A single brain area previously thought to control only the movement of the hands and arms also may control movement across all four limbs, a study found. This discovery opens new avenues for the use of brain-computer interfaces in people who have lost control of their arms and legs, including those with amyotrophic lateral sclerosis (ALS).
The study, “Hand Knob Area of Premotor Cortex Represents the Whole Body in a Compositional Way,” was published in the journal Cell.
The motor cortex — a region in the brain’s frontal lobe that controls voluntary movements — is thought to be divided into distinct areas, each controlling the muscles of a different body part. However, studies exploring these brain regions have so far used low-resolution techniques that allow for only a rough mapping of such areas.
In a recent collaboration between Stanford University and Brown University, researchers used microelectrode arrays to map the function of individual nerve cells in a region of the brain believed to control only the arms and hands — called hand knob area.
The neuroscientists implanted the arrays in two patients included in BrainGate2 (NCT00912041), a pilot clinical trial designed to test medical devices that help people with tetraplegia (also known as quadriplegia) control movements using only their thoughts.
One of these patients had a severe spinal cord injury and was paralyzed from the neck down. The other had ALS and was still able to move most of his limbs, although some movements in the arm were limited due to muscle weakness.
The electrodes helped determine which nerve cells in the hand knob area were active when participants performed — or attempted — a variety of tasks displayed on a computer screen, such as moving the head, face, arms or legs.
Interestingly, nerve cells in this region were activated not only by movements in the hands and arms, but also when the participants moved other parts of the body, such as their legs or face.
“This research shows for the first time that an area of the brain previously thought to be connected only to the arm and hand has information about the entire body,” Frank Willett, PhD, first author of the study said in a press release.
Although it previously was recognized that brain areas controlling nearby body parts could overlap (such as those regulating movement in the wrists and fingers), the team found an unexpected link between all four limbs.
When participants moved a limb, there were two patterns of neuronal activation: one that represented the limb that was being moved, and another representing the movement being made.
This meant that when patients made similar movements of the arms and legs (hand grasp and toe curl), a similar set of motor neurons coding for the movement was activated, but the nerve cells used to code the limb were distinct.
“We would have expected the resulting patterns of neural activity in motor cortex to be different, because they are a completely different set of muscles. We actually found that they were much more similar than we would have expected,” said Willett, a postdoctoral fellow in the Neural Prosthetics Translational Laboratory at Stanford and the Howard Hughes Medical Institute.
The existence of a movement-specific pattern also suggests that the brain needs to learn a certain motor skill only once, then transferring the skill to another limb by changing the limb-specific activation pattern.
The findings have major implications for brain-computer interfaces (BCIs), such as those where a person’s thoughts are used to move a device. To date, it was thought that implants were needed in many different areas of the brain to control movement of different body parts, but “now we can explore controlling movements throughout the whole body with an implant in only one area,” Willett said.
“The results (…), which show strong whole-body tuning in just one patch of the cortex, make it possible for current iBCIs to decode movements from all four limbs” and possibly “restore continuous control of leg and arm movements across both sides of the body,” the researchers concluded.
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