Brain regions behind reading difficulties differ between cultures.
There is no one cause for dyslexia: rather, the causes vary between languages. So conclude researchers who have found that Chinese children with reading difficulties have different brain anomalies to their Western counterparts1.
The finding explains why one can be dyslexic in one language but not another. The team also hopes the work will aid the design of culturally specific strategies for learning to read and write that could benefit everyone.
People with dyslexia often find it difficult to recognize and understand words. Speakers of alphabetic languages, such as English or Russian, can have a problem converting letters into sounds. Dyslexics in these languages have reduced activity in a brain region called the left temporoparietal cortex.
But Chinese readers must learn the meanings of around 5,000 different characters, each corresponding to a word. Instead of letter-to-sound conversion problems, Chinese dyslexics have difficulties extrapolating from a symbol's shape to its sound and meaning.
Neuroscientist Li Hai Tan from the University of Hong Kong, China, and his colleagues speculated that dyslexia in Chinese readers might affect a different brain region. They used functional magnetic resonance imaging (fMRI) to monitor the brain activity of eight impaired and eight normal readers as they performed two language tests.
In the first task, the children were asked whether two Chinese characters had the same pronunciation. In the second, they were shown one real and one bogus character, and asked about their meaning.
The dyslexic children performed worse, but both groups showed the same activity in their left temporoparietal cortex. Instead, children with reading difficulties showed less brain activity in another region, the left middle frontal gyrus. This area helps to coordinate shapes, pronunciation and meaning, says Tan. The results are reported in this week's Nature1.
Most people thought that dyslexia is rooted in one brain region, Tan says. "But previous studies never looked at Chinese children."
"The study shows that the neural basis of reading is complex and differs depending on the nature of the writing system," agrees Guinevere Eden of Georgetown University, Washington, who studies dyslexia.
The finding could help researchers develop treatments for dyslexia that target the relevant problem, such as converting characters into meanings, or letters into sounds. Similar approaches could help non-dyslexics to read better, or benefit those learning a second language.
"We need to be more open minded about diverse treatment approaches for different aspects of the reading process," says Eden.
The finding could also explain the rare cases of people who read normally in one language, but are dyslexic in another. For example, one bilingual boy has reading problems in English, but none in Japanese2.
Japanese is a halfway house between alphabetic languages and Chinese. Readers often have to match shapes to syllables, a different task that is likely to involve a third, as yet unidentified brain region. So the boy?s left temporoparietal cortex was probably under active, whilst the unknown Japanese language-related area was fine, says Eden.
SlokW. T, PerfettiC. A., JinZ. & TanL. H. Nature, 431. 71 - 76(2004).
WydellT. N. & ButterworthB. B. Cognition, 70. 273 - 305(1999).
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