Monkeys imitate what they see, but so do humans, only more discreetly. Whether any muscles actually flex, our brains fire up the same pathways needed to perform any action we observe another person perform. But new work on disabled volunteers indicates that the brain instead activates alternate circuits when faced with an action its body cannot physically copy. The research suggests that the brain's motor system may be wired to work toward a goal rather than just duplicating a movement.  
Every time you watch someone press a computer key or pick up a cup, regions of your brain unconsciously respond, mapping what you see onto the motor pathways you would use to carry out that same motion. Researchers believe that this so-called mirror neuron system, which consists of a subclass of motor neurons, is critical to learning new behaviors, and perhaps for developing skills like recognizing facial expressions. But neuroscientists have long wondered how the brain reacts if the body lacks the ability to replicate the action.  
In a study reported 12 July in Current Biology, a team from the Netherlands, Italy, and France showed videos of hands performing simple actions, like grasping a cup, to 16 "normal" people and two aplasic people born without hands or arms. While the volunteers watched the video, the researchers spied on their brain activity using MRI.