Health

Not only can people who have lost a limb continue to sense its presence, they can also move their phantom limb in ways that would be anatomically impossible with the real limb. With practice, they can even learn to envisage their bodies as having changed to an entirely unnatural shape.

Lorimer Moseley of the University of New South Wales in Sydney, Australia, and Peter Brugger of University Hospital Zurich, Switzerland, investigated seven amputees who sensed a phantom arm.

In one task, the amputees were shown pictures of a hand and were asked to say if it was a left or right hand. Because the hand was shown in an awkward position, the subjects had to imagine moving their phantom hand into the position shown. In a second task, they were shown pairs of pictures of a hand in two different positions, and asked to imagine making their phantom hand move from one to the other.

In both tasks, it was quicker to make the necessary movement by doing something that would be impossible with a real hand, such as bending it back through the wrist. So by timing their subjects' responses, Moseley and Brugger could tell whether they were moving their hand in an anatomically correct or anatomically impossible way.

Mind over matter

After practising, four of the seven subjects learned how to move their phantom limbs in "impossible" ways, and were able to say how their wrist allowed them to do this. The joints they described not only allowed them to move their phantom arms in strange ways, but also got in the way of normal movement. Essentially, they constructed a new body image of their phantom limb. "The brain truly does change itself," Moseley and Brugger say.

"The idea that just thinking about movements can change body image is quite surprising," says Henrik Ehrsson, a neuroscientist at the Karolinska Institute in Stockholm, Sweden.

Brugger hopes the findings can be used to help patients with phantom-limb pain. An amputee might feel as if their phantom hand is always clenching a metal bar, causing pain. The discovery that phantom limbs are highly flexible ups the chance that amputees can learn to get out of such painful positions.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0907151106

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