Cultured readers: Chinese kids show new neural side of dyslexia.
Neuroscientist Li Tai Han of the National Institute of Mental Health in Bethesda, Md., and his colleagues say their data challenge the view that the reading difficulties considered central to dyslexia spring from a common biological source (SN: 3/31/01, p. 205). "Rather than having a universal origin, the biological abnormality of impaired reading is dependent on culture" the investigators conclude in the Sept. 2 Nature.
Prior brain-imaging studies of dyslexia among readers of letter-based languages have highlighted disturbances in a brain network with its hub in tissue toward the back of the left hemisphere (SN: 5/24/03, p. 324). Scientists have tied that neural region to the ability to match written letters to corresponding sounds.
In Chinese readers with dyslexia, however, the locus of trouble lies in a vertical fold of tissue near the front of the brain. This area assists in recognizing sounds and meanings denoted by Chinese characters and other abstract symbols, Han and his coworkers say.
They studied 16 children, ages 10 to 12, who attended a Beijing elementary school. All the youngsters scored well on intelligence tests, but half of them had severe reading problems.
A functional magnetic resonance imaging (fMRI) scanner measured blood flow throughout each child's brain during two experiments. In one of them, kids judged whether pairs of Chinese characters presented on a computer screen had the same pronunciations. In the other test, children tried to discern whether each of a series of written characters was a real Chinese character or a fake one with no designated meaning or pronunciation.
Compared with the fMRI scans of good readers, the scans of poor readers indicated substantially lower blood flow, and therefore reduced brain-cell activity, in and around the left-brain tissue fold. Several other left-brain areas associated with reading non-alphabetic script also showed minimal activation in poor readers. In addition, two right-brain areas involved in the visual analysis of written characters displayed weak activity in poor readers.
Similar findings emerged in an unpublished brain-imaging study of 65 more Chinese children, Tan says.
Neuroscientist Guinevere Eden of Georgetown University Medical Center in Washington, D.C., says that the new study shows how different writing systems can direct the development of distinct brain networks for reading.
It's too early to say whether different writing systems fundamentally change dyslexia's neural basis, cautions Bennett A. Shaywitz of Yale University Medical School. He notes that Tan's "milestone" fMRI data also reveal that several brain regions for dyslexia are common to children in China and the United States.
Tan argues that his findings of cultural differences in dyslexia's biology are sound. One test, he says, would be to see whether English readers with dyslexia profit from instruction in recognizing whole words, as Chinese readers do. This might open an otherwise-unused neural pathway to effective reading.
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|Title Annotation:||This Week|
|Date:||Sep 4, 2004|
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