Phantoms take many forms — headless horseman, ghost ships, murdered fathers — and they can even reach out and grab the living: many people who have had an arm or leg amputated feel the limb is still present. The phantom pain that often accompanies these limbs has been successfully treated by using visual feedback from mirrors to trick the brain. Now similar instances of mind over non-matter have been achieved without external help — amputees have learned to mentally manipulate their phantom limbs into anatomically impossible configurations through thinking alone, scientists report October 26 in the Proceedings of the National Academy of Sciences.
“It is very surprising that anybody — amputees or not — can learn impossible movements just by thinking about it,” comments neuroscientist Henrik Ehrsson of the Karolinska Institutet in Stockholm.
Treatment of people with phantom limb pain usually requires starting a new conversation between the brain and the environment, typically accomplished through visual feedback, Ehrsson says.
The work suggests that people with a distorted body image — such as those with anorexia — may be able to alter their self-image by imagining a change to the body, says Lorimer Moseley of the Prince of Wales Medical Research Institute in Randwick, Australia. And those getting neural reconstructive surgery may be able to practice using their new body parts by simply imagining their use, says Moseley, who coauthored the work with colleague Peter Brugger of the University Hospital Zurich.
Seven people who had an arm that had been amputated above the elbow were encouraged to learn a particular arm movement that defies biomechanics — turning a hand that’s bent 90 degrees at the wrist the last quarter of a full turn that the hand won’t do. The study participants practiced by imagining that they were moving the phantom limb for five minutes per hour every day until they had achieved the impossible movement or had given up (this took one to four weeks depending on the individual). Four of the participants were successful in feeling the sensation of the impossible movement, the researchers report.
“This shows that body image is constructed in a dynamic manner — it can be changed,” says V.S. Ramachandran, director of the Center for Brain and Cognition at the University of California, San Diego. Previous work by Ramachandran and others has shown that the sensation of a perpetually clenched and painful wrist that often accompanies a phantom limb can be relaxed with a mirror-based therapy: the patient clenches and then unclenches the remaining hand while looking at a boxed mirror that makes it appear both arms are intact. By visualizing both hands unclenching, the patient feels a release in the phantom limb.
To corroborate that the individuals had really learned the new movement (after all, the scientists couldn’t see the phantom limbs) the researchers had them perform a task known as left-right hand judgement before and after their training. The ability to twist the phantom wrist in a new way allowed the participants to react to this task faster than they could before they had learned the impossible move.
Each of the participants who achieved the impossible move also described developing a new wrist joint that allowed the impossible movement. And three of the four reported that moves that were previously possible for the phantom limb were now difficult with their new wrist.
Even though the new movements suggest that “I think, therefore I can” is the operating principle in phantom limbs, the fact that some movements became harder with the new wrist suggests that Newtonian physics still govern perceived motion, says Moseley. “We have an inbuilt sense of what’s physically impossible and what is not,” he says.
Even so, “Body image turns out to be extraordinarily plastic,” says Ramachandran. “We think of ourselves as stable people with a stable body image — but we can inhabit a body that cannot exist in the physical world.”