Is There A Limit to Brain Plasticity

Posted by

Dov Michaeli

First published 3/3/11 on The Doctor Weighs In

Stevie Wonder attended this year’s NBA all-star game. He was accompanied by someone who described the action to him. You start wondering: Stevie was blind since birth, so obviously he has never seen a basketball court, or a basketball, or a basketball player. Did he visualize what was going on? If so, did it have any resemblance to the real game, or was it a totally idiosyncratic, sui generis game? What was his visual cortex “doing” when he “watched” the game? Tantalizing questions, leading to tantalizing experiments.

The case of the seeing blind ferrets

An April 20, 2000, article in Nature titled “Induction of Visual Orientation Modules in Auditory Cortex” by MIT scientists. What Dr. Sur and her colleagues did was astounding in its simplicity and audacity: they cut the nerves leading from ferret retinas and rerouted them to their respective auditory complexes. What was expected did not happen: the ferrets did not go blind, they retained their vision. What happened? The auditory cortex of the ferrets reorganized itself along the same lines as the visual cortex. This was a profound observation; it meant that experience can determine shape of the brain. It was just a question of time before other experiments would probe the limits of brain plasticity.

What about us, humans?

A paper in the Proceedings of the National Academy of sciences (1 March, 2011) by Marina Bedny and her team of MIT and Harvard asked a “simple” question: how do people who are congenitally blind process language? In sighted people we know that there are connections between the centers that process language (they are located in the left frontal and temporal lobes, and are called Brocca’s area) and the visual cortex. Each one of us experienced these connections when we read a vivid description of a place or person and had a “mental picture” of it. In fact, even the auditory centers are connected to the visual cortex. I find myself frequently imagining what an unknown person on the other side of a phone conversation would look like, with details of height, weight, facial features etc, judging by the timbre, cadence and accent of speech. Wouldn’t you associate a deeply resonant bass with a six foot six broad-shouldered hulk or with a five foot pimpled computer nerd? In fact, I suspect that the visual cortex is in direct communication with numerous centers in the brain. Remember Einstein’s story how he came up with the theory of relativity? He had a mental image of himself riding on the nose of a rocket, which led him to ponder the relationship between time and space. But all these suggest extensive connectedness between brain centers. Can one center assume the function of another?

Back to the PNAS paper. Bedny’s team examined the brains of congenitally blind people. They observed their brain activity, using fMRI, in response to various sentence comprehension tasks. Lo and behold, in addition to the expected activity in Brocca’s area, their visual cortex lit up as well. Imagine: cells specialized in sensing points of light switched their specialty and are now analyzing words and sentences. I find it mind boggling.

More questions than answers

Of course we will never know what Beethoven, who was stone deaf in his later years, “heard” (or see?) when he wrote his beautiful late piano sonatas, or the soaring ninth symphony. But this study raises questions for which an answer may exist. Is the visual cortex of sighting people equally activated when presented with language tasks? Is memory connected the visual cortex? Memory whizzes remember incredible amount of information by constructing a visual story (they call it a “memory palace”) of the information.

But the most profound question still to be studied is: how “hard” is the hard wire of the brain? What happens to Chomsky’s theory that syntax is hard-wired in our brain? It seems that the brain is not a collection of pre-determined hard-wired centers of tasks; it is the tasks that determine the centers.

There is another caveat that Jonah Lehrer point out in his blog “The Frontal Cortex”’:

“It’s also worth pointing out that these dramatic examples of structural plasticity come with a crucial caveat: the rewiring takes place in young children. Once we become adults, we exchange an endlessly malleable brain for one that can execute complex decisions and movements with ease. Although we might not be able to train our visual cortex to contemplate a novel, we can drive a car while daydreaming, or catch a baseball, or play chess. This is the tradeoff of maturity: we can exchange our innate flexibility for wisdom and habits”.

This may be true for our present state of knowledge, but we don’t know if there is not a mechanism hidden in the brain to allow it to revert to its “immature state” of almost infinite plasticity. A case in point: differentiated cells, be it heart muscles or brain neurons, were thought to be “terminal”, that is they had no way of reverting to their undifferentiating state. We now know that this is not the case; these tissue cells can be reverted, through cell biology techniques, to their undifferentiated state. We used to think that once a stroke destroyed a brain function, there was no way of regaining it. Modern neurobiology and physical therapy proved it wrong.

These “brave new world” experiments are bound to change our sense of ourselves and what we can accomplish. We are very lucky to be alive and witness this amazing biological revolution unfold.

Dov Michaeli MD PhD writes at the Doctor Weighs In.

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