Researchers at the Cleveland Clinic have created a new technology to trick the brain into thinking it can sense a prosthetic limb moving, just like it might sense an actual muscle moving.
“One of the things motorized prosthetics are missing is a sense of movement,” said Paul Marasco, a biomedical engineer at the Cleveland Clinic who led the new research, which was published this week in Science Translational Medicine.
When a muscle moves, there’s a feedback loop at work between the brain and the rest of the body. In an able-bodied person, the brain signals to a muscle to move, and that movement sends feedback to the brain that allows it to sense how a muscle is moving. It’s how we inherently know how far to reach, for example, to pick up a glass.
In a person with a motorized prosthetic limb, though, there’s no loop — the brain can tell the prosthetic limb to move, but the prosthetic limb isn’t sending any messages back. That means it’s almost impossible to sense how a prosthetic is moving without looking directly at it.
Marasco and his colleagues designed a system that aims to give people with prosthetic limbs more precise control. To do so, they played a magic trick on the brain using what’s known as a “perceptual illusion.”
“If you vibrate a muscle mechanically, you’ll get a sense in the joint that’s crossed by that muscle that it’s moving, even though it’s not,” Marasco explained. One example: the “Pinocchio illusion.” If you pinch your nose while your bicep is being hit with vibrations, you’ll feel like your arm is moving. To make sense of that, your brain will sense that your nose is extending, because it thinks your arm is moving. Like if you pinch your nose and vibrate your biceps, you get the sense your arm is moving.
“These illusions override reality, and they’re really powerful,” Marasco said.
Working with patients who’d had upper limb amputations, the researchers built a brain-machine interface by redirecting the nerves from an amputated hand to the muscles in a residual limb. They can vibrate the muscles in that limb to create a perceptual illusion, and programmed the prosthetic to match that movement.
“So as the muscles vibrate and the prosthetic moves, the amputees can feel it like it’s moving,” Marasco said. “It’s a representation of what the prosthesis is doing.”
Now, Marasco and his colleagues are working to tie together the new technology with their past work to improve the sense of touch in patients with prosthetic limbs. They’re also hoping to test the system in patients with other types of amputations.