A pair of partially paralyzed monkeys regained the ability to move their wrists when researchers wired individual neurons directly to the monkey’s arm muscles, according to a study published online in Nature on Wednesday.
The researchers, led by Eberhard Fetz, a professor of physiology and biophysics at the University of Washington, temporarily paralyzed each monkey’s arm. Then they rerouted brain signals around the blocked nerve pathway by running wires from a single neuron in the motor cortex–the brain area responsible for movement–through a computer and into a muscle in the arm. Whenever the neuron fired above a certain rate, the computer translated the signal into a jolt of electricity to the arm muscle, causing it to contract.
As a test of the rewiring, the researchers had each monkey play a simple video game. By moving its wrist, the monkey could manipulate a cursor on a computer screen. Moving the cursor into a box at the side of the screen earned the monkey a reward. Even though the rewired brain cell was chosen at random, the monkeys quickly learned to move their paralyzed wrists.
“We found, remarkably, that nearly every neuron that we tested in the brain could be used to control this type of stimulation,” says Chet Moritz, a senior research fellow at the University of Washingtonand coauthor of the paper. “Even neurons which were unrelated to the movement of the wrist before the nerve block could be brought under control and co-opted.”
Normally, arm movement–even the contraction of a single arm muscle–would not result from the firing of a single neuron but from the coordinated action of many neurons in the motor cortex. Those neurons would initiate an electrical signal that propagated down the spinal cord and through peripheral nerves to trigger arm movements tailored to the monkey’s intention.
Other groups have recorded those complex neuron-firing patterns and used computer algorithms to translate them into action–for instance, moving a computer cursor. Instead, the University of Washington group linked a single neuron to a single muscle. “Our approach is to create the raw connectivity between single neurons in the brain and muscles, or groups of muscles, and let the monkey learn how to use that connectivity.”
Using a single neuron has its advantages, says Moritz. Translating one cell’s firing rate into an electrical shock is a straightforward computation, easily accomplished with a device the size of a cell phone. Translating simultaneous measurements into a suite of coordinated muscle movements takes far more computing power.