Neurons have minds of their own

STANFORD (US) — Neurons in the brain trigger physical movements in the body, but at times seem to fire in a crazy quilt pattern just before and during the movement.

A new study finds there may be a method to the apparent madness. The neurons aren’t contrary after all, they just have a different way of getting to their goal.

“A classic idea is that the neurons are coded according to a sort of blueprint, in which each neuron has a movement that it ‘prefers,'” says Mark Churchland, a postdoctoral researcher in electrical engineering at Stanford University.

That means that a given neuron would be most active before and during its preferred movement. For example, if you wanted your leg to make a rightward movement, all your neurons would be active, but the rightward-preferring neurons would be the most active.

“But what we found is that a neuron could be very active before, say, a rightward movement, but then actually shut down just before the rightward movement,” Churchland says.

Or it could be completely inactive before a leftward movement, but then become strongly active during the leftward movement.

“If you’re trying to relate the activity of a single neuron to the action that takes place, it looks crazy,” he says.

“If you said that the neuron was effectively voting for its preferred movement, you’d say it is voting for moving left at this time and a tenth of a second later it is voting for moving right and a tenth of a second after that it is voting for something else,” Churchland says. “It would not make any sense at all.”

But if you compare the neuron’s behavior to a pendulum in a clock, things begin to make sense, he explains. In order to get a pendulum to swing to the right, you first have to pull it to the left. And as a pendulum swings back and forth, it will be moving in different directions at different times, even as all its activity is directed at keeping the proper time.

“Whereas a vote is something that should stay nice and consistent across time, a pendulum may swing different directions at different times. But a pendulum has dynamics that are consistent across time even though the position of the pendulum is not,” Churchland says.

“It basically comes down to don’t think of planning a movement as something that involves creating an explicit blueprint,” Churchland says. “Think of it as getting your motor system wound up in just the right way so that when you release it, it does just the right thing.”

The research is published in the Nov. 4 issue of Neuron. Krishna Shenoy, associate professor of electrical engineering, is the senior author.

Shenoy and Churchland conducted their research with rhesus macaque monkeys, that were connected to a computer monitoring their neural activity.

While the monkeys were monitored, they played a video game that required them to move their hand from an initial point on the screen to a small square that appeared elsewhere on the screen.

“The square initially jiggles onscreen, sort of like a buzzing fly. Then it stops jiggling, which we call landing, and then the monkey’s job is to swat the fly,” Churchland says.

“The important thing is that the tasks allowed us to record neural activity not just during the movement, but also during the period when the monkeys are getting ready to make the movement.”

So if there is no blueprint for your brain to follow each time you decide to move a muscle, does the brain have to figure everything out from scratch, starting at the beginning each time?

“We do know that the brain has difficulty winding up its ‘neural pendulum’ exactly the same way every time,” Churchland says. “That is probably part of what makes golf so challenging. However, we really don’t have a full answer to that question yet. It is very much an avenue for future research.”

The research was principally supported by the National Institutes of Health and the Burroughs Wellcome Fund Career Awards in the Biomedical Sciences.

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