A new study has found that motor neurons encode the world differently from other types of brain cells.
"This work is a prime example of engineering principles and methods being used to advance our understanding of the brain."
—Zoubin Ghahramani, Professor of Information Engineering in the Department of Engineering
The recently published work reports how nerve cells in the brain control movement, which may help unlock the secrets of the motor cortex, a critical region that has long resisted scientists' efforts to understand it.
For years, scientists have been trying to establish a one-to-one relationship between a neuron's behaviour and factors such as muscle activity or speed of movement. A team of scientists have shown that the motor cortex's effects on movement can be much more easily understood by looking at groups of motor cortex neurons instead of individual nerve cells. In the study, published in the scientific journal 'Nature', researchers identified rhythmic brain cell firing patterns coordinated across populations of neurons in the motor cortex. They linked those patterns to different kinds of shoulder muscle movements.
One of those involved in the research work was Dr John Cunningham, who carried out his research whilst he was a Postdoctoral Fellow in the Department of Engineering's Computational and Biological Learning Laboratory. He explained: "Populations of neurons in the motor cortex oscillate in beautiful, coordinated ways. These patterns advance our understanding of the brain's control of movement, which is critical for understanding disorders that affect movement and for creating therapies that can restore movement."
Until now, scientists had based their studies of the motor cortex on decades-old insights into the workings of the visual cortex. In this region, orientation, brightness and other characteristics of objects in the visual field are encoded by individual nerve cells.
However, researchers could not detect a similar direct encoding of components of movement in individual nerve cells of the motor cortex.
"We just couldn't look at an arm movement and use that to reliably predict what individual neurons in the motor cortex had been doing to produce that movement," said Dr Cunningham.
For the new study, scientists monitored motor cortex activity as participants reached for a target in different ways. They showed that the motor cortex generated patterns of rhythmic nerve cell impulses.
"Finding these brain rhythms surprised us a bit, as the reaches themselves were not rhythmic," said Dr Cunningham's associate researcher, Dr Mark Churchland, Assistant Professor of Neuroscience at Columbia University. "In fact, they were decidedly arrhythmic, and yet underlying it all were these unmistakable patterns." The third member of the team was Dr Krishna Shenoy, Associate Professor of Electrical Engineering at Stanford University.
Dr Cunningham compared the resulting picture of motor cortex function to an automobile engine. The engine's parts are difficult to understand in isolation but work toward a common goal, the generation of motion. "If you saw a piston or a spark plug by itself, would you be able to explain how it makes a car move?" said Dr Cunningham. "Motor-cortex neurons are like that, too. They are understandable only in the context of the whole."
Researchers are now applying their new approach to understanding other puzzling aspects of motor cortex function.
Zoubin Ghahramani, Professor of Information Engineering in the Department of Engineering, commented: "This work is a prime example of engineering principles and methods being used to advance our understanding of the brain."
Dr Cunningham added:"This research involved heavy experimental and computational/analytic components. The computational aspects were accomplished at Cambridge in the Department of Engineering's Computational and Biological Learning Laboratory. The Information Engineering Division provided the ideal environment for these scientific advances. The fundamental neuroscientific finding was enabled by novel data analysis and machine learning techniques that were developed over the last two years at Cambridge."
Dr. Cunningham, who worked as a Research Fellow in Cambridge under Professor Ghahramani and Dr Rasmussen, is now assistant professor of biomedical engineering at Washington University.
The full paper can be found in Nature.