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Study Shows Amputees Can Move Phantom Limbs

October 27, 2009

New research shows that 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, Reuters reported.

The researchers say the findings demonstrate that the brain alone can alter how we perceive our bodies, without input from our senses.

The report in the Proceedings of the National Academy of Sciences also raises the possibility of using similar strategies to treat certain movement problems and pain syndromes.

Dr. G. Lorimer Mosely of the University of Oxford in the UK and Dr. P. Brugger of University Hospital Zurich in Switzerland noted that our sense of our own body, sometimes called our “body image,” is something we take for granted.

But, they add, this image can be altered in many neurological or psychiatric conditions and can even be manipulated in healthy patients.

For the study, 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 in order to investigate whether it would be possible to change the image by thought alone, and find out whether it would be limited by the normal physics of the human body if the body part in question didn’t exist in real life.

The subjects were basically taught to spin the hand 360 degrees as if the hand and forearm were joined with a pivot rather than a wrist.

All of the study participants said they felt like they were watching someone else’s arm do the movement at first. However, the four who succeeded in learning the movement eventually said the rotating arm felt like part of their own body.

Each test subject 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 study participants went through a test of reaction time that required them to judge whether an image was of a left or a right hand. Previous 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 said.

All of the study participants responded faster before the training if the image of the hand was in a position similar to their own hand. However, the four successful participants got much faster at reacting to a hand shown in a position opposite from their own phantom hand after training — meaning they could mentally “flip” it thanks to its new joint.

A separate test flashed pictures of hands at the beginning and the end of the impossible movement for the participants.

In most cases, 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. But they would see the normal, longer movement that one’s hand would make to change from one position to the other if the images were flashed more slowly.

No matter how quickly the images were flashed, the people who successfully learned the impossible movement always saw the hand making that movement.

“It suggests that the brain truly does change itself,” Mosely and Brugger concluded.

The researchers 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.

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