Inside every human brain lies a detailed map of the body, with different regions dedicated to different body parts – the hands, lips, feet and more. But what happens to this map when a body part is removed?
For decades, scientists believed that when a body part is amputated, the brain’s body map dramatically reorganises itself, with neighbouring body parts taking over the area once represented by the missing limb.
This idea of large-scale brain reorganisation became a central pillar of what neuroscientists call adult brain plasticity: the ability of the brain to change its structure and function in response to injuries, new experiences or training.
Our new study, published in Nature Neuroscience, shows the opposite is true: the brain’s body map remains strikingly stable, even years after amputation.
To test what happens in the brain after a person loses a body part, we took a unique approach.
Working with NHS surgeons, we followed three adult patients who were preparing to undergo lifesaving arm amputations for medical reasons, such as cancer or severe problems with blood supply. We scanned their brains with functional magnetic resonance imaging (MRI) before the amputation and repeatedly afterwards – in some cases for as long as five years.
During the MRI scans, we asked patients to move different body parts: tapping their individual fingers, curling their toes or pursing their lips. This allowed us to map brain activity and to construct the brain’s body map.
After the surgery, we repeated the scans, this time asking them to move their missing (phantom) fingers. Phantom movements are not imaginary: most amputees continue to feel vivid sensations of their missing limbs, even though they are physically no longer there. Doing so gave us a rare opportunity to directly compare the brain’s hand map before and after amputation in the same person.
We discovered that, across all three patients, the map of the hand in the brain remained remarkably unchanged and did not get overwritten by other body parts, such as the face. This neural stability helps explain why so many amputees continue to feel their missing limbs so vividly.
For most amputees, however, phantom sensations are not neutral sensations; they are painful and described as burning, stabbing or itching. For years, the dominant explanation for these painful sensations came from the idea that the brain’s body map has reorganised itself. In turn, this theory inspired therapies such as mirror box therapy, virtual reality training, or sensory-discrimination exercises, all aimed at fixing supposedly broken maps.
Our findings show the brain’s body map is not broken. This helps explain why these therapies consistently fail to outperform placebo treatments in clinical trials. If the map remains intact, trying to fix it is a dead end.
The real culprit
Instead, our results suggest we should look elsewhere, for example, in the nerves that are cut during surgery. Severed nerves can form tangled clusters that misfire signals back to the brain. New amputation surgical techniques are being developed to preserve nerve signalling and maintain stable connections to the brain.
Our findings have important implications for developing prosthetic limbs and brain-computer interfaces. Invasive next-generation brain-computer interfaces can tap directly into the preserved map of the amputated body part to decode what movements are being attempted or even deliver electrical stimulation to the map to enable amputees to feel their missing limb.
These technologies are in development and could, one day, restore natural and intuitive control and sensations of a prosthetic limb, by using the preserved body map.
Our results show that our brains have a resilient model of the body that maintains the representations, even when the sensory input is lost. For amputees, this means that the missing limb lives on in the brain, sometimes as a source of discomfort, but also as a resource for future technologies to use.
This article is republished from The Conversation under a Creative Commons license. Read the original article.