Awakening Paralyzed Limbs

Brain signals can drive arm movement in a monkey with a paralyzed arm.

A monkey with a paralyzed arm can still grasp a ball, thanks to a novel system designed to translate brain signals into complex muscle movements in real time. The research, presented at the Society for Neuroscience conference in Chicago this week, could one day allow people with spinal cord injury to control their own limbs.

“This is a big leap forward–they show the monkey using the ability to artificially contract his hand to actually pick up a ball,” says Krishna Shenoy, a neuroscientist at Stanford University. “I think it’s the first demonstration of a cortically controlled electrical stimulation system performing a task that would ultimately be useful for a human patient.”

While spinal cord injury keeps the brain’s electrical signals from reaching muscles, people paralyzed by these injuries often have intact nerves and muscles in their limbs. A technique called functional electrical stimulation (FES), in which implanted electrodes deliver electrical current to trigger muscle contractions, provides a way to reconnect this loop.

Devices that can restore hand function and bladder control to some paralyzed patients have already been approved by the U.S. Food and Drug Administration. Patients use residual muscle movement to consciously control these systems–a system that works well for some applications but limits the complexity of the movement that can be performed. For example, an FES device allows people to shrug a shoulder to trigger a grasping motion with their hand, but they cannot control how tightly to grasp..

Now, by pairing FES technology with brain implants, scientists are trying to create a more intuitive system for controlling paralyzed limbs, such that thinking about moving an arm or grasping with a hand would automatically be translated into the pattern of electrical activity needed to perform that movement. “It’s much more natural, and if you can decode activity in enough muscles, you could move multiple joints simultaneously,” says Robert Kirsch, a neuroscientist at Case Western Reserve University, in Cleveland, OH. Normal hand and arm motion involves fluid movement of multiple joints, rather than the limited movements possible today.

Monkey think, monkey do: By translating electrical signals from a monkey’s brain into muscle contractions via implanted electrodes, an animal with a paralyzed arm was able to grasp a ball. Credit: Christian Ethier, Lee Miller

Christian Ethier, a researcher in neuroscientist Lee Miller’s lab at Northwestern University, in Chicago, has demonstrated the first steps toward this kind of system in monkeys. Researchers gave each monkey a local anesthetic to temporarily block the function of the flexor nerves in its arm. The animals had wires implanted into their arms to deliver electrical stimulus to the muscles, much like nerves would, and an array of electrodes implanted in the brain to record electrical activity from the motor cortex.

The monkeys were first trained to pick up a ball and put it in a hole to earn a reward. Using brain activity recorded during this task, the scientists developed specialized decoder algorithms that would translate brain activity linked to the movement of different muscles into an electrical stimulus for each of five flexor muscles in the arm in real time, enabling the monkey to grasp its hand. “We can predict what the monkey is trying to do with his muscles and stimulate the muscles accordingly, essentially giving the monkey voluntary control through the computer instead of his nerves,” says Miller.

Normally, with the paralyzed arm, the animals had a difficult time completing the task, getting the ball into the target only about 10 percent of the time, compared to 100 percent before the nerve block. Turning on the brain-controlled FES system boosted the paralyzed animals’ success rate to 77 percent. The researchers also showed they could get the monkey to move its wrist in different directions–they now want to see if they can repeat the results with the muscles that control reaching.

Human tests might not be far off. Cortical implants are already being tested in human patients. Case Western’s Kirsch presented research at the conference showing that a paralyzed patient with a cortical implant could control a sophisticated computer model of an arm. Kirsch and Miller don’t yet have a specific timeline to put the two systems–the cortical implant and the FES implant–together in humans, but Miller says it would be technically feasible in a year. However, they want to wait until scientists have developed a wireless and fully-implantable version of the cortical implant, which is now underdevelopment at Brown University. Current implants have protruding wires that increase risk of infection and limit patients’ mobility.

Previous research has shown that patients with these implants can control a computer cursor and make some movements with a robotic arm. While that research is exciting for people whose limbs have been amputated, the new research is applicable to patients with spinal cord injury. “Many people would strongly prefer to have their arm reanimated in some way,” says Shenoy. “This is a big step forward for that patient population.”

By Emily Singer
Copyright Technology Review 2009.

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