The bionic hand closes slowly. Its slender metal digits whirr as they jitter into a loose fist, as though they are wrapping around an invisible baton. "OK, closed," says the test subject.

The test subject is Amanda Kitts. In 2006, a Ford F350 hit her Mercedes sedan head-on. The collision rent the truck's tire from its chassis and shoved the axle into Kitts' car, where it nearly severed her arm. "It wasn't completely off, but it was mincemeat," she says. "There was no saving it. So the surgeons pretty much cut it straight off, like you would a piece of wood."

More than a decade later, she's sitting in a white-walled room inside Cleveland Clinic medical center controlling a bionic hand via a thermoplastic prosthetic socket, which wraps around her upper body. It envelops what remains of her left arm, which ends a few inches below her shoulder. There, nerves that once commanded her hand and forearm—remnants of her amputation—now innervate her biceps and triceps muscles. The socket relays electrical signals from those muscles to the computerized, motor-driven hand, which she operates with her mind. I want to close my hand, she thinks, and the hand complies.

"You can provide someone a feeling of a hand movement, and a sense of authorship over that movement." Todd Kuiken

But what's impressive here isn't that Kitts can manipulate the hand. Motorized prostheses have improved dramatically in recent years, providing increasingly sophisticated options for restoring dexterous movement.

What's remarkable is that she knows what her bionic appendage is up to, in spite of her blindfold and the noise-cancelling headphones that cover her ears. Kitts can feel the hand’s movement, sense its position in space, and it's unlike anything she's ever experienced. "Being able to close the hand and feeling that it’s closed and knowing that it’s closed. That’s what’s amazing," she says. As the robotic fist loosens, she mirrors its conformation with her right hand. “Open,” she says, when its fingers—which feel like her fingers—reach their full extension.

Neurophysiologists call awareness of the movement and position of one's body parts kinesthesia. (The more general term is proprioception, though it refers more to position than movement.) When an able-bodied person moves her hand, sensorimotor signals inform her brain where and how it's moving. Kinesthesia is what lets her seize a falling bottle of shampoo in the shower, or shoulder her backpack with her right hand while staring at the phone in her left. The sensation is commonly regarded as a sixth sense; it's entirely distinct from touch, yet kinesthesia is equally if not more important for complex motor tasks.

But today, even the most sophisticated prosthetic hands provide no kinesthetic feedback. "That means the only way to know where your prosthesis is, is to watch it," says Paul Marasco, a neuroscientist and—deep breath—sensorineural physiologist at the Cleveland Clinic. An upper limb amputee does not simply open a door. Rather, he sees the door handle. Watches his prosthetic hand reach for the handle. Watches his prosthesis grasp the handle. Turn the handle. Pull the handle. And so on.