How does the brain react to the loss of a limb?

The brain has a particular structural organization, divided into areas and segments, where each area performs a specific function. Thus, there are brain regions related to language, smell, sight, different limbs, and so on. This organization is called "brain map". However, for example, what happens to the area assigned to a hand if the body is amputated? In a recent study, researchers have shown that in such a case, the brain rearranges this map so that the area in question is reallocated to the remaining hand. These results confirm the exceptional capacity of the brain to adapt to changes.

Researchers at the University of Missouri have discovered new clues about the functioning of the complex neuronal structure of the human brain. Similar results have already been reported in animal studies, but this is one of the first studies where such a result has been observed in humans. The study was published in the journal NeuroImage.

"When a person touches something with their right hand, a specific" hand-allocated area "in the left side of the brain activates,  " says Scott Frey, a researcher in cognitive neuroscience. " A similar but opposite reaction occurs with the left hand. But when someone loses a hand, we discover that the two areas of the brain's hand - left and right - are dedicated to the remaining hand. This is a striking example of the functional reorganization or plasticity of the human brain ."

Loss of a limb: the brain reorganizes to compensate for amputation

Researchers used the functional brain MRI (brain imaging) MRI to scan the brain of 48 people, 19 of whom had lost a hand. They created a computer-controlled system to deliver a light feel to the hands and face. Functional MRIs are similar to conventional MRIs, but are sensitive to minute changes in blood oxygen levels in the brain that occur when areas of the brain process information.

Complete cerebral scans obtained by fMRI for three patients: (A) Hand-amputated patient, (B) Finger-only amputee patient, and (C) Control patient (unaffected). The colors indicate the intensity of the brain reorganization. Credits: Kenneth F.Valyear et al. 2019

Scientists have observed in scans that when the brain is deprived of information from a lost hand, it reorganizes its neural map and redirects these functions to the remaining hand. This discovery could help neurologists and health professionals better understand the mechanisms underlying plasticity of the brain when body trauma occurs, for example, when returning wounded veterans to the battlefield.

" We could think that the areas of the brain that treat the sensations of our body are organized as a map with separate territories dedicated to specific body regions such as hands, face or feet. We've known for a long time that injuries such as amputation or spinal cord injuries change the way this card is organized, "explains Frey.

" If you lose a hand, for example, the area of ​​the associated hand may be partially supported by the card's neighboring functions involved in the treatment of arm or facial sensations: it is a form of "Cerebral plasticity". This work demonstrates that such plasticity also occurs over large distances between the left and right hemispheres of the brain.

Additional work is underway to determine how, and if, these changes affect the way in which amputees experience sensations, including pain. Neuroscientists hope their discoveries could also help inform efforts to develop prosthetics that provide users with a tactile experience.


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