Cracking an egg is challenging—apply too much pressure and the shell shatters, apply too little and it won’t break. For amputees, such tasks that involve “just right” amounts of pressure are next to impossible because of the body’s inability to receive feedback from prosthetic devices. Now, new research is helping two men with amputations feel tactile pressure, a breakthrough that could one day change the way prosthetic devices work with the body.
The study, published today in the journal Science Translational Medicine, details a new type of prosthetic arm that integrates touch-sensing electrodes into the body's nervous system. The researchers used the device to learn more about how the nervous system transmits information about tactile intensity—the intensity of touch—to the body.
Two men with amputations were equipped with the prosthetic system, which includes pressure sensors on the prosthetic hand and a stimulator that transmits that pressure information to cuffs of electrodes implanted around three sets of nerves that control hand motion. When the men use their prosthetic hands, they send electrical impulses with information about pressure directly to those hand-controlling nerves, which in turn stimulate the brain. The brain then interprets the intensity information sent by the electrodes.
During a series of tests, the researchers found that their subjects can differentiate 20 levels of tactile intensity. Not only were they able to distinguish between similar intensity levels, but they also were able to rate touch intensity and compare and even match it to the same intensity on the non-prosthetic hand.
Given that the men don’t have nerve fibers in their prosthesis, these results are a big deal. But even more important, the researchers used their observations to determine how neural responses to touch actually work.
These experiments suggest that when it comes to neurons activated by touch, both the firing rate and the number activated deliver information about what’s going on. That information opens up potential ways that future prostheses could better incorporate tactile information, which would be a coup for amputees.
“When you lose your sense of touch, you have to use one of the other senses to establish a connection to the object you’re picking up,” Jared Howell, director of orthopedics and prosthetics at Baylor College of Medicine who was not involved in the study, tells Smithsonian.com.
Howell, who works with amputees in his clinical practice and heads up research to create better prostheses, says that amputees are forced to rely on sight instead of touch, which then prevents them from participating fully in other activities that require their sense of sight. “They actually lose a level of independence and a level of function that they would otherwise have,” he says.
In the future, the researchers hope to use their work as a foundation for research that could make better prosthetics possible or even allow robots to transmit tactile information to humans. But for now, the experimental prostheses are allowing two men to do complex lathe work, walk dogs, and yes, crack eggs—tasks that once seemed insurmountable with relatively clunky, non-neural prostheses.
“We never will restore true function until we get the brain to interact with the prosthetic system,” Howell says. The newly-announced research could be a vital step on that road—that is, if this technology is eventually accessible to more than two people.
In the meantime, researchers will continue to crack the code on how the body interacts with its sense of touch, one neuron (and one egg) at a time.