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Credit: Pixabay/CC0 Public domain
Credit: Pixabay/CC0 Public domain
Contrary to popular belief, the brain does not have the capacity to rewire itself to compensate for loss of vision, amputation or stroke, for example, say scientists from the University of Cambridge and Johns University. Hopkins.
Write in eLife, Professors Tamar Makin (Cambridge) and John Krakauer (Johns Hopkins) argue that the idea that the brain, in response to injury or deficit, can reorganize itself and reassign particular regions to new functions, is fundamentally erroneous, although it is commonly cited in scientific publications. manuals. Instead, they argue that what is happening is simply the brain being trained to use already existing, but latent, abilities.
One of the most common examples is when a person loses their sight – or is born blind – and the visual cortex, previously specialized for processing vision, is rewired to process sounds, allowing the individual to use a form of “echolocation” to navigate. a cluttered room. Another common example is people who have suffered a stroke and are initially unable to move their limbs and re-use other areas of the brain to allow them to regain control.
Krakauer, director of the Center for the Study of Motor Learning and Brain Repair at Johns Hopkins University, said: “The idea that our brains have an incredible ability to rewire and reorganize itself is appealing. It gives us hope and fascination, in particular. when we hear extraordinary stories of blind individuals developing almost superhuman echolocation abilities, for example, or of stroke survivors miraculously regaining motor skills they thought they had lost.
“This idea goes beyond simple adaptation, or plasticity: it involves a global repurposing of brain regions. But while these stories may be true, the explanation for what is happening is, in fact, false.”
In their article, Makin and Krakauer review 10 seminal studies purporting to demonstrate the brain’s ability to reorganize itself. They argue, however, that while studies do show the brain’s ability to adapt to change, it does not create new functions in previously unrelated areas, but rather uses latent abilities present since birth.
For example, one of the studies – research conducted in the 1980s by Professor Michael Merzenich of the University of California, San Francisco – examined what happens when a hand loses a finger.
The hand has a particular representation in the brain, with each finger appearing to correspond to a specific brain region. Remove the index finger and the area of the brain previously allocated to that finger is reassigned to processing signals from neighboring fingers, Merzenich explained. In other words, the brain has rewired itself in response to changes in sensory input.
That’s not the case, says Makin, whose own research provides an alternative explanation.
In a study published in 2022, Makin used a nerve blocker to temporarily mimic the effect of index finger amputation in his subjects. It showed that even before amputation, signals from neighboring fingers were mapped to the region of the brain “responsible” for the index finger. In other words, while this region of the brain may have been primarily responsible for processing signals from the index finger, it was not exclusively the case. All that happens after amputation is that existing signals from the other fingers are “compounded” into that region of the brain.
Makin, from the Medical Research Council (MRC) Cognition and Brain Sciences Unit at the University of Cambridge, said: “The brain’s ability to adapt to injury is not about commandeering new brain regions for completely different purposes. »
“These regions don’t begin to process entirely new types of information. Information about the other fingers was available in the brain area examined even before the amputation. It’s just that in the original studies, the researchers didn’t didn’t pay much attention, because it was weaker than for the finger about to be amputated.
Another compelling counterexample to the reorganization argument comes from a study of congenitally deaf cats, whose auditory cortex – the area of the brain that processes sound – appears to be repurposed to process vision. But when fitted with a cochlear implant, this brain region immediately begins processing sound again, suggesting that the brain has not in fact been rewired.
Reviewing other studies, Makin and Krakauer found no compelling evidence that the visual cortex of individuals born blind or the uninjured cortex of stroke survivors ever developed a new functional capacity that did not otherwise exist.
Makin and Krakauer don’t rule out stories of blind people being able to navigate based on their hearing alone, or people who have suffered strokes regaining motor functions, for example. Rather, they argue that rather than completely reassigning regions to new tasks, the brain improves or modifies its pre-existing architecture through repetition and learning.
Understanding the true nature and limits of brain plasticity is crucial, both for setting realistic expectations for patients and for guiding clinicians in their rehabilitation approaches, they say.
Makin added: “This learning process speaks to the brain’s remarkable, but limited, capacity for plasticity. Reflection rather than reality. It is a slow, gradual journey, requiring persistent effort and practice. Recognizing this helps us appreciate the hard work behind each recovery story and adapt our strategies accordingly.
“So many times the brain’s ability to rewire itself has been described as ‘miraculous,’ but we are scientists and we don’t believe in magic. These amazing behaviors we observe are rooted in hard work, repetition and training, not in the magical reallocation of brain resources.
Tamar R Makin et al, Against cortical reorganization, eLife (2023). DOI: 10.7554/eLife.84716
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