All dyslexics are not alike. According to new research, Chinese- and English-speaking people with the disorder have impairments in different regions of their brains. The findings shed light on the neurological basis of dyslexia and reveal fundamental differences in how brains process the two languages.
Dyslexics, about 5% to 10% of the population in both the United States and China, have trouble making the connection between the sight and sound of a word. In English, this results in word distortions or transpositions of letters. "Dyslexia," for example, might be read as "Lysdexia." In Chinese, the problem can affect how a person converts a symbol into both sound and meaning. Brain imaging with reading-impaired Chinese children has shown that these functions are mediated by a different part of the brain than is reading and writing in English (ScienceNOW, 1 September 2004).
Now a group headed by Li-Hai Tan at the University of Hong Kong has shown that these functional differences between Chinese and English speakers are rooted in the actual anatomy of the brain. The team did an analysis, called voxel-based morphometry, to get precise three-dimensional brain measurements from 16 dyslexic Beijing schoolchildren and compared them with those from 16 “normal” Chinese readers. The data showed that although total volume of gray matter--the part of the brain devoted to higher cognitive functions--did not differ between the two groups, the dyslexics had significantly less gray matter in the left middle frontal gyrus, an area important for identifying images and shapes, as well as recalling memory. The brains of English-speaking dyslexics, in contrast, have less gray matter in the left parietal region, according to a study reported by a different group last year. This region is more involved in converting letters to sounds than in the interpretation of shapes. "The finding is very surprising," says Tan, whose team's work appears online this week in the Proceedings of the National Academy of Sciences. "We had not ever thought that brains are structurally different for dyslexic children in two cultures."
The findings make sense based on the vastly different nature of the Chinese and English written languages, says neuropsychologist Robert Desimone of the Massachusetts Institute of Technology (MIT) in Cambridge. Whereas Chinese relies on complex images to represent entire words, English is an alphabetic language that relies more on rules and less on pattern recognition and memory. Now that differences in actual brain anatomy have been revealed, scientists are "a step closer to the underlying problem" of dyslexia, adds MIT cognitive psychologist John Gabrieli, who was involved in the English dyslexia study.
So if you are dyslexic in one language, would you be dyslexic in the other? That's "a totally fascinating question," says Gabrieli. But Han doubts it, because he says that his group thinks "different genes may be involved in Chinese and English dyslexic readers."