A new brain imaging study has found a part of the brain specifically attuned to the shape of written words. And unlike other similar areas, this one develops its abilities through learning and experience.
Over the course of evolution, certain parts of our brain have been specifically tuned to faces, human bodies and landscapes. These structures turn up in the same places in very different people. Their roles are so fundamental to the way we (and our ancestors) experience the world, that they have been long since hardwired into our genetic plans.
Words on the brain
But not everything we see is like this. Words, for example, are an exception. Even though reading and writing are such central parts of our lives now, they have only been around for a few millennia. And for most of that time, they were skills available only to a learned elite.
The entire history of writing is a mere blip in evolutionary time, certainly not long enough to evolve a specialised, genetically determined brain region dedicated to processing written words. Nonetheless, one such region exists.
Chris Baker and colleagues from the National Institute of Mental Health, Bethesda, have found that a small part of the brain specifically recognises written words. And unlike the areas that recognise faces and bodies, its origins lie in learning and experience.
Where words are recognised
Baker examined the brain activity of several English speakers using functional magnetic resonance imaging (fMRI), a technique that measures the flow of blood and oxygen in the brain. He found that a small region at the back of the brain – no bigger than a piece of sweet corn – responds strongly and specifically to English words.
Strings of consonants worked just as well but strings of numbers or Hebrew words (right), which use unfamiliar characters, triggered much weaker responses. And the region responded even more weakly to line drawings of common objects or Chinese characters, which obviously perform the same function as English words but are very different in appearance.
Baker gave the region the slightly unwieldy name of ‘candidate letter string-selective region’ or cLSSR for short. He had its location, but it was still unclear if its properties are innate of the product of experience. Indeed, the cLSSR lies very close to the fusiform gyrus, a part of the brain genetically programmed to recognise faces and numbers.
Things got interesting when Baker repeated his experiments in people who were fluent speakers and readers of both Hebrew and English. Their cLSSRs responded equally strongly to both English and Hebrew words. But in all other ways, they behaved identically to the cLSSRs of those who just spoke English.
Nature and nurture
These results provide powerful evidence that experience shapes the abilities of this part of the brain. It’s obvious that experience breeds familiarity. But this is the first time that someone has shown that a part of the brain becomes specifically attuned to a type of visual through experience and learning alone.
Obviously, a genetic influence on the cLSSR cannot be ruled out. After all, genes control the structure of the developing brain, and in different people, the cLSSR is consistently found in the same place. This is most often in the left hemisphere. If it is damaged in adults, the right hemisphere can’t pick up the slack, and people suffer form problems in reading.
All this suggest that this particular bundle of neurons may develop its taste for words, but it is somehow predisposed to do so. Baker speculates that the cLSSR’s lies along the route that nervous signals take from visual areas to language areas, and gradually learns from the signals it carries.
The discovery of the cLSSR is just the beginning, and the questions practically ask themselves. Is it more diffuse or less responsive in dyslexic children? And how does it grow and develop over time? More research and new methods even more accurate than fMRI will help to provide the answers.
Reference: Baker, Liu, Wald, Kwong, Benner & Kanwisher. 2007. Visual word processing and experiential origins of functional selectivity in human extrastriate cortex. PNAS 104: 9087-9092.
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