On a bitter, soul-shivering, damp, biting gray February day in Cleveland—that is to say, on a February day in Cleveland—a handless man is handling a nonexistent ball. Igor Spetic lost his right hand when his forearm was pulped in an industrial accident six years ago and had to be amputated. In an operation four years ago, a team of surgeons implanted a set of small translucent “interfaces” into the neural circuits of his upper arm. This afternoon, in a basement lab at a Veterans Administration hospital, the wires are hooked up directly to a prosthetic hand—plastic, flesh-colored, five-fingered, and articulated—that is affixed to what remains of his arm. The hand has more than a dozen pressure sensors within it, and their signals can be transformed by a computer into electric waves like those natural to the nervous system. The sensors in the prosthetic hand feed information from the world into the wires in Spetic’s arm. Since, from the brain’s point of view, his hand is still there, it needs only to be recalled to life.
Now it is. With the “stimulation” turned on—the electronic feed coursing from the sensors—Spetic feels nineteen distinct sensations in his artificial hand. Above all, he can feel pressure as he would with a living hand. “We don’t appreciate how much of our behavior is governed by our intense sensitivity to pressure,” Dustin Tyler, the fresh-faced principal investigator on the Cleveland project, says, observing Spetic closely. “We think of hot and cold, or of textures, silk and cotton. But some of the most important sensing we do with our fingers is to register incredibly minute differences in pressure, of the kinds that are necessary to perform tasks, which we grasp in a microsecond from the feel of the outer shell of the thing. We know instantly, just by touching, whether to gently squeeze the toothpaste or crush the can.”
With the new prosthesis, Spetic can sense the surface of a cherry in a way that allows him to stem it effortlessly and precisely, guided by what he feels, rather than by what he sees. Prosthetic hands like Spetic’s tend to be super-strong, capable of forty pounds of pressure, so the risk of crushing an egg is real. The stimulation sensors make delicate tasks easy.