A rare circulatory problem required Emily Wheldon to have her left arm amputated three years ago. Her brain still thinks it's there.
"Most days, it just feels like I've got my arm next to me," she says.
The perception is so compelling that Wheldon had to train herself not to rely on the missing limb.
"When I first had the amputation," she says, "I was trying to put my arm out to stop myself from falling."
Now, a study of Wheldon and two other people with arm amputations may help explain why they are living with phantom limbs.
Brain scans showed that in all three, "the phantom hand representation is exactly similar to the pre-amputated hand representation," even five years after surgery, says Hunter Schone, a postdoctoral associate at the University of Pittsburgh who started the project as a doctoral student at University College London.
The finding, which appears in the journal Nature Neuroscience, challenges decades-old research in monkeys and people suggesting that after losing sensory input from a limb, the brain dramatically reorganizes the areas linked to that limb.
"I'm not sure the [new] study really negates that research," says Dr. Krish Sathian, the chair of neuroscience at Penn State Health, who was not involved in the research. "But the plot thickens, which is always the case in science."
Sathian and Schone agree the finding bodes well for people who rely on a surgically implanted brain-computer interface to control a prosthetic or robotic limb. The interface depends on the brain maintaining for many years the circuits once used to move an arm, hand, or leg.
A brain map of the body
The news study involved three people who knew they were going to have an amputation because of cancer or some other disease.
Researchers performed MRI scans before and after the amputation to look for changes in the somatosensory cortex, an area of the brain that maintains a detailed map of the body.
"When you touch something with your hand, a certain region's activated," Schone says. "If you feel something with your toes, a different region is activated."
Before the amputation, participants in the scanner would move their fingers, allowing scientists to see which brain regions responded. Up to five years after the amputation, participants imagined moving their missing fingers.
Earlier studies had suggested that after the loss of a hand, the brain would shift the borders of its body map. The area responding to the missing hand would shrink, while the neighboring area linked to the lips would expand.
But that's not what the team found.
"There's no evidence that the map of the lips is changing," Schone says, "which goes completely against all of those old studies that suggest if you lose this body part, this region of the brain is going to completely reorganize."
Earlier studies were limited because they compared the brains of people who'd already lost a limb with the brains of typical people. The new study appears to be the first to look at the same person's brain before and after an amputation.
Prosthetic arms and phantom limb pain
Like many people who've had an amputation, Wheldon often feels pain in her phantom arm and hand.
"It's like a throbbing pain that becomes quite unbearable at times," she says. Sometimes it feels like her wrist is sore, other times it's like her fingers are cramping.
Previous research suggested phantom limb pain was the result of changes in the brain's body map. But the new study suggests it occurs because the map hasn't changed, and the brain is still expecting signals from the missing body part.
"Imagine if you had a nerve that was receiving a highly detailed information for the body and suddenly now it's receiving some strange, atypical input," Schone says. "How the brain would deal with something like this?"
It might interpret the input as pain, he says.
If so, he says, the solution may lie in finding a new home for a nerve ending, rather than just leaving it exposed.
An unchanging body map could be a huge boost for the emerging field of brain-computer interfaces, which can allow a paralyzed person to speak or move a robotic arm.
Many of these interfaces place electrodes in the same area of the brain that maintains the body map. So they depend on that map remaining constant over many years.
The new evidence for this "offers a lot of hope for patients with neurological conditions," Sathian says.
Emily Wheldon isn't seeking a brain-computer interface that could control a prosthetic left arm.
But she says it's helpful just to have a scientific explanation of why her missing limb still seems like it's attached, and sometimes hurts.
"A lot of people don't realize that you can actually still feel the limb," she says, "and they're shocked when I say I'm suffering from phantom pain."
Wheldon has been able to control that pain with electrical stimulation and a therapy that uses a visual representation of the missing limb. And she says the phantom pain is much less severe than the pain she felt when her arm was still there.
"Back then, the pain was so intense I couldn't look after my newborn daughter," she says. Now she's back at work and able to help her kids get dressed and ready for school.
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