June 11, 2012
In a surprising finding, researchers have proposed a new model that says motor neurons send basic rhythmic patterns down the spine to drive movement.
The new theory proposed by neuroscientists at Stanford University is a significant departure from existing understanding and helps to explain, in relatively simple and elegant terms, some of the more perplexing aspects of the activity of neurons in the motor cortex.
In their paper published in Nature, electrical engineering associate professor Krishna Shenoy and postdoctoral researchers Mark Churchland, now a professor at Columbia University, and John Cunningham of Cambridge University, now a professor at Washington University in St. Louis, have shown that the brain activity controlling arm movement does not encode external spatial information—such as direction, distance, and speed—but is instead rhythmic in nature.
Neuroscientists have long known that the neurons responsible for vision encode specific, external-world information—the parameters of sight. It had been theorized and widely suggested that motor cortex neurons function similarly, conveying specifics of movement such as direction, distance and speed, in the same way the visual cortex records color, intensity, and form.
“Visual neurons encode things in the world. They are a map, a representation,” says Churchland, who is first author of the paper. “It’s not a leap to imagine that neurons in the motor cortex should behave like neurons in the visual cortex, relating in a faithful way to external parameters, but things aren’t so concrete for movement.”
Speedometer or engine?
Scientists have disagreed about which movement parameters are being represented by individual neurons. They could not look at a particular neuron firing in the motor cortex and determine with confidence what information it was encoding.
“Many experiments have sought such lawfulness and yet none have found it. Our findings indicate an alternative principle is at play,” says co-first author Cunningham.
“Our main finding is that the motor cortex is a flexible pattern generator, and sends rhythmic signals down the spinal cord,” says Churchland.
To employ an automotive analogy, the motor cortex is not the steering wheel, odometer, or speedometer representing external-world information. It is more like an engine, comprised of parts whose activities appear complicated in isolation, but which cooperate in a lawful way as a whole to generate motion.
“If you saw a piston or a spark plug by itself, would you be able to explain how it makes a car move?” asks Cunningham rhetorically. “Motor-cortex neurons are like that, too, understandable only in the context of the whole.”
In monitoring electrical brain activity of motor-cortex neurons, researchers found that they typically exhibit a brief oscillatory response. These responses are not independent from neuron to neuron. Instead, the entire neural population oscillates as one in a beautiful and lawfully coordinated way.
Player in a band
The electrical signal that drives a given movement is therefore an amalgam—a summation—of the rhythms of all the motor neurons firing at a given moment.
“Under this new way of looking at things, the inscrutable becomes predictable,” says Churchland. “Each neuron behaves like a player in a band. When the rhythms of all the players are summed over the whole band, a cascade of fluid and accurate motion results.”
In the new model, a few relatively simple rhythms explain neural features that had confounded science earlier.
“Many of the most baffling aspects of motor-cortex neurons seem natural and straightforward in light of this model,” says Cunningham.
The team studied non-rhythmic reaching movements, which made the presence of rhythmic neural activity a surprise even though, the team notes, rhythmic neural activity has a long precedence in nature. Such rhythms are present in the swimming motion of leeches and the gait of a walking monkey, for instance.
“The brain has had an evolutionary goal to drive movements that help us survive. The primary motor cortex is key to these functions. The patterns of activity it displays presumably derive from evolutionarily older rhythmic motions such as swimming and walking. Rhythm is a basic building block of movement,” explains Churchland.
To test their hypothesis, the engineers studied the brain activity of monkeys reaching to touch a target. According to the researchers, experiments show this “underlying rhythm” strategy works very well to explain both brain and muscle activity. In their reaching studies, the pattern of shoulder-muscle behavior could always be described by the sum of two underlying rhythms.
“Say you’re throwing a ball. Beneath it all is a pattern. Maybe your shoulder muscle contracts, relaxes slightly, contracts again, and then relaxes completely, all in short order,” explains Churchland.
The researchers say that although the activity may not be exactly rhythmic, it can be created by adding together two or three other rhythms. The team asserts that this may be how the brain solves the problem of creating patterns of movement.
“This surprised us a bit. In decidedly arrhythmic movements, there were these unmistakable patterns,” says Churchland.
Accordingly, the seemingly complex system that is the motor cortex can now be at least partially understood in more straightforward terms.