In a dramatic display of the potential of prosthetic arms, a monkey  was able to use his brain to directly control a robotic arm and feed himself a marshmallow.
To achieve the feat, two monkeys had a grid of microelectrodes implanted into the motor cortex, [the] part of the brain that controls motor planning and execution. The animals had previously been trained to move an anthropomorphic robotic arm  using a joystick. To learn to control the prosthesis with their minds, the monkeys had their arms temporarily restrained as they watched a computer move the arm through the required motions . ‘They imagine themselves doing the task, like athletes do for sports,' says Schwartz. ‘The neurons are active as they observe the movement, and then we can capture the [neural signals] and use them for our own control.'