The Brain Can Adopt Prosthetics As Its Own, Study Says

Using electric brain stimulation, researchers tricked people into accepting a rubber hand as part of their body.

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Dec 30 2016, 3:00pm

Gary Macie's foot turned at an impossible angle. 

The Texas Instruments electrical engineer hit an icy patch while glacading (essentially controlled sliding) down a frozen volcano in Tlaxcala, Mexico, and crashed into a pile of boulders. His tibia was shattered, restricting blood flow to his foot. After being transported to a Mexican hospital, where he paid $32 for surgery, American doctors at Loyola University Medical Center in Chicago eventually amputated the limb. Eight months later, he took his first step in a prosthetic foot. 

"It felt like a ton of bricks," Macie says of his first replacement appendage. That was in 1992; he has since retired and sails the world in a 49-foot vessel with a contemporary blue fiberglass foot he screws on each morning. 

Prosthetics technology has come a long way since Macie's accident, with recent research focusing on neuroprosthetics, tools that allow the brain to control fake limbs. Hand amputees, for instance, now have neuroprosthetic appendages that move in accordance with their thoughts. Yet the user's brain still perceives the hand as a foreign object, a sort of tool attached to the body. 

That may soon change, suggests a preliminary study from the University of Washington. Researchers developed and tested a method of electrical stimulation that tricks the user's brain into believing it's picking up sensory input from a foreign object. The study was small, but it shows promise in making the brain take ownership over a prosthetic.

To carry out the study, the researchers drew on the rubber hand illusion, a classic demonstration in neuroscience in which a subject comes to regard a fake hand as connected to his or her brain. The subject's real hand is hidden from view and touched repeatedly, while at the same time, the rubber hand is kept in full view and touched simultaneously. The congruent touching tricks the brain, giving the subject a sensation of "feeling" from the rubber hand.

The trick doesn't traditionally work for amputees with nerve or spinal cord damage, because there's no real hand to hide or touch, no sensory nerves to shoot signals to the brain. So for the patients in the study, the research team bypassed the peripheral nervous system entirely, modifying the rubber hand illusion by stimulating the brain directly. 

The researchers recruited two volunteers. Neither were amputees, but both were undergoing invasive surgery for epilepsy, wherein invasive electrodes were being implanted into the brain. Apart from providing more localized imaging than an MRI, the electrodes are also able to provide small electric shocks.  

When the researchers touched a rubber hand, the probe sent the electrical current through the electrode to the part of the brain that controls the hand, and the participants reported feeling light, if slightly unnatural, pressure and vibrating sensation in parts of their real hand, which was out of sight during the experiment. 

"Surprisingly, the illusion worked very well in both of our participants, and it was actually equally vivid as the classical rubber hand illusion," says Arvid Guterstam, who co-authored the study.

While the researchers say the findings shouldn't be generalized to the entire amputee population due to the small sample size, they do represent a big step toward eventually building a functional robotic prosthesis that would fully replace a human limb.

"This suggests that if an artificial/robotic prosthesis were equipped with tactile sensors, which were connected to a permanently implanted version of the electrode array we used in the experiments, that users would be able to quickly establish a sense of ownership for a satisfying and dextrous movement," says lead researcher Kelly Collins.

Sailing off the coast of Indonesia, Macie says that his peg leg is more fitting to pirate lore than it is to the Six Million Dollar Man. But in the quickly developing world of neuroprosthetics, doctors may someday be able to give him a foot as sophisticated as the one he lost. The technology is getting there.