Dyslexic people show an abnormal pattern of brain function when reading - under-activity in some regions, over-activity in another - that accounts for the difficulty they have in extracting meaning from the printed word, researchers say.
The findings provide dramatic evidence that people with dyslexia are not poorly taught, lazy, or stupid, but have an inborn brain abnormality that has nothing to do with intelligence, say the scientists from Yale School of Medicine.
"I feel really very gratified that these individuals can now say there us good evidence that this is a neurobiological disorder," said Dr. Sally Shaywitz, a Yale pediatrician who headed the study. The evidence has been accumulating for a number of years in research on dyslexia, which is believed to afflict 10 percent or more of all children.
Dyslexia is defined as a significant reading disability in people with normal intelligence.
The new picture, literally, of brain abnormalities was obtained by placing normal and reading impaired volunteers inside computerized MRI scanners while they were presented with successively harder tasks.
Shaywitz said the study "means we've identified a system in the brain that allows us to go from looking at printed words to connecting it to the sound structure of words," and that dyslexics have "glitches" in the system.
Shaywitz said the poorly performing brain centers are the ones that are crucial to what is called phonological processing. This processing is the ability to crack the "code" that underlies written language, that is, the relationship between letters and letter groups with the spoken sounds they represent.
Learning this code is difficult for dyslexics, who also have trouble breaking down unfamiliar words into their letter-sound segments. As a result, reading is slow and filled with errors.
When normal readers were challenged with the tasks of increasing difficulty, brain scans revealed greater and greater activity in crucial brain areas, reports the Yale team in today's issue of the "Proceedings of the National Academy of Sciences".
In dyslexic volunteers, the same brain centers in the rear part of the brain, centers that involve visual perception, failed to become more active in response to the phonological tasks, the authors said.
At the same time, a region in the front of the brain became overactive, as if trying to compensate for the failure of the others, they said. This frontal region said Shaywitz enables the person to convert visual information into sounds. It's been known that damage to this brain center can cause adults to lose the ability to read.
The initial tasks only required the volunteers to say whether patterns of lines were similar or different, or whether patterns of upper and lower-case letters were the same. In a subsequent task, they were asked whether single letters , like T and V, rhymed - a task that begins to test the brain's ability to match letters and their sounds.
In subsequent tests, the volunteers had to say whether nonsense words like "leat" and "jete" rhyme. To answer correctly, the person has to know that both ea and e (in a word ending in a silent e) have the same long-e sound. People with dyslexia can learn these relationships with intense phonics training, but have trouble doing it naturally. The most difficult task was to determine whether two words, like "rice" and "corn" belong in the same category, which meant the volunteer had to know the meanings.
The Yale researchers tested 29 dyslexic and 32 non-impaired volunteers, and found significant differences in brain activation between the two groups.
Paula Tallal, a neuroscientist at Rutgers University, said the new study "beautifully pulls together" recent findings about phonological processing difficulties in dyslexia. And, she added, "it focuses on how important the phonics approach is" in teaching children to read.
Dr. Albert Galaburda, a neurologist at Beth Israel Deaconess Medical Center who has studied structural differences in the brains of dyslexics, said the findings clearly show the perceptual problems dyslexics have, but he called the study "a very pretty confirmation of things we already knew."