'Phantom Limbs' May Not Be What We Thought, Surprising Study Finds

Inside every human brain sits a finely detailed map of the body, with distinct patches devoted to different parts - the hands, lips, feet and beyond. But when someone loses a limb, what becomes of that internal map?

For many decades, researchers assumed that amputation triggers a dramatic reshuffle of the brain’s body map, so that neighbouring body parts gradually occupy the territory that used to represent the missing limb.

That proposed large-scale remapping became a cornerstone of what neuroscientists describe as adult brain plasticity: the capacity of the adult brain to alter its structure and function in response to injury, fresh experience or training.

Our new study, published in Nature Neuroscience, indicates the reverse: the brain’s body map stays strikingly stable, even many years after an amputation. To examine what changes (and what does not) after the loss of a body part, we used an unusual study design.

In collaboration with NHS surgeons, we tracked three adult patients who were due to have life-saving arm amputations for clinical reasons, including cancer or severe disruption to blood supply. Using functional magnetic resonance imaging (MRI), we scanned their brains before surgery and then repeatedly afterwards - in some instances for up to five years.

During the MRI sessions, participants were asked to move various body parts: tapping individual fingers, curling toes, or pursing lips. These tasks enabled us to chart brain activity and build a detailed picture of each person’s body map.

Following the operation, we repeated the scans - but this time we asked them to move their missing (phantom) fingers. Phantom movements are not make-believe: most amputees continue to experience vivid sensations in a limb that is no longer physically present. This gave us a rare chance to compare, within the same person, the brain’s hand map before and after amputation.

Across all three patients, we found that the hand map in the brain remained largely unchanged and was not taken over by representations of other body parts, such as the face. This neural stability helps to explain why so many amputees continue to sense their missing limb so clearly.

For many amputees, however, phantom sensations are far from neutral; they can be painful and are commonly described as burning, stabbing or itching. For years, the prevailing account of this pain drew on the idea that the brain’s body map had reorganised. That view, in turn, helped motivate treatments such as mirror box therapy, virtual reality training, and sensory-discrimination exercises - approaches intended to correct maps thought to have become disrupted.

Our findings suggest the brain’s body map is not broken. This may help clarify why these therapies repeatedly fail to beat placebo treatments in clinical trials. If the map is preserved, attempts to “repair” it are unlikely to address the true source of the problem.

The real culprit behind the brain’s body map after amputation

Instead, our results point to other mechanisms - for example, changes in the nerves severed during surgery. Cut nerves can develop tangled bundles that send abnormal signals back towards the brain. New surgical approaches for amputation are being developed with the aim of preserving nerve signalling and keeping stable connections with the brain.

These findings also matter for the future design of prosthetic limbs and brain-computer interfaces. Invasive, next-generation brain-computer interfaces may be able to access the preserved map of the amputated body part, decoding the movement a person is trying to make - or even applying electrical stimulation to that map so that amputees can experience sensations in the missing limb.

Such technologies are still under development, but they could one day provide more natural, intuitive control of a prosthetic limb - and more lifelike sensations - by making use of the brain’s retained body map.

Overall, our results indicate that the brain holds a robust internal model of the body, maintaining these representations even when sensory input disappears. For amputees, this means the missing limb persists in the brain - sometimes as a source of distress, but also as a foundation that future technologies may be able to harness.

Malgorzata Szymanska, PhD Candidate, Cognition and Brain Science, University of Cambridge and Hunter Schone, Postdoctoral Research Fellow, University of Pittsburgh

This article is republished from The Conversation under a Creative Commons licence. Read the original article.