New research shows the brain’s map of the body remains unchanged after amputation

The brain holds a “map” of the body that remains unchanged even after a limb has been amputated, contrary to the prevailing view that it rearranges itself to compensate for the loss, according to new research from scientists in the UK and US.

The findings, published in Nature Neuroscience, have implications for the treatment of “phantom limb” pain, but also suggest that controlling robotic replacement limbs via neural interfaces may be more straightforward than previously thought.

Studies have previously shown that within an area of the brain known as the somatosensory cortex there exists a map of the body, with different regions corresponding to different body parts.

These maps are responsible for processing sensory information, such as touch, temperature and pain, as well as body position. For example, if you touch something hot with your hand, this will activate a particular region of the brain; if you stub your toe, a different region activates.

For decades now, the commonly-accepted view among neuroscientists has been that following amputation of a limb, neighboring regions rearrange and essentially take over the area previously assigned to the now missing limb. This has relied on evidence from studies carried out after amputation, without comparing activity in the brain maps beforehand.

But this has presented a conundrum. Most amputees report phantom sensations, a feeling that the limb is still in place—this can also lead to sensations such as itching or pain in the missing limb. Also, brain imaging studies where amputees have been asked to ‘move’ their missing fingers have shown brain patterns resembling those of able-bodied individuals.

To investigate this contradiction, a team led by Professor Tamar Makin from the University of Cambridge and Dr. Hunter Schone from the University of Pittsburgh followed three individuals due to undergo amputation of one of their hands.

This is the first time a study has looked at the hand and face maps of individuals both before and after amputation. Most of the work was carried out while Professor Makin and Dr. Schone were at UCL.

Prior to amputation, all three individuals were able to move all five digits of their hands. While lying in a functional magnetic resonance imaging (fMRI) scanner—which measures activity in the brain—the participants were asked to move their individual fingers and to purse their lips. The researchers used the brain scans to construct maps of the hand and lips for each individual. In these maps, the lips sit near to the hand.

The participants repeated the activity three months and again six months after amputation, this time asked to purse their lips and to imagine moving individual fingers. One participant was scanned again 18 months after amputation and a second participant five years after amputation.

The researchers examined the signals from the pre-amputation finger maps and compared them against the maps post-amputation. Analysis of the ‘before’ and ‘after’ images revealed a remarkable consistency: even with their hand now missing, the corresponding brain region activated in an almost identical manner.

Professor Makin, from the Medical Research Council Cognition and Brain Science Unit at the University of Cambridge, the study’s senior author, said, “Because of our previous work, we suspected that the brain maps would be largely unchanged, but the extent to which the map of the missing limb remained intact was jaw-dropping.

“Bearing in mind that the somatosensory cortex is responsible for interpreting what’s going on within the body, it seems astonishing that it doesn’t seem to know that the hand is no longer there.”

As previous studies had suggested that the body map reorganizes such that neighboring regions take over, the researchers looked at the region corresponding to the lips to see if it had moved or spread. They found that it remained unchanged and had not taken over the region representing the missing hand.

The study’s first author, Dr. Schone from the Department of Physical Medicine and Rehabilitation, University of Pittsburgh, said, “We didn’t see any signs of the reorganization that is supposed to happen according to the classical way of thinking. The brain maps remained static and unchanged.”

To complement their findings, the researchers compared their case studies with 26 participants who had their upper limbs amputated, on average, 23.5 years beforehand. These individuals showed similar brain representations of the hand and lips to those in their three case studies, suggesting long-term evidence for the stability of hand and lip representations despite amputation.

The researchers offer an explanation for the previous misunderstanding of what happens within the brain following amputation. They say that the boundaries within the brain maps are not clear cut—while the brain does have a map of the body, each part of the map doesn’t support one body part exclusively.

So, while inputs from the middle finger may largely activate one region, they also show some activity in the region representing the forefinger, for example.

Previous studies that argue for massive reorganization determined the layout of the maps by applying a “winner takes all” strategy—stimulating the remaining body parts and noting which area of the brain shows the most activity; because the missing limb is no longer there to be stimulated, activity from neighboring limbs has been misinterpreted as taking over.

The findings have implications for the treatment of phantom limb pain, a phenomenon that can plague amputees. Current approaches focus on trying to restore representation of the limb in the brain’s map, but randomized controlled trials to test this approach have shown limited success—today’s study suggests this is because these approaches are focused on the wrong problem.

Dr. Schone said, “The remaining parts of the nerves—still inside the residual limb—are no longer connected to their end-targets. They are dramatically cut off from the sensory receptors that have delivered them consistent signals. Without an end-target, the nerves can continue to grow to form a thickening of the nerve tissue and send noisy signals back to the brain.

“The most promising therapies involve rethinking how the amputation surgery is actually performed, for instance grafting the nerves into a new muscle or skin, so they have a new home to attach to.”

Of the three participants, one had substantial limb pain prior to amputation but received a complex procedure to graft the nerves to new muscle or skin; she no longer experiences pain. The other two participants, however, received the standard treatment and continue to experience phantom limb pain.

The University of Pittsburgh is one of a number of institutions that is researching whether movement and sensation can be restored to paralyzed limbs or whether amputated limbs might be replaced by artificial, robotic limbs controlled by a brain interface. This study suggests that because the brain maps are preserved, it should—in theory—be possible to restore movement to a paralyzed limb or for the brain to control a prosthetic.

Dr. Chris Baker from the Laboratory of Brain & Cognition, National Institutes of Mental Health, said, “If the brain rewired itself after amputation, these technologies would fail. If the area that had been responsible for controlling your hand was now responsible for your face, these implants just wouldn’t work. Our findings provide a real opportunity to develop these technologies now.”

Dr. Schone added, “Now that we’ve shown these maps are stable, brain-computer interface technologies can operate under the assumption that the body map remains consistent over time.

“This allows us to move into the next frontier: accessing finer details of the hand map—like distinguishing the tip of the finger from the base—and restoring the rich, qualitative aspects of sensation, such as texture, shape, and temperature. This study is a powerful reminder that even after limb loss, the brain holds onto the body, waiting for us to reconnect.”