The point of his experiment was to use the illusion as a tool, to force the brain to change its perception of body size so that the scientists could use a scanner to spy on the mental work in progress. This kind of mapping to find the origins of body image had not been done before, Dr. Ehrsson said. "It's a little bit of a forgotten sense," he said. "We know about touch, pain and position, but the sense of size of body parts has been a mystery. There are no receptors in skin or muscle that tell the brain the size of body parts, so the brain has to figure it out by comparing signals."
