The extraordinary capacity for the brain to rewire itself after a stroke, an amputation, or sudden loss of vision or hearing has been shown repeatedly in studies over decades. At least, that's what we all thought.
Now, writing in eLife, two neuroscientists – Tamar Makin and John Krakauer – argue that the most influential experiments in this field don't conclusively show that the brain can functionally reorganize itself.
"The idea that our brain has an amazing ability to rewire and reorganize itself is an appealing one. It gives us hope and fascination, especially when we hear extraordinary stories," says Krakauer from Johns Hopkins University.
"This idea goes beyond simple adaptation, or plasticity – it implies a wholesale repurposing of brain regions. But while these stories may well be true, the explanation of what is happening is, in fact, wrong."
In their view, none of the key studies meet the strictest definition for cognitive reorganization, where a part of the brain usually dedicated to one type of computation becomes capable of an entirely different type of cognition, marked by a change in function or behavior.
"We conclude that none of the canonical studies we reviewed convincingly fulfill these criteria," they write.
Makin is a professor of cognitive neuroscience at the University of Cambridge and her research focuses on the limits of neuroplasticity in differently abled adults, such as people with prosthetic limbs.
Together, Makin and Krakauer – who has an interest in stroke rehabilitation – have seen first-hand the "surprising and impressive behavioral changes" that can be made following "neurological insults, such as congenital blindness, deafness, amputation, and stroke".
A striking example of apparent cognitive rewiring comes from a study on newborn ferrets published in 2000.
In this experiment, the neural inputs from the ferrets' eyes were surgically connected to the auditory cortex of the brain instead of the visual cortex. Despite this mix-up, the ferrets had some vision in a follow-up study. The auditory neurons had reorganized themselves to perform a new function.
"But is this true reorganization…?" ask Makin and Krakauer. The type of processing done in the visual cortex might be just like that done by the auditory cortex, meaning that this surgical rewiring isn't really challenging the brain to change its functions.
If the same input were to be delivered to a part of the brain responsible for completely different processes, such as the prefrontal cortex, the results might be far less impressive.
When a study participant miraculously recovers cognitive functions that were thought to be lost due to injury or impairment, it's likely that the brain is adding computational capacity by leaning on neural connections or functions that previously existed but were very quiet or under-utilized, the authors argue.
For instance, when a mouse is still able to move a whisker even after the nerves connecting that particular whisker have been severed, it's likely that nerves on neighboring whiskers were always tuned into the damaged whisker, the authors argue. No rewiring necessary!
Similarly, when newborn kittens had one eye temporarily stitched shut, this strengthened the active eye and weakened the unavailable eye, resulting in some "very clumsy kittens" once the second eye was opened.
If it looks like part of the brain is doing something it never did before, that could just be an illusion produced by the fact that we didn't know the brain had that additional capability in the first place, the authors propose.
The researchers also doubt that the brain 'takes over' neurons that aren't being used and 'rewires' them to perform other functions.
For example, children with congenital cataracts (who are born blind) can have their sight immediately restored following surgery.
"If the visual cortex is re-appropriated to support new functions, then it follows that restoration of visual input will be futile (or will at least require substantial reversal of reorganization)," the authors write.
"But this is not the case. Not only are the children immediately able to perceive some visual information, they show susceptibility to visual illusions."
While the brain is more interconnected and "fuzzy" than we give it credit for, Makin and Krakauer argue different parts of the brain are destined to perform certain functions, and it's not possible to deviate from this underlying "architecture" or "blueprint", even in early development.
"So many times, the brain's ability to rewire has been described as 'miraculous' – but we're scientists, we don't believe in magic," says Makin.
"These amazing behaviors that we see are rooted in hard work, repetition and training, not the magical reassignment of the brain's resources."
This paper was published in eLife.