Building a Better Bionic Arm by Teaching the Brain a New Signal

The Body’s Sense of Spatial Awareness

The human body has a built-in sense for where its various parts are in space, explains Philip Sabes, a professor and neurophysiologist at UC San Francisco.

“There are sensors that live in your muscles, in your joints, even in your tendons,” Sabes says, “that tell your brain about where you body is.”

This is why you can touch your nose even with your eyes closed; proprioception tells you where your nose is with respect to your hand.

As you move, proprioception gives the brain feedback, and this feedback lets you refine your movement, moment by moment. But amputees controlling a prosthetic have to do without this sense. Generally, they rely on sight instead, which isn’t as fast.

“Visual feedback is much slower to get to the parts of the brain that control movement than proprioception is,” Sabes says.

Sabes’ research is backed by the Department of Defense research agency, DARPA. There are hundreds of thousands of amputees in the U.S., including some 1,500 veterans of the wars in Iraq and Afghanistan.

Sabes’ Experiment

Sabes is researching how an artificial limb might be able to send useful feedback to the brain of an amputee. What if the signal were artificial, he asked, could the brain figure out what it meant and put it to use?

Sabes began with a monkey that was trained to move his hands in response to a dot on a screen, that is, a visual signal. Then Sabes added a signal to the brain.

The signal went to a tiny chip, surgically implanted just inside the monkey’s skull. It delivered tiny fractions of a volt to the motor cortex — an amount of electricity so small, Sabes says, you probably wouldn’t feel it on your hand.

“And so that signalled to the monkey,” Sabes says, “that maybe there was something lightly touching his arm or his elbow or his shoulder.”

The researchers paired this stimulation alongside the visual signals the monkey was already used to reading. Sabes says initially, it didn’t make much difference.

“Over days and weeks,” he says, “we noticed that the monkey started to perform better and better when he had both cues.”

The monkey’s brain was incorporating the new signal — putting it to use, so he had to rely less on visual cues alone.

“After a few weeks, he was so good we actually let him do the task with just stimulation alone,” Sabes says. “So then he was able to sit in a dark room with no visual feedback, nonetheless he could reach from one target to the next to the next, because he could feel where those targets were, via the stimulation.”

What It Means

The results show that the primate brain, given an arbitrary kind of signal, can adapt to understand it as a kind of faux proprioception. Knowing that the brain can do this could someday lead to giving amputees a sense of where their artificial limbs are in space. Such signals could be delivered through feedback, Sabes says, via either the nerves or small implants in the brain.

So how long until this kind of technology is far enough along it can help amputees control their prosthetics well enough to move like the real thing?

“I’m going to be bullish,” Sabes said. “Fifteen to 20 years until it’s as natural as normal movement.”