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Researchers now say they've figured out a way to rewire the brain and reduce pain coming from a phantom limb, according to a new study.
The technique essentially involves distracting the brain from mixed signals it may receive as a result of losing the limb, said co-author Ben Seymour. He's a neuroscientist with the Department of Engineering at the University of Cambridge in England.
Phantom pain occurs in about half of patients who have either lost a limb or have lost nervous system contact with the limb, said Seymour, who worked on this project with researchers from Osaka University in Japan.
A popular theory holds that people experience phantom pain because the part of the brain responsible for sensing and moving the hands, arms and legs -- the sensorimotor cortex -- becomes confused by the sudden loss of a limb, researchers said in background notes.
The brain senses a mismatch between its attempt to move the now-lost limb and the feedback it receives, and interprets that confusion as pain -- the body's most basic message that something is not right.
"The brain knows the signals that should be coming from that hand," said Dr. Joseph Herrera, chair of rehabilitation medicine for the Mount Sinai Health System in New York City. He was not involved with the study.
"The problem with having a prosthetic limb is that when you try to control that hand, it does not translate. You use other parts of your body, whether it's the thigh or elbow or shoulder, to control a prosthetic limb, and the sensation does not match the feedback the brain is accustomed to receiving," Herrera explained.
In the study, Seymour and his colleagues delved into phantom limb pain by training 10 amputees to control a robotic arm with their brains.
The research team used a brain-machine interface to decode the neural activity of the mental action needed for a patient to move their "phantom" hand, and linked those signals to a robot prosthetic limb.
Patients experienced an increase in phantom pain if they tried to control the prosthetic arm by willing the movement of their missing arm.
But patients' phantom pain decreased if they were trained to move the robot arm using the "wrong" side of the brain. For example, patients missing the left arm experienced reduced pain if they moved the prosthetic arm through neural signals associated with their right arm, researchers said.
"At first, the patients find it quite difficult to control the robotic limb, but they get better with training because the brain adapts the way it sends information to the robot during learning," Seymour said.
In essence, the researchers diverted the brain's attention away from the confusing signals that were being interpreted as pain, Herrera said.
"The brain can only perceive a limited number of sensations at any one time," Herrera said. "Pressure, temperature and pain all compete for the brain's attention. Training the other side makes sense because you're making the other side stronger, and decreasing your perception of pain associated with the missing limb."
Seymour hopes this research will provide an alternative to painkillers for people experiencing phantom limb pain.
"This shows how technology can offer an alternative to drugs for patients," he said. "Actually, we recently surveyed patients with chronic pain and asked them how they felt about technology-based treatments compared to drugs. To our surprise, we found that patients were remarkably positive about technology, considerably more than drugs."
Why is that? "Probably because people are wary of side effects of drugs, and feel much more in control with technology-based treatment," Seymour added.
The study was published Oct. 27 in the journal Nature Communications.
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