Our brain is not reorganized after an amputation

Our brain may not be as capable of reclassifying after an amputation as we thought, which could have serious implications on how we treat a common complication called pain of ghosts.

Part of the brain called the Somatosensoriel cortex receives and processes sensory information through the body, such as touch and temperature. Some studies suggest that the zones of the cortex are mapped to different parts of the body, so a different area lights up if you burn your hand compared to your toe, for example.

It has also been suggested that the Somatosensory Cortex reorganizes in the case of a cut amputation or nerve. For example, in a study of macaques whose nerves of the arm had been cut, the neurons of the somatosensory cortex which normally respond to the stimulation of the hand were rather activated by touching the face. The researchers concluded that part of the zone of the cortex which responds to the affected hand had been reassigned to the face.

But for the first time, Tamar Makin at the University of Cambridge and his colleagues compared people’s brain activity before and after an amputation – and found that it does not really change.

The researchers used MRI to scan three people’s brains before their arms were amputated for medical reasons. During the scans, they were asked to purge their lips and type their fingers.

When the team repeated these three and six months after the amputation and asked the participants to try to move the fingers they no longer had, their brain signals remained the same. “As far as we can measure, they are identical,” explains Makin.

Two of the participants were also followed at 18 months and five years, respectively, after the amputation, without having shown a significant change to their brain signals before.

The researchers validated their results by first forming an AI model to recognize the preampted brain scans linked to the participants who moved to each finger. When they returned post-shooting and imagined stirring each finger in a random order, the model could identify from the cerebral activity which finger they tried to move, demonstrating that it remained constant.

In another part of the experience, the researchers measured the activity of the participants’ somatosensory cortex as they moved their lips and tried to move their fingers after the amputation. This was also done for 26 people whose weapons was amputated on average 23 years ago, and the researchers found that the activity was comparable.

“This study definitively confirms that this idea that the brain is capable of remapry, reclassify or reorganize – that the cortex can simply make a switcheroo – is incorrect,” John Krakauer told Johns Hopkins University in Maryland.

Researchers argue that the results could change the treatment of the pain of ghost limbs, a common condition among people who have suffered an amputation where they always perceive pain or discomfort in an arm or a leg that is no longer there.

Certain efforts to treat the condition consists in using visual clues, such as virtual reality, to encourage the brain to reorganize. This has mixed results, with any advantage possibly due to the placebo effect, explains Makin.

Instead, researchers say that the condition could possibly be avoided by greating the nerves in new tissues which are sometimes added during amputations. Otherwise, the remaining parts of the nerve which are cut off from their target can develop and cause the thickening of the nervous tissue, which can contribute to the ghost pain of the limbs.

“The theory of unsuitable plasticity of the pain of ghost limbs was based on the idea that the brain can reorganize in a way that it does not do it,” explains Krakauer. “In a sense, the way people think of treating ghost members will now change simply because the theory on which it was based is wrong.”