Touching things, feeling them, is one of the most neurologically complex things human beings do. The sense of touch is the responsibility of what is called the somatosensory system. This is the collection of nerves receptors, and brain connections that gathers data about the outside world—and the body’s state—from the nerve endings on the skin and elsewhere and interprets the data in order to recognize textures and how things feel.
Among the problems faced by people with prosthetic limbs is the loss of a crucial component of this system. Prosthetic limbs, after all, lack these nerve endings, and can’t gather data to give to the brain. The science of neuroprosthetics is looking to change that. Neuroprostheses are artificial limbs wired directly into the brain, not merely to allow natural movement, but to allow natural sensation as well. Research into such devices is only in its infancy, but it is hoped that artificial limbs that function like natural ones will soon be on the horizon.
Indeed, experimental devices are already being tested. A Danish man who lost part of his arm in a fireworks accident is among the first subjects. With his experimental prosthetic, which he used for a month after it was attached in Switzerland nine years after the accident, Dennis Aabo Sørensen was able to feel things for the first time in nearly a decade. He was able to identify things he was holding while blindfolded, using only the sensation of touch from the artificial hand.
Scientists implanted electrodes in his arm to convey sensory signals and nerve impulses from hand to brain and brain to hand, respectively. To allow Sørensen to feel with the hand, artificial tendons in the prosthesis register pressure much as natural ones do. The biggest technological advance was the ability to do this at the same speed as the human brain, allowing for touch—and such related activities as adjusting the grip—to occur naturally. The model of limb tested used implanted electrodes, whereas previous models had used electrodes that were attached on the outside, a significant improvement in useability. Further challenges, however, lie ahead. The technology still can’t detect temperature, or subtle, as opposed to large, differences in texture. In addition, the limbs need to be suitable for long-term use.