Research Shows Motor Neurons Adjust to Control Tasks

Image courtesy of CMU Materials Science and Engineering.

New research from the College of Engineering at Carnegie Mellon University (CMU) and the University of Pittsburgh reveals that motor cortical neurons optimally adjust how they encode movements in a task-specific manner. The findings enhance the understanding of how the brain controls movement and have the potential to improve the performance and reliability of brain-machine interfaces, or neuroprostheses, that assist patients with paralysis and amputations. The study was published in the April 18 issue of the journal eLife.

"Our brain has an amazing ability to optimize its own information processing by changing how individual neurons represent the world. If we can understand this process as it applies to movements, we can design more precise neural prostheses," said Steven Chase, PhD, assistant professor in the CMU Department of Biomedical Engineering and the Center for Neural Basis of Cognition. "We can one day, for example, design robotic arms that more accurately implement a patient's intended movement because we now better understand how our brain adjusts on a moment-by-moment basis when we are in motion."

Our visual system is equipped with a trait similar to a camera's auto-contrast feature that enables it to take high-quality pictures in a wide range of lighting conditions. Neurons in the visual system increase or decrease their sensitivity to light as appropriate to enable us to see in both dimly lit rooms and dazzling sunshine. This process, which is known as dynamic range adaptation, also occurs in neurons that are sensitive to sound or touch. The researchers wanted to know if the motor cortical neurons would automatically adjust their sensitivity to direction when presented with a wide range of possible directions instead of a narrow one.

For the study, the researchers trained two rhesus macaque monkeys that were implanted with Utah Arrays to use their brain activity during simple motor tasks, in this case, to move a cursor on a virtual reality screen in either 2D and 3D. Studying this brain activity showed that neurons became less sensitive to the cursor's direction of movement when the task switched from 2D to 3D. This makes sense, the authors stated, because in a 3D task, which also features depth, the neurons have a greater range of possible movement directions to encode. Conversely, the neurons became more sensitive to the direction of movement when the task switched from 3D to 2D. Under these circumstances the neurons can use activity that was previously dedicated to encoding depth to instead represent the 2D space in finer detail.

The results revealed that dynamic range adaptation did indeed occur in the motor cortical neurons. Based on these findings, the researchers concluded that this feature is widespread throughout the brain.

"We found that dynamic range adaptation isn't restricted to sensory areas of the brain. Instead, it is a ubiquitous encoding feature of the cortex," explained Andrew Schwartz, PhD, distinguished professor of neurobiology and chair in systems neuroscience at the University of Pittsburgh School of Medicine, and a member of the University of Pittsburgh Brain Institute. "Our findings show that it is a feature of information processing, which your brain uses to efficiently process whatever information it is given-whether that is light, sound, touch, or movement. This is an exciting result that will motivate further research into motor learning and future clinical applications."

Editor's note: This story was adapted from materials provided by CMU and eLife.