Amputees Maintain Detailed Brain ‘Map’ of Amputated Limb

After having a limb amputated, amputees may experience phantom sensations—either a tingle or an itch coming from a limb that is no longer there—that are connected to the brain and the spinal cord. Now, researchers have revealed more insights into the phenomena: Activity in the brains of amputees showed detailed information about the hand they were missing, even though they had them amputated two to three decades before.

The study was published in the journal eLife. The group’s research prior to this study used an ultra-high-power MRI scanner and found that—though the amputees had less brain activity related to fingers—the amputees’ brains had patterns that made up the composition of the hand image, which ended up matching the specific patterns of activity in brains of two-handed participants. The scanner also showed that the hand picture in the cortex of the amputees was stable, even after two to three decades.

The new study, which highlights the previous research, sheds further light on how this brain activity of hand representation affects the phantom sensations that some amputees experience.

Using a brain-decoding technique, the researchers observed the pattern of brain activity of 18 amputees who had lost their hands over almost two decades ago, including groups of amputees that have varying phantom sensations—from feeling it consistently to hardly feeling it at all.

The team also observed 13 people who were born without a hand, but did not have an amputation, to understand whether the neural “fingerprints” require once having a hand. While under the MRI scanner, both groups were required to move the fingers of their missing and intact hands, and then the two were compared.

The brain activity of the amputees showed to have the strongest sensations of being able to move phantom fingers, and retained the clearest image of the missing hand in the brain. But another factor of the study was more surprising—amputees who did not experience phantom pain or sensations had similar brain activity. There was no evidence of symptoms that would indicate that their brain still harbored detailed information about the missing limb.

"We've shown that once the hand 'picture' in the brain is formed, it is generally unlikely to change, despite years of amputation and irrespective of the vividness of phantom sensations," said Tamar Makin, senior author, associate professor and Sir Henry Dale Fellow at the UCL Institute of Cognitive Neuroscience.

The group born with only one hand showed some brain activity of a neural “fingerprint,” but it was not similar to the two amputee groups.

Prosthetics that tap into the brain’s control center—neuroprosthetics—could benefit from this new research while developing neuroprosthetics for the group born without a hand could potentially be more challenging, based on what was revealed in the study.

"Our work suggests that daily life experience could shape the fine-grained aspects of hand representation, but that the large-scale functional organization of the hand area is fundamentally stable,” Makin said.

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