A BILINGUAL boy who speaks fluent Japanese and English yet is dyslexic in English is helping scientists to shed fresh light on the causes of word blindness.

The boy was born in Japan and went to school there, where he learnt to speak and read in Japanese, but he spoke English at home with his British mother and Australian father.

Although he reads Japanese to undergraduate level and has successfully passed university entrance exams set in Japanese, his reading ability in English is well below his age level. His mother, a teacher, taught him English using conventional methods and his inability to read properly is due to dyslexia rather than poor tutoring.

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Brian Butterworth, professor of cognitive psychology at University College London, and Taeko Wydell, of Brunel University, said it is the first time a bilingual person who can read and write in two languages has been found to be severely dyslexic in one and normal in the other.

"It shows that dyslexia is not purely a reading problem because this boy can read, it's just that he can't read in English," Professor Butterworth said.

"The reason is very straightforward. Most dyslexias are due to a deficit in analysing syllables into their component sounds. To read English you need to be able to break up the syllable into phonemes to learn how to map letters onto sounds. In Japanese, each character stands for a whole syllable, so this deficit won't be expressed in learning to read Japanese.

"Our study shows that dyslexia is a very specific interaction between something that's gone wrong in your brain and what you do with it. It is a nice demonstration about the real nature of dyslexia, since there is a perfect control - the same subject. The research could help language teachers to devise new methods of teaching dyslexics based on treating words as wholes, just as Japanese treat characters."

The boy has always spoken English well but even at high school his written English was below the average for his Japanese-speaking classmates.

He was only diagnosed as dyslexic at the age of 13 when he would often confuse words, such as "mother" for "mouth", as well as mistaking the letter "b" for a "d".