For years, neuroscientists have operated under the assumption that the brain undergoes critically important reorganization following the amputation of a body part. However, a recent study challenges this long-held belief, suggesting that the brain’s map of the body remains surprisingly stable even after limb loss.

The study, which examined brain activity in three individuals before and after hand amputation, revealed that brain images were remarkably similar both pre- and post-surgery. This finding suggests that the brain’s representation of the body is largely preserved, even after a significant physical change. The research, published in Nature neuroscience, has potential implications for the development of advanced prosthetics and the treatment of phantom limb pain, according to the researchers.

“It solves one of the main problems with these kinds of patients, namely the fact that normally you don’t know how they are before amputation,” said Eraldo Paulesu, a neurologist at the University of milano-Bicocca, commenting on the study’s innovative approach. “So the longitudinal study in this particular case addresses a essential methodological problem.”

The somatosensory cortex, responsible for processing sensory data, contains a detailed map of the body. Previous studies on monkeys suggested that this map dramatically reorganizes itself after limb loss, leading to the widespread belief that the brain undergoes significant changes following amputation.

According to Tamar Makin,senior author of the study and a cognitive neuroscientist at the University of Cambridge,researchers previously believed that neighboring brain regions would encroach upon and repurpose the area of the map representing the lost limb.However, this theory has been challenged by the persistent sensations, including pain, experienced by amputees in their missing limbs, a phenomenon known as phantom limb pain. This new study offers a potential resolution to this contradiction and may pave the way for new treatment strategies.

Stable Brain Maps and Prosthetic Development

The research team recruited three patients scheduled for hand amputations. Using MRI scans, they monitored brain activity as participants moved, or attempted to move, their fingers and lips, both before and after the surgery. The hand and lip regions were examined as they are located close to each other in the brain map. The conventional theory suggested that hand amputation would cause the lip region to expand into the area previously occupied by the fingers, altering both maps.

However, the MRI scans revealed a different picture. The brain maps for the hand and lip regions remained consistent before and after limb loss. Post-amputation images were taken three and six months after surgery for all patients. One participant was scanned again at 18 months, and another at five years, with all images showing the same stable result.

“The body map in the brain is highly preserved in the sensory cortex, or somatosensory cortex, despite a very drastic change to the sensory input that’s going back to the brain,” explained lead author Hunter Schone, a postdoctoral research fellow at the University of Pittsburgh.

Jacob George, director of the Utah NeuroRobotics Lab, who was not involved in the study, believes the research has significant implications for the field of prosthetics.The stability of the brain map after amputation suggests that individuals retain the concept of the missing limb, making them suitable candidates for neural prosthetics or brain-computer interfaces, even long after the amputation. These systems translate brain activity into commands for external devices.

“The body map in the brain is highly preserved… despite a very drastic change to the sensory input.”

Implications for Phantom Limb Pain Treatment

Hunter Schone suggests that these findings could also reshape the treatment of phantom limb pain. Current therapies are frequently enough based on the idea of reversing brain map reorganization after amputation. Such as, some patients use mirror therapy to trick the brain into seeing the missing hand moving.

“These therapies continue to perform the same as placebos,which tells us that we’re not actually targeting the mechanistic target of phantom limb pain,which aligns with what our data shows,” Schone said. He proposes that the underlying cause of the condition might potentially be related to nerve structures outside the brain, rather than changes in higher cortical structures.

Rory Cooper, director of the Human Engineering Research Laboratories at the University of Pittsburgh, noted that the study’s small sample size of three participants is a limitation. To address this, the researchers analyzed data from a larger group of upper-limb amputees who had lost their limbs an average of two decades prior. According to Tamar Makin, there were no significant differences in brain activity between this group and the three study participants.

Rory Cooper suggests that future research should investigate whether these findings apply to individuals with lower-extremity amputations.Jacob George also proposes exploring the impact of amputation at an earlier age,noting that individuals born without limbs never develop a hand image in their brain map. He suggests further research to determine the point at which the brain loses the plasticity needed to fundamentally change after limb loss.

Jacob George believes that this study “will be a key piece of the clinical documentation as we move towards what I would refer to as neural prosthetic type systems.” He added that “this could ultimately change the way that we think about delivering therapy, and it could also change the way that we’re prescribing these prostheses.”