People with dyslexia cannot read or spell properly because of communication problems in their brain and not because they fail to form mental images of the sounds that make up a language, scientists have found.
Dyslexia has long been thought to result from an inability of the brain to learn all the small sound units or “phonemes” used to build up words, but a study suggests that this is false and that the real problem lies in one part of the brain not being able to link up with another, researchers said.
About one in ten people suffer from dyslexia which can lead to severe reading problems and difficulties with educational achievement even in highly intelligent people. Famous high-achieving dyslexics include Richard Branson, Steve Jobs, Walt Disney and Eddie Izzard.
Dyslexia is not associated with any obvious impairment in vision, hearing or general intelligence and this has led some to suggest that “word blindness”, as it is sometimes known, cannot be a genuine medical problem.
However, for about 40 years neuroscientists have argued that the problem is real and lies with the brain’s ability to acquire accurate representations of the various phonemes which are used in the communication of language.
More recently, other researchers have suggested that these phoneme representations are in fact there in the brain, it is just that they are not accessible by the other regions of the brain involved with language processing.
“The two hypotheses are very difficult to disentangle. This is because cognitive or behavioural tasks always tap both the representation and the access to this representation simultaneously,” said Bert Boets, a clinical psychologist at the Catholic University of Leuven in Belgium.
The study, published in the journal Science, managed to separate the two competing theories by using real-time brain scanning to observe how the brains of dyslexics and non-dyslexics coped with a set of mental tasks aimed at distinguishing between various sounds.
The scientists found that the dyslexic group was just as accurate as the non-dyslexics in completing the tasks and that the “crispness” of their brain scans were equal or even better to the people who could read normally.
Dr Boets said the findings demonstrated that the phonetic representations of the dyslexic subjects were “perfectly intact”. However, the dyslexics were about 50 per cent slower than the normal readers in making their responses, suggesting some kind of communication problem.
When the researchers analysed the overall activity of the brain, the dyslexics were notably different to the non-dyslexics. The dyslexics showed less coordination between the 13 brain regions that process basic phonemes and a region of the brain called Broca’s area, which is involved in higher-level language processing.
“Our neuroimaging findings suggest that it is not a deficit in underlying representations that characterise dyslexia,” the study concluded.
“Instead, our results suggest that a dysfunctional connection between frontal and temporal language areas [of the brain] impedes efficient access to otherwise intact representations of speech sounds, thus hampering a person’s ability to manipulate them fluently,” it found.
Dr Boets said that the findings are important because they could affect the way dyslexic children are trained to overcome their handicap. Traditional educational techniques for instance are designed to improve the quality of phonetic representations, not to increase access to them.
In the longer term, the results may also lead to new treatments based on improving these communication channels in the brain between various parts of the brain involved with the processing of language, he said.
“With this new knowledge, it is not inconceivable that we could design more focussed and effective interventions that specifically target improving the specific connections between frontal and temporal language regions [of the brain],” Dr Boets said.
Not everyone is convinced, however. Michael Merzenich, a neuroscientist at the University of California, San Francisco, said that previous scientific evidence has pointed to a lack of phoneme representation in dyslexic people and it would be wrong to just ignore the large body of scientific literature.
Q&A: Dyslexia explained
What is dyslexia?
It is one of the most common learning disorders, caused when children have difficulty with reading and spelling. They have problems in “decoding” the individuals sounds or phonemes that make up the words of a language.
Is it linked with low intelligence?
Dyslexia is called a learning disorder but it can affect all intellectual abilities. Not everyone on the dyslexia spectrum suffers to the same extent, so highly intelligent children with low-level dyslexia may read just as well as non-dyslexic children of lower intellectual ability.
How common is dyslexia?
Estimates range from 4 per cent to as many as 10 per cent. Dyslexia may become more apparent in certain cultures where the sounds and spelling of a language are not clear cut. Italian for instance, where the spelling follows the pronunciation quite closely, is probably easier for dyslexics than English, where words like “cough” and “dough” are spelt similarly but spoken very differently.