The ghost in the machine
After a limb is amputated, the brain's internal body map does not always update. Up to 80% of amputees experience phantom limb pain (PLP), where they feel often excruciating sensations in a limb that is no longer there. This occurs because the sensory and motor areas of the brain that once controlled the limb can remain active. The brain sends motor commands, but receives no feedback, creating a conflict that can be interpreted as pain. Studies show a lifetime prevalence for phantom limb pain between 76% and 87% among amputees. The pain is often described as burning, stabbing, or cramping, and is typically projected to the distal parts of the missing limb, like the fingers or toes.
In the 1990s, neuroscientist Vilayanur S. Ramachandran, director of the Center for Brain and Cognition at the University of California, San Diego, proposed a new theory. He hypothesized that some phantom pain resulted from a "learned paralysis." If a limb was paralyzed before amputation, the brain learned that it could not move. After amputation, every time the brain sent a command to the phantom limb, it received no visual or sensory confirmation of movement, reinforcing the paralysis and causing pain. This mismatch between the brain's motor output and the lack of sensory input is an important aspect of the phenomenon.