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In the late 1990s, a team of neuroscientists at MIT led by Mriganka Sur undertook an audacious experiment: they rewired the brain of a ferret, so that the information from its retina was plugged into its auditory cortex. The assumption was that the animal would be blinded, unable to make sense of all the incoming pixels. To Sur’s astonishment, however, the ferrets could still see. Furthermore, their auditory cortex now resembled the typical ferret visual cortex, complete with spatial maps and neurons tuned to detect certain slants of light. At the time, Michael Merzenich, a leading plasticity researcher at UCSF, called this experiment “The most compelling demonstration you could have that experience shapes the brain.” Our mental hardware wasn't hard at all.
A brand new paper in PNAS by a group of MIT and Harvard researchers (Marina Bedny, Alvaro Pascual-Leone, Rebecca Saxe, et.al.) extends this line of thinking to humans, demonstrating that people who are blind from birth are able to process language in their unused visual cortex. (The brain abhors wasted space.) What makes this discovery so interesting is that language has long been seen as a prime example of brain localization. Ever since the neurologist Paul Broca began studying Tan - a patient with a brain lesion unable to speak any words other than "Tan" - we've assumed that our linguistic skills depend on particular chunks of matter in the left frontal and temporal lobes. These are the folds of flesh that make words and sentences possible, which is why even the slightest bit of damage to these areas - say, after a stroke - can render people utterly languageless. We suddenly lose our most uniquely human talent.
This latest study involved studying the brain activity of congenitally blind individuals with fMRI in response to a variety of sentence comprehension tasks. While the scientists observed increases in activity in the classic language areas, they also observed a spike in blood flow in the visual cortex. Furthermore, this visual excitement exhibited "functional connectivity" with the other regions, suggesting that cells normally responsive to points of light were now plugged into our linguistic circuits, helping the blind subjects analyze phonological, lexical and even semantic information.
Needless to say, this strange and fascinating finding raises more questions than it answers. I'm most interested in whether or not this increased language processing activity leads to improved language comprehension. Perhaps blind people are better at difficult linguistic tasks, such as parsing complex sentences or reading while distracted? Maybe their active visual cortex makes them ideal literary critics, able to detect subplots and themes that escape the rest of us? The brain, as always, remains a machine of tradeoffs. Our losses are often shadowed by surprising gains, or what neurologists call "paradoxical functional facilitations."
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 exchange our innate flexibility for wisdom and habits.
