People who have had limbs amputated often have the sensation of a “phantom” limb, but getting feedback from a powered prosthesis is a difficult challenge. Users need to know where their prosthetic hand is in order to make natural movements. It turns out that stimulating muscles can trick the brain into sensing movement. The “Pinocchio illusion” occurs when subjects close their eyes and hold their nose. If their bicep is vibrated, it feels as if the arm is extending, and thus the subjects perceive that their nose is getting longer.
Researchers at the Cleveland Clinic have harnessed this effect to give patients the sensation of motion in prosthetic arms. A computerized system provides kinesthetic signals to the brain in response to the movements. Without any training on the system, subjects report that they feel as if their arm is present. The system relies on reattaching both the motor and sensing nerves from the absent arm to other muscles in the arm. The nerves are stimulated in response to corresponding movements. The motor nerves drive actuators in the powered prosthesis, and the sensor nerves send signals to the brain when the muscle tissue is vibrated. The result is a more intuitive control of the prosthetic arm and hand.
Currently, subjects must be connected to a computer in the lab in order to use this system. The researchers are working to miniaturize the components in order to create a mobile system. Further refinement will be needed to provide better control of individual finger motions. This system may eventually be able to provide limb control for paralyzed subjects.