The truth behind phantom limbs has been revealed in a new study published in Nature Communications.
Missing limbs leave behind painful sensations— even when they are gone, one can still feel them, a phenomenon called phantom limb. This is the case for most people having gone through amputations: they will experience different kinds of sensations, most of which will be riddled with pain, in the body part that has been removed. Why do limbs that are absent make their ‘presence’ known, why the chronic pain? A team of researchers from the University of Cambridge working in collaboration with a group of Osaka University, Japan, have found the cause thereof: a ‘reorganisation’ in the brain.
“Even though the hand is gone, people with phantom limb pain still feel like there’s a hand there— it basically feels painful, like a burning or hypersensitive type of pain, and conventional painkillers are ineffective in treating it,” says neuroscientist Dr Ben Seymour, one of the authors of the study. “We wanted to see if we could come up with an engineering-based treatment as opposed to a drug-based treatment.”
The scientists discovered a cross-wiring in a region of the brain, the one linked with movement and sensation, called the sensorimotor cortex. They reached this conclusion after they investigated movement in 10 individuals with amputations or nerve injuries. The participants were to control a robotic arm with their brains. One of the tasks that they were to do was to associate the movement of a prosthetic with their missing limb: surprisingly, this caused the patients to experience increased pain. However, when the participants would associate the movement of the prosthetic with the other hand, their pain would be reduced. The pain is said to be the result of crossed wires in the brain. A ‘defect’ in these links (‘wires’) would entail a malfunction between movement and perception of that movement.
The researchers reached this interpretation when, using a brain-machine interface, they ‘read’ the neural activity involved in the mental actions of the patients to move their phantom limb. They then converted the decoded information into movement of the robotic neuroprosthetic 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,” says lead author, Takufumi Yanagisawa. “The movement part of the brain is working fine, but they are not getting sensory feedback— there’s a discrepancy there.”
The chronic pain characterising phantom limbs is currently without treatment. But, this finding will hopefully change this as the study authors are positive that the ‘defective’ wiring they have discovered can be fixed to prevent the disruption. Moreover, this new knowledge could also be used for the treatment of other conditions with chronic pain like arthritis.
“Ideally, we’d like to see something that people could have at home, or that they could incorporate with physio treatments,” says Dr Seymour. “But the results demonstrate that combining AI techniques with new technologies is a promising avenue for treating pain, and an important area for future UK-Japan research collaboration.”