Amputees can "learn" to move their missing arm in an anatomically impossible way, in some cases making normal movement of that "phantom limb" more difficult, new research shows.
The findings show that the brain can alter how we perceive our bodies all by itself, without input from our senses, the researchers say, and raise the possibility of using similar strategies to treat certain movement problems and pain syndromes.
Our sense of our own body, sometimes called our "body image," is something we take for granted, Dr. G. Lorimer Mosely of the University of Oxford in the UK and Dr. P. Brugger of University Hospital Zurich in Switzerland note in their report in the Proceedings of the National Academy of Sciences. But this image can be altered in many neurological or psychiatric conditions, they add, and can even be manipulated in healthy people.
To investigate whether it would be possible to change this image by thought alone, and find out whether this image would be limited by the normal physics of the human body if the body part in question didn't exist in real life, Mosely and Brugger had seven amputees with a "vivid phantom" try to learn to move the wrist of their missing hand in an anatomically impossible way.
Basically, they were taught to spin the hand 360 degrees-as if the hand and forearm were joined with a pivot rather than a wrist. At first, all of the study participants said they felt like they were watching someone else's arm do the movement. But the four who succeeded in learning the movement eventually said the rotating arm felt like part of their own body. Each of them also said that they now perceived their phantom arm differently, while two said it had become more difficult for them to move their phantom hand from side to side because of the changes in their arm's shape.
The researchers used a test of reaction time that required study participants to judge whether an image was of a left or a right hand. Studies have shown that response time corresponds to how long it would take a person to move their arm to match the position of the image. "That is, correct left-right hand judgments require the participant to mentally rotate their own hand to match that shown in the picture," the researchers explain.
Before the training, all of the study participants responded faster if the image of the hand was in a position similar to their own hand. But after training, the four successful participants got much faster at reacting to a hand shown in a position opposite from their own phantom hand-meaning they could mentally "flip" it thanks to its new joint.
In another test, the researchers flashed study participants pictures of hands at the beginning and the end of the impossible movement. Normally, a person perceives a small "impossible" movement between the two hand positions if the images are alternated faster than their own hand would be able to move. If the images are flashed more slowly, a person will see the normal, longer movement that one's hand would make to change from one position to the other.
The people who successfully learned the impossible movement always saw the hand making that movement, no matter how quickly the images were flashed.
The findings suggest that "the brain truly does change itself," Mosely and Brugger say.
The findings raise the "speculative, but not outrageous" possibility that patients could cope with movement problems due to stroke, back pain, or pain in other regions of the body, by being trained to change the image of that body part, according to the researchers.