Culture affects dyslexia too
Doctors who say dyslexia has a universal biological cause linked to a small, specific part of the brain are wide off the mark, according to the first study to say culture also shapes this reading problem.
The conventional view is that dyslexia is caused by an abnormality in the data-processing part of the brain that assesses a letter, attributes a block of sound called a phoneme to it, then a meaning.
But, researchers say in today's issue of the journal Nature, this theory has only ever been tested fully on alphabet-based languages, such as English, French, Italian and German, not on logographic languages like Chinese or Japanese.
The team asked young Chinese people with dyslexia to perform various reading tasks with Chinese characters. As they did this, they were scanned by functional magnetic resonance imaging (fMRI), which takes a 3D picture of their brain activity.
Various characters were used in the experiments.
They could be real ones with a single meaning; homophones or characters that are pronounced the same but have different meanings; or bogus ones that look like Chinese characters but are really meaningless.
The fMRI pictures showed that rather than the left temporoparietal region of the brain being activated, as other researchers had found, a different part of the brain, the left middle front gyrus, was activated among Chinese people with dyslexia.
The researchers, led by Li Hai Tan, from the University of Hong Kong and the U.S. National Institute of Mental Health, believed this region is implicated because reading Chinese is a different mental task compared with reading an alphabetic language.
Reading between the lines
With an alphabetic language, reading is done sequentially. Letters are recognised, broken up into phonemes that are then matched to a known meaning.
But with Chinese, reading is more like parallel processing. The brain has to seize the meaning of the pictogram almost as simultaneously as it figures out its sound.
"Rather than having a universal origin, the biological abnormality of impaired reading is dependent on culture," the authors said.
They said the finding was important because it supported the idea that strategies can be developed to stimulate the brain regions involved in dyslexia.
About 10% of the population may have some form of dyslexia, which causes difficulty in reading, writing and spelling. It tends to run in families but scientists say environmental factors may also contribute to it.
Last month Finnish scientists said they had found a gene called DYXC1 that they believe could be important in causing dyslexia. Other genes have also been implicated.