A woman born with eight fingers feels as if she has 10, oddly enough after losing a hand.
The 57-year-old woman, nicknamed "RN" in a new case study, was born with no right thumb or forefinger. But after a car accident claimed her underdeveloped hand, she felt a five-fingered phantom in its place.
"A phantom is the vivid perceptual experience that a limb that's been amputated is still there," said Dr. Paul McGeoch at the Center for Brain and Cognition at the University of California at San Diego. "They don't see it, but they feel it."
The strange and often painful sensation is thought to stem from the loss of sensory input to the brain.
"Even though the limb is gone, its representation in the brain remains," said McGeoch, describing how activity in neighboring brain areas sets off the sensation. "That cross-activation generates the perception of a normal limb."
But the case of "RN" is curious. Even though her thumb and forefinger failed to develop in the womb, her phantom had both.
"Her brain developed to represent her body as it was supposed to be: with five fingers instead of three," said McGeoch. "The presence of her deformed hand was enough to suppress that representation. But when the hand was removed, its influence on the representation was gone and the five fingers sprang forth. They had always been waiting in the representation of her brain."
"RN's" phantom also came with "crushing and throbbing" pain, and her newly perceived thumb and index finger felt shorter than the rest. But she could ease the pain and "elongate" the phantom fingers by using a mirror box, a device designed to trick the mind into seeing the phantom hand.
"Historically, phantom pain has been difficult to treat," said McGeoch, explaining how traditional painkillers tended to fall short. "But the mirror box has shown to be effective by many trials over the years."