, a neuroscientist at John Hopkins University who wasn’t involved in the study.
The study was published Wednesday in the journal Nature.
Before the “speed cell,” there was the grid cell. From the Nobel Prize-winning research of John O’Keefe and Edvard Moser and May-Britt Moser, we know that grid cells help map an animal’s movement; When grid cells work together, they form the coordinate system of the internal map — the longitude and latitude lines. Grid cells primarily work to define the rat’s location, but they also give us information about speed.
“But the field hadn’t found the cells that specifically encoded the speed of movement,” said Jim Knierim, a neuroscientist at John Hopkins University who did not participate in the study.
To find the speedometer neurons, Edvard Moser and his colleagues built a Flintstones-like car for rats. The vehicle was bottomless, so the rat’s paws tread across the floor as the car rolled forward. Researchers controlled the speed of the rat vehicle. If the car moved slowly, the rat strode leisurely. When the speed increased, the rat broke into a sprint. The car dawdled or zipped across a runway multiple times at different speeds, and during each trip, the researchers recorded the brain activity in the entorhinal cortex.
They found that 15 percent of the neurons in the brain’s entorhinal cortex responded specifically to the rat’s speed. If the rat went faster, the neurons fired more signals. But if the rodent slowed down, so did the cell’s response.
A Long-Evans rat at the end of the track that runs a rat sized “Flintstone-like” car. Researchers used the car to detect speed cells in the rat brain. Photo by Raymond Skjerpeng
Speed cells behaved likewise when rats roamed freely in a large box, firing in response to how fast the rat traveled, regardless of location.
Speed cells also appeared to be slightly prophetic. They set the pace “60 to 80 milliseconds ahead in time,” Edvard Moser said. In other words, they seemed to anticipate behavior before the rat accelerated, helping to create a cognitive map slightly before it’s needed.
After a neuron sends a message, it needs time recover before sending a new one. However, some neurons recover more easily than others. About a quarter of the speed cells in the study were “fast spiking,” meaning they require less recovery time and they’re more likely to fire. Knowing the characteristics of these speed cells should help scientists create computer algorithms that model how we steer through space.
If the rat traded in his Flintstones car for a rollercoaster, Moser and Knierim agree that the speed cells would still react, but the response could be different. A roller coaster takes running out of the equation, leaving our other senses to calculate speed. As the rat gains momentum, these sensations — how its muscles shift, how much wind brushes past its fur, and how imagery changes in front of its eyes — likely contribute to a speed cell’s response. Scientists suspect that humans have mental maps too, but less is known about the neurons involved.
Moser’s team intends to study how the various senses contribute to these speed cells.
Experiments in virtual environments could help scientists determine how speed cells react when a stationary rat sees it’s surroundings moving past him on a screen, for example, Moser said. Or they can record from speed neurons while the rat runs in place and the virtual scene remains the same.
Catherine Woods is a 2015 mass media science and engineering fellow at the American Association for the Advancement of Science. She worked with the PBS NewsHour in the summer of 2015.
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