The brain holds space for a missing limb, even years after it’s gone.
For three women who underwent planned hand amputations, brain scans revealed remarkably durable maps of hand areas, lasting for five years in one case. The results, published August 21 in Nature Neuroscience, counter the long-held idea that the adult brain remakes itself in prominent ways after a change to the body, such as an amputation.
Earlier research, much of it on rodents and nonhuman primates, suggested that after a limb was gone, the brain’s real estate shifts. It was thought that the brain area that used to receive input from a missing hand would be taken over by neighboring areas, particularly those corresponding to the face and lips.
But that idea doesn’t fit with a common phenomenon: People often retain vivid sensations of their missing limb, including what’s known as phantom pain, says Hunter Schone, a neuroscientist at the University of Pittsburgh. “These two ideas of ‘The brain is completely changing,’ and the amputee experience, saying, ‘I can still feel the limb,’ are very much in conflict with each other.”
Schone and his colleagues set out to see how the brain adjusts — or doesn’t — after amputation. The study depended on three women facing planned hand amputations, each of whom agreed to functional MRI brain scans before and after their surgeries. “They offered their time at one of the most difficult periods of their lives,” Schone says.
During the scans, the women moved their fingers one at a time, curled their toes and pursed their lips. These small movements triggered changes in blood flow in the brain that marked activity in the primary somatosensory cortex, a neural strip that runs up each side of the brain, from just above the ear to the top of the head. This area handles signals from the body, including touch, pain and spatial position.
The neural real estate there remained unchanged after amputation, follow-up brain scans showed. When the women attempted to move their missing fingers one at a time, the same brain activity patterns showed up as before, even though the fingers weren’t there. And pursing their lips didn’t lead to any new activity in the hand area, suggesting that the lips didn’t expand into this territory.
The hand maps were sturdy. When one woman was scanned five years after surgery, her hand map showed no overt differences. Those results fit with scans of 26 other amputees that the team analyzed. “There is really strong, consistent evidence in pretty much every amputee I scan that there is persistent representation of their missing hand,” says Tamar Makin, a cognitive neuroscientist at the University of Cambridge.
By comparing the three women’s brains before and after surgery, the study design “is a powerful way to say, at the macroscopic, global scale, we can’t really detect the strong reorganization,” says Dan Feldman, a neuroscientist at the University of California, Berkeley, who was not involved in the new research.
However, he cautions, “that doesn’t mean [reorganization] is not there at the local scale and may be doing important things.” But on bigger scales, such as those detected by fMRI, “maybe the maps are not changing.”
The finding may have implications for the design of better prosthetics and treatments for phantom limb pain, which is common and sometimes debilitating for people who have lost limbs. Some approaches aim to stop or shift purported brain rewiring, but Schone says they’re based on flawed reasoning. “To me, our study says we need to stop chasing or trying to fix broken brain maps that aren’t actually broken.”