Humanity’s greatest invention
Reading supplies our brains with an external hard drive and gives us access to our species’s past: In the words of Francisco de Quevedo, it enables us “to listen to the dead with our eyes.”
But how, in such a short time, did the human species evolve this unique skill, one that requires the brain to decode written words visually and process their sounds and sense rapidly? In this fascinating and scholarly book, French neuroscientist Stanislas Dehaene explains what scientists now know about how the human brain performs the feat of reading, and what made this astonishing cultural invention biologically possible.
Amazingly, most children become proficient readers during elementary school (although learning to read Italian is easier, and learning to read Chinese harder, than learning to read English). In recent years, new imaging techniques have allowed researchers to watch normal brains in the act of reading, and studies have shed light on why the brains of dyslexic children, as well as those of certain stroke victims, fail to process written words successfully.
“Only a stroke of good fortune allowed us to read,” Dehaene writes near the end of his tour of the reading brain. Experiments in monkeys show that, within this area, individual nerve cells are dedicated to respond to a specific visual stimulus: a face, a chair, a vertical line. Research suggests that, in humans, a corresponding area evolved to become what Dehaene calls the “letterbox,” responsible for processing incoming written words. Located in the brain’s left hemisphere near the junction of the temporal and occipital lobes, the letterbox performs identical tasks in readers of all languages and scripts. Like a switchboard, it transmits signals to multiple regions concerned with words’ sound and meaning — for example, to areas that respond to noun categories (people, animals, vegetables), to parts of the motor cortex that respond to action verbs (”kiss,” “kick”), even to cells in the brain’s associative cortex that home in on very specific stimuli. Children learn reading in a stepwise process: first, awareness that words are made up of phonemes or speech sounds (ba, da); then the discovery that there’s a correspondence between these speech sounds and pairs or groups of letters.
Later the child begins to recognize entire words, and after a few years, reading speed becomes independent of word length. Dehaene deplores the whole-language approach to teaching reading in which beginning readers are presented with entire words or phrases in the hope of fostering earlier comprehension of text. He cites research showing that children who first learn which sounds are represented by which letters, and how pairs or groups of letters correspond to speech sounds, make steadier progress and achieve better reading scores than those taught using the whole-language method. For example, in preparation for learning to read, young Montessori students are often asked to trace with their fingers the shapes of large letters cut out of sandpaper. Between 5 percent and 17 percent of U.S. children suffer from dyslexia, or severe difficulty in reading.
Several susceptibility genes have been identified, most of them influencing the migration of nerve cells within the developing brain of the fetus. Research suggests that, even as infants, many dyslexic children have trouble hearing the difference between similar-sounding consonants such as b and d; but about one in four dyslexics has primarily visual difficulties with word-processing. Although there is no prospect of a cure for dyslexia, Dehaene points to promising results with various intervention strategies aimed at strengthening awareness of speech sounds and letter differences.
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