Prosthetics that feel

Amputees whose “sense of touch” was rerouted from their missing limbs view their prosthetic not as a tool, but as part of the body, a study to appear in Brain suggests. Such enhanced sense of ownership, scientists say, might lead to prosthetics that operate seamlessly in place of a missing limb.

The new study was conducted with two arm amputees who had undergone a surgery called targeted reinnervation, in which the remaining nerve ends from the severed arm were rerouted to an area on the arm above the site of amputation. This patch of skin serves as a proxy — touching different parts of the area makes the amputee feel as though distinct parts of his or her missing arm were being touched.

The research “tells us about the brain — that the brain can take this abnormal sensation and attribute it to the hand, to the arm,” says neuroscientist Steven Hsiao of Johns Hopkins University in Baltimore, who wasn’t involved in the work. “These people are feeling something. They feel like they’re really touching something, presumably.”

To create that feeling of limb ownership, or “embodiment,” researchers in Chicago led by Paul Marasco designed a pressure-sensing system for the prosthetics. Each time a sensor on the prosthetic hand detected a touch, it would send a signal to a small robot that would poke a targeted area of the reinnervated skin.

Using the robot system, Marasco and his team had each subject sit at a table, with the prosthetic arm unattached but arranged in a natural position. As the subject watched a researcher touch the prosthetic hand, the robot would simultaneously press on the reinnervated skin.

Seeing and feeling the touch at the same time created a powerful illusion in both amputees that the prosthetic hand was part of the body. When they saw but did not feel the researcher’s touch, the subjects didn’t feel a sense of ownership over the prosthetic, says Marasco, now at the Advanced Platform Technology Center at the Veterans Affairs Medical Center in Cleveland. “It was really when the touch matched what they saw, that we saw these changes.”

Also, when one of the subjects both saw and felt a touch on her prosthetic arm, the temperature of the arm just above the site of the amputation rose. This boost, Marasco says, may reflect the body adopting the prosthetic. When sensory information is blocked from a limb, such as an arm that suffers nerve damage after a stroke, the limb’s temperature drops slightly.

Although the new study is “a baby step,” it’s important, says bioengineer Michael Goldfarb of Vanderbilt University in Nashville, Tenn. “What makes you feel something is a part of you is not just being able to move it.” Sensory input can’t be overlooked, he says, when creating good prosthetics.

“People feel like these are tools attached to their body. Even though they are very sophisticated, they are tools,” Goldfarb says. “So the idea of trying to get sensory information back in would help integrate this limb as part of the body, help [people] control it and, one would argue, help them feel more whole.”