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We have shown, for the first time, that the brains of developmental dyslexics differ depending on the language they speak. Developmental dyslexia affects 7% to 9% of children in Hong Kong, and up to 17% throughout the world. It results in a severe learning disability in acquiring reading skills. Previous studies using magnetic resonance imaging (MRI) and functional MRI (fMRI) have revealed that dyslexic readers of alphabetic languages have decreased gray-matter volume and weak reading-related activity in posterior brain systems. These brain regions are responsible for letter-to-sound conversion and are spatially close to the auditory cortex. For alphabetic readers, reading and listening are very closely related.

In order to assess whether these abnormalities are universal, or culture-dependent, our research team studied dyslexic Chinese children in this RGC GRF research project. Unlike alphabetic languages such as English which are learnt using letter-to-sound conversion rules, written Chinese is a non-alphabetic writing system which is composed of square-shaped or picture-like characters. A Chinese character cannot be pronounced by any letter-to-sound conversion rules and must be memorized by rote. For Chinese readers, reading and writing may be more closely related.

We used two brain imaging techniques. Firstly, we used MRI to investigate brain structure (gray matter volume) of 16 11-year-old Chinese dyslexic subjects and 16 age-matched normal children. The children, who were studying in Beijing primary schools, were all native speakers of Putonghua. We found that the gray-matter volume in a left anterior brain region (the left middle frontal region) was significantly smaller in dyslexic children than in normal subjects. This brain region has been shown to play a part in Chinese reading and writing, and is also important for working memory. Their posterior gray matter, however, was unaffected.

Chinese and Western dyslexics show structural and functional abnormalities in different brain regions, suggesting that dyslexia may be two different brain disorders in the two streams of culture. Figure 1: Normal Chinese subjects have stronger brain activity during reading tasks and have bigger gray-matter volume in the left middle frontal gyrus region than the dyslexia children. Figure 2: When compared with normal children, dyslexic readers of alphabetic languages have decreased gray-matter volume in posterior brain systems, and have weak reading-related activity in the left temporoparietal and occipitotemporal regions of the brain.

Secondly, an fMRI experiment was conducted on a subset of 12 of each of the dyslexics and normal-reading groups. They were asked to decide whether two Chinese characters viewed simultaneously rhymed with each other. The rhyme judgment task involves reading and speech sound processing. It was found that the normal subjects had much stronger activation of the left middle frontal gyrus region during the task than the dyslexic group. The posterior brain systems, again, did not show differences between the normal-reading and dyslexic groups.

The fact that Chinese and Western dyslexics show brain abnormalities in different brain regions suggests that dyslexia may even be two different brain disorders in the two streams of culture. This research, published in the Proceedings of the National Academy of Sciences, could have important implications for teaching and learning. One implication suggests we should use different treatment methods for Chinese children but cannot simply adopt western treatment methods for alphabetic reading. Our research may help tailor-making therapies for children who grow up in different cultures and may well provide useful clues for further genetic studies in dyslexia.

Dr. SIOK Wai Ting
Department of Linguistics
The University of Hong Kong