A few stray hairs

The part of the mouse brain devoted to sensory input is moonlighting as a whisker-flicker, scientists have found. The result may prompt researchers to rethink strict descriptions of certain brain regions.

The new study, published November 26 in Science, shows that it’s not the brain’s motor cortex, which is in charge of voluntary motion, but rather the sensory cortex that tells a mouse to pull its whisker away from danger.

“This study furthers the whole line of thinking about the brain — that really, all these systems are deeply interconnected,” says neuroscientist Michael Graziano of Princeton University. “There’s a growing realization that it’s difficult to chop the brain up into little pieces and study them separately.”

In the new work, scientists led by Ferenc Matyas of the Hungarian Academy of Sciences in Budapest saw that when a mouse deflected its whisker away from an object, neurons in the sensory cortex were the first to fire. This region of the mouse brain is thought to be responsible for sensing a soft, warm nest or a painful prick from a twig, but not initiating motor activity.

When Matyas and his team blocked the activity of the sensory cortex with a toxin, the mice could no longer move their whiskers away from a signal. What’s more, inactivating the motor cortex with the toxin had no effect on the whisker flick.

To confirm the toxin results, the team genetically engineered mice to have sensory cortex neurons that responded to blue light. When the light was shone in that brain region, the whisker retracted, even though the motor cortex was inactivated. This told the researchers that the sensory — and not the motor — cortex was moving the whisker away.

“This was a big surprise because most people thought that the motor cortex is the one that controls voluntary movements and the sensory cortex is important for perception,” Matyas says.

The motor cortex wasn’t completely off-duty, though. Whisker motion toward an object was still controlled by the motor cortex, the team found.

This job mingling might have important consequences, Matyas speculates. The “move whisker away” signal that comes from the sensory cortex is slightly faster than the opposite signal that comes from the motor cortex. “It’s a few milliseconds’ difference, but in neuronal processes, milliseconds mean a lot,” Matyas says. “Mice observe their environment through the whiskers,” so quickly sensing an aversive object and pulling away could be a matter of survival.

It’s unclear whether the same brain-region blurriness is present in humans, but the new study may point out places of overlap to explore, says Ron Frostig of the University of California, Irvine. “One needs breakthrough research like this to highlight for other researchers that such blurriness exists in principle,” he says. “I think it is time to start looking at the cortex in a new, integrative way.”