The neurologist VS Ramachandran demonstrates a mirror box, his invention to treat phantom limb pain. Ramachandran is the author of the book, “Phantoms in the Brain”.

Explanation (from Wikipedia):

Based on the observation that phantom limb patients were much more likely to report paralyzed and painful phantoms if the actual limb had been paralyzed prior to amputation (for example, due to a brachial plexus avulsion), Ramachandran and Rogers-Ramachandran proposed the “learned paralysis” hypothesis of painful phantom limbs (Ramachandran & Blakeslee 1998). Their hypothesis was that every time the patient attempted to move the paralyzed limb, he or she received sensory feedback (through vision and proprioception) that the limb did not move. This feedback stamped itself into the brain circuitry through a process of Hebbian learning, so that, even when the limb was no longer present, the brain had learned that the limb (and subsequent phantom) was paralyzed. Often a phantom limb is painful because it is felt to be stuck in an uncomfortable or unnatural position, and the patient feels he or she cannot move it.

Using A Mirror Box To Treat Phantom Limb Pain – VideoSift