When you think about it, that anyone can read at all is something of a miracle. Reading requires your brain to rejigger its visual and speech processors in such a way that artificial markings, such as the letters on a piece of paper, become linked to the sounds they represent. It's not enough simply to hear and understand different words. Your brain has to pull them apart into their constituent sounds, or phonemes. When you see the written word cat, your brain must hear the sounds /k/ ... /a/ ... /t/ and associate the result with an animal that purrs.

Unlike speech, which any developmentally intact child will eventually pick up by imitating others who speak, reading must be actively taught. Linguists believe that the spoken word is 50,000 to 100,000 years old. But the written word has probably been around for no more than 5,000 years. "That's not long enough for our brains to evolve certain regions for just that purpose," says Guinevere Eden, a professor of pediatrics at Georgetown University in Washington, who also uses brain scans to study reading. "We're probably using a whole network of areas in the brain that were originally designed to do something slightly different." As Eden puts it, the brain is moonlighting — and some of the resulting glitches have yet to be ironed out.

To understand what sorts of glitches we're talking about, it helps to know a little about how the brain works. Researchers have long been aware that the two halves, or hemispheres, of the brain tend to specialize in different tasks. Although the division of labor is not absolute, the left side is particularly adept at processing language while the right is more attuned to analyzing spatial cues. The specialization doesn't stop there. Within each hemisphere, different regions of the brain break down various tasks even further. So reading a sonnet, catching a ball or recognizing a face requires the complex interaction of a number of different regions of the brain.

Most of what neuroscientists know about the brain has come from studying people who were undergoing brain surgery or had suffered brain damage. Clearly, this is not the most convenient way to learn about the brain. Even highly detailed pictures from the most advanced computer- enhanced X-ray imaging machines could reveal only the organ's basic anatomy, not how the various parts worked together. Researchers needed a scanner that didn't subject patients to radiation and that showed which parts of the brain are most active in healthy subjects as they perform various intellectual tasks. What was needed was a breakthrough in technology.

That breakthrough came in the 1990s with the development of a technique called functional magnetic resonance imaging (FMRI). Basically, FMRI allows researchers to see which parts of the brain are getting the most blood — and hence are the most active — at any given point in time.

	Boys and girls are equally likely to suffer from dyslexia

Neuroscientists have used FMRI to identify three areas of the left side of the brain that play key roles in reading. Scientifically, these are known as the left inferior frontal gyrus, the left parieto-temporal area and the left occipito-temporal area. But for our purposes, it's more helpful to think of them as the "phoneme producer," the "word analyzer" and the "automatic detector." We'll describe these regions in the order in which they are activated, but you'll get closer to the truth if you think of them as working simultaneously, like the sections of an orchestra playing a symphony.

Using FMRI, scientists have determined that beginning readers rely most heavily on the phoneme producer and the word analyzer. The first of these helps a person say things — silently or out loud — and does some analysis of the phonemes found in words. The second analyzes words more thoroughly, pulling them apart into their constituent syllables and phonemes and linking the letters to their sounds.

As readers become skilled, something interesting happens: the third section — the automatic detector — becomes more active. Its function is to build a permanent repertoire that enables readers to recognize familiar words on sight. As readers progress, the balance of the symphony shifts and the automatic detector begins to dominate. If all goes well, reading eventually becomes effortless.

In addition to the proper neurological wiring, reading requires good instruction. In a study published in the July issue of Biological Psychiatry, Shaywitz and colleagues identified a group of poor readers who were not classically dyslexic, as their phoneme producers, word analyzers and automatic detectors were all active. But the three regions were linked more strongly to the brain's memory processors than to its language centers, as if the children had spent more time memorizing words than understanding them.

The situation is different for children with dyslexia. Brain scans suggest that a glitch in their brain prevents them from easily gaining access to the word analyzer and the automatic detector. In the past year, several FMRI studies have shown that dyslexics tend to compensate for the problem by overactivating the phoneme producer.

Here at last is physical evidence that the central weakness in dyslexia is twofold. First, as many dyslexia experts have long suspected, there is an inherent difficulty in deriving sense from phonemes. Second, because recognizing words does not become automatic, reading is slow and labored. This second aspect, the lack of fluency, has for the most part not been widely appreciated outside the research community. Imagine having to deal with each word you see as if you had never come across it before, and you will start to get the idea. That's exactly what Abbe Winn of Atlanta realized her daughter Kate, now 9, was doing when she first went to school. "I noticed that when her teacher sent home a list of spelling words, she had a real hard time," Winn says. "We'd get to the word the and come back five minutes later, and she had no idea what it was."

So much for what dyslexia is. What many parents would like to know is what can be done about it. Fortunately, the human brain is particularly receptive to instruction. Different people respond to different approaches, depending on their personality and the nature of their disability. "The data we have don't show any one program that is head and shoulders above the rest," says Shaywitz. But the most successful programs emphasize the same core elements: practice manipulating phonemes, building vocabulary, increasing comprehension and improving the fluency of reading.

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