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Team May Have Found Cause of Phantom Limb Pain

According to researchers, a “reorganization” of the wiring of the brain may be the underlying cause of phantom limb pain, which occurs in the vast majority of individuals who have had limbs amputated. They have also found a potential method of treating it using artificial intelligence techniques.

The researchers, led by a group from Osaka University in Japan in collaboration with the University of Cambridge, used a brain-machine interface to train a group of ten individuals to control a robotic arm with their brains. They found that if a patient tried to control the prosthesis by associating the movement with their missing arm, it increased their pain, but training them to associate the movement of the prosthesis with the unaffected hand decreased their pain.

Their results, reported in the journal Nature Communications, demonstrate that in patients with chronic pain associated with amputation or nerve injury, there are “crossed wires” in the part of the brain associated with sensation and movement. By mending that disruption, the pain can be treated. The findings could also be applied to other forms of chronic pain, including pain due to arthritis.

A popular theory of the cause of phantom limb pain is faulty “wiring” of the sensorimotor cortex, the part of the brain that is responsible for processing sensory inputs and executing movements. In other words, there is a mismatch between a movement and the perception of that movement.

In the study, Ben Seymour, PhD and his colleagues, led by Takufumi Yanagisawa, MD, PhD, from Osaka University, used a brain-machine interface to decode the neural activity of the mental action needed for a patient to move their “phantom” hand, and then converted the decoded phantom hand movement into that of a robotic neuroprosthesis using artificial intelligence techniques.

“We found that the better their affected side of the brain got at using the robotic arm, the worse their pain got,” said Yanagisawa. “The movement part of the brain is working fine, but they are not getting sensory feedback-there’s a discrepancy there.”

The researchers then altered their technique to train the “wrong” side of the brain. For example, a patient who was missing their left arm was trained to move the prosthetic arm by decoding movements associated with their right arm, or vice versa. When they were trained in this counterintuitive technique, the patients found that their pain significantly decreased. As they learn to control the arm in this way, it takes advantage of the plasticity-the ability of the brain to restructure and learn new things-of the sensorimotor cortex, showing a clear link between plasticity and pain.

Although the results are promising, Seymour warns that the effects are temporary and require a large, expensive piece of medical equipment to be effective. However, he believes that a treatment based on their technique could be available within five to ten years.

This article was adapted from information provided by the University of Cambridge.

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