Neurons in the brain have puzzled scientists for some time. Why, as they begin to trigger movements in the body, do some seem to spontaneously carry out the opposite action to what is expected?
Stanford scientists think they’ve got it figured out.
If you compare a neuron’s behavior to a pendulum in a clock, things start to make sense, said Mark Churchland, Ph.D., a postdoctoral researcher in electrical engineering.
“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,” he said.
This means that a neuron becomes most active before and during its preferred movement, he explains. For example, if a person wanted to move his leg to the right, all neurons would be active, but the rightward-preferring neurons would show the most activity.
“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 said.
“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.”
Churchland further explains the pendulum analogy by saying that in order to make a pendulum swing to the right, you first need to pull it to the left. As the pendulum swings left and right, it will be moving in different directions at different times, but all of the action is still working at keeping the proper time.
“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 said. “It would not make any sense at all.”
“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 said.
Churchland and Krishna Shenoy, Ph.D., an associate professor of electrical engineering, were able to study these neural pendulum actions with rhesus macaque monkeys, whose neural activity was monitored by a computer.
During the test, the monkeys played a “swat the fly” video game in which a square would jiggle around on the screen, like a buzzing fly. When it landed (stopped jiggling), the monkey would swat at it.
The game allowed researchers to record neural activity not just during the fly-swatting movement, but also during the time when the monkeys were getting ready to move.
The research is found in the November 4 issue of Neuron.
Source: Stanford University