In up to 80% cases of amputations, a strange phenomenon occurs where the amputee reports sensation or even severe pain where the limb has been amputated. It was noticed in field hospitals during wars when a soldier would wake up and ask someone to scratch his leg – which was no longer attached to his body. The sensation can be so powerful that victims actively believe that their phantom limb can interact with real objects. For example, there have been case reports of patients trying to pick up a cup with an amputated arm and becoming frustrated with their inability to.
Phantom limb pain may persist even after the amputee realises the limb is no longer there. The basis for phantom limb pain is a neurological system called the cortical homunculus. The cortical homunculus is a concept that the part of the brain responsible for sensation and movement is mapped out so that each part corresponds to a part of the body (see picture). For example, the top of the primary somatosensory and motor cortices (said parts of the brain) is responsible for foot sensation and movement while the side receives information from and sends signals to the face. It lets the brain construct an image of what the body looks like from sensory information it collates from various body parts. It is suggested that phantom limb pain is caused by a remapping of the cortical homunculus, fooling the brain to think that the limb is there even if it has been physically cut off. This also explains a similar condition called supernumerary phantom limb, where the brain believes there is an extra limb (e.g. a third arm).
As the homunculus concept is a recent idea, treatment options had not advanced much until the late 1990s. In 1998, a neuroscientist called Dr. Vilayanur S. Ramachandran devised a method called the mirror box treatment. He noticed that victims of phantom limb pain (PLP) had paralysis or pain in the limb just before the amputation (such as tightly gripping something before the arm got blown up by a mine), suggesting that PLP may be a form of learned paralysis. This means that the brain believes that the arm is still paralysed and any movement causes an uncomfortable sensation as the brain thinks the limb is contorted into a painful position. To fix this problem, Dr. Ramachandran invented a box with two holes, each going into a separate compartment. One compartment is for the good arm while the other has a mirror positioned on an angle to reflect the other arm (instead of seeing the stub they put in the hole). He would then instruct the patient to perform symmetric movements with both hands while looking at the reflected arm. For example, he would tell the patient to squeeze their “fists” tightly as possible and then let go. Through this procedure, the brain is retrained to let go of the perceived paralysis and pain as it is tricked in to thinking that the arm is healthy again. The mirror box therapy drastically improved the outcome and quality of life of PLP patients through the power of illusions.