Scientists in China and the United States have discovered that the brain problems that underlie the disorder are not always the same.

The findings indicate that dyslexia probably does not have a universal biological cause, but can be triggered in at least two distinct ways.

This means that the propensity of some people to develop the condition may depend on the culture in which they grow up, as well as the detailed biology of their brains.

While scientists have long understood that rates of dyslexia vary among speakers of different languages, it has generally been thought that the difference is one of degree that depends on the complexity of particular tongues.

Native speakers of Italian, for example, are slightly less likely to be dyslexic than English speakers, apparently because the language has simpler pronunciation and transcription rules.

Among both language groups, however, those who do suffer dyslexia tend to show a characteristic abnormal pattern of activity in the same part of the brain — the left temporoparietal region — suggesting that the same biological process is provoking the disorder.

The new research, details of which are published today in the journal Nature, indicates that this is not true of Chinese speakers. Although they also suffer dyslexia, those affected show abnormal brain activity in a different region — the left middle frontal gyrus.

Li Han Tan of the University of Hong Kong said that the results appear to reflect different writing methods. Western languages are based on an alphabet, while Chinese is based on pictures or symbols representing complete words or phrases. The brain, the research suggests, responds to the systems in different ways, leading to separate manifestations of dyslexia that depend on the language concerned.

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