They still say they feel hot or cold, tingling, sometimes severe pain, or even feel like they can continue to move... even though they have lost a limb. This phenomenon of " phantom limb", reported by people who have undergone amputation, has been the subject of much research, sometimes contradictory. This time, a new study published in Nature Neuroscience closes the debate. It shows that in the brains of amputees, the " mind map » of their bodies remains unchanged. The areas that controlled the now-lost limb are still there and do not fade over time. A discovery that paves the way for better pain management and the design of more effective prosthetics.
A " mind map » intact after more than 20 years
In our brain there is a kind of " plan » or « map » of our entire body. It is nestled in the somatosensory cortex, the region of the brain that processes all sensory information: touch, pain, temperature, and even body position. When we touch something hot with our hand, one region activates; if we bump our elbow, another will. Previous research assumed that after an accident or amputation, the brain rearranges this body map by removing the area attributed to the lost limb and compensating with other areas to cushion the loss.
Read alsoPhantom limb pain deciphered by brain imaging
Yet many amputees report phantom limb sensations. On imaging, individuals asked to move missing fingers showed distinct brain activity for each of them. To dig deeper, a team from the University of Cambridge performed brain MRI scans on volunteers preparing to undergo hand amputation. The first scan, the baseline scan, was taken before the amputation, when the participants could still move all their fingers. Then, the scans continued at different intervals: once just after the amputation, then three months later, then six months, and so on up to a year and a half after losing their limbs. And the results were surprising.
Over the months, the brain region activated almost identically, without any noticeable decrease. To drive the point home, they compared these MRIs with those of 26 participants who had lost their limbs an average of 23.5 years earlier. Once again, the images had not changed. The brain's representation of the hand remains remarkably stable and preserved, even years after the limb was amputated. The brain therefore does not erase the memory of the missing limb. It maintains its presence.", tells Science and Future Dr. Hunter Schone, a neuroplasticity specialist at the University of Pittsburgh, who said, " blown away » to discover the results of his study.
Brain activity related to the hand (in red) remains the same before and after amputation. For comparison, activity in the mouth (in blue) also remains similar. Photo credit: Tamar Makin / Hunter Schone
Making life easier for amputees
This discovery could make life easier for amputees in many ways. Starting with therapies to treat phantom limb pain, the current approach is to try to restore the brain's representation of the limb. There are probably better approaches to consider. One would have to start by understanding where phantom pain comes from, perhaps from changes in the nerves at the amputation site or perhaps in the spinal cord.“, explains the researcher.
Furthermore, this " mind map » of the body well preserved in the brain opens the way to more advanced work on prosthetics. They assume that the brain could control them with all the more ease if it is « remembers » with precision of the initial limb. A big advantage for the use of neuroprostheses, with a brain-machine interface. The brain could produce control signals for limb prostheses that require very fine muscle contractions. But decoding these output signals remains a major challenge.", Dr. Schone ponders. For now, the team wants to tackle the central question that emerged from this work: why does the brain maintain an intact representation of the lost limb over the years? By finding these answers, they hope to one day enable people who have lost a hand to regain real sensations: textures, posture, and even temperature.
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