Perhaps one of the most exciting findings was the surprisingly refined sensory ability the patient developed in her chest. (The patient described in the paper was the third to undergo the nerve-transplantation procedure, but she was the first to have sensory nerves transplanted in addition to motor nerves.) When the area was touched, she felt as if her missing hand had been touched, and she eventually developed a faint sensation of her middle finger when touched on a particular part of her chest.

Scientists say this sensory ability is an important step for the next generation of prosthetic limbs. Sensors or haptics technology could be placed in the fingers of a robotic arm and transmit signals to the chest, allowing the patient to feel the sensation encountered by the prosthetic limb. This would provide the sensory feedback--not present in standard prosthetics--that allows us to grip a Styrofoam coffee cup without crushing it or put down a cup of soup if it's too hot. "Instead of doing commands like a robot, it might actually feel like a part of the body," says Kirsch.

Other scientists are now developing similar implantable devices, potentially allowing a finer level of control. Kirsch, for example, is developing a device that would be implanted onto the muscle to directly detect muscle activity and then wirelessly transmit the activity signals to a prosthetic, an approach that he says will provide more-stable input to the robotic limb.

Richard Normann, a neuroscientist at the University of Utah who has pioneered the development of small electrode arrays that can record sophisticated neural signals, is working on a device that, when implanted onto the nerve, could record signals from individual axons within the nerve fiber, thereby providing a more nuanced set of control signals. He hopes to have a working version to test in some of Kuiken's patients in about two years. "It is not unreasonable to believe an amputee could have an arm that he will come to believe and use just like an existing arm," says Normann. "It's not the reality today, of course, but it's not a fantasy anymore."