A woman born missing a right hand finger and a thumb "grew" them back as part of a phantom limb after her hand was amputated.
University of California neuroscientists in San Diego said the experience of the woman - known as "RN" - showed the brain has its own internal template of how the body should look, regardless of what they actually look like.
The woman was born with only three fingers on her right hand and had the hand amputated after a car accident when she was aged 18. She later began to feel that her missing limb was still present and developed a "phantom" hand.
"But here's the interesting thing," Dr. Paul McGeoch at the University of California, San Diego, told New Scientist. "Her phantom hand didn't have three digits, it had five."
However RN's phantom thumb and index finger were less than half the usual length and were painful.
McGeoch and Professor V.S Ramachandran used a mirror box which reflected the woman's left hand to make it look like she had a pair of limbs.
After two weeks of training RN was able to extend the short fingers on her phantom limb, which relieved her pain.
McGeoch said the study indicates there is a hardwired representation in the brain of what the body should look like, regardless of how it actually appears in real life. It showed more about the balance between the external and innate representations of a limb, he said.