Dyslexia's broken bridge

Rita Carter

DYSLEXIA is caused by a faulty connection between two areas of the brain which process language, according to British cognitive scientists. The research may finally resolve years of debate over whether dyslexia is at root a biological, cultural or learning problem. It may also lead to better diagnosis and education for dyslexics.

The research team, led by Uta Frith of the Medical Research Council's Cognitive Development Unit in London, recruited five dyslexic and five nondyslexic adult volunteers of above-average intelligence. While the volunteers performed linguistic tests, the researchers studied brain activity using positron emission tomography (PET), which reveals which parts of the brain are most active by measuring blood flow.

The people with dyslexia had problems creating spoonerisms from familiar combinations of words: converting "John Lennon" to "Lohn Jennon", for example. The PET scans revealed differences between the two groups in a part of the brain called the left perisylvian area-a chunk of cerebral cortex just above the ear. This contains Broca's and Wernicke's areas, which are involved with processing words, plus a lump of tissue called the insula.

Wernicke's area is thought to be involved in the recognition of complete written words, says Frith, while Broca's breaks the same words down into segments and creates a mental image of their sound. Until now the function of the insula was unknown, but the PET scans suggest that it forms a crucial bridge between the two areas. In the nondyslexic volunteers, the insula and both language areas lit up together during the linguistic tests. In the dyslexics' scans, however, the insula did not light up and each language area was activated in isolation (Brain, vol 119, p 143).

"Each of the language areas deals with a specific aspect of word processing and in normal people the insula synchronises this work," says Frith. So when most people see a written word they automatically "hear" it in their head at the same time. "In dyslexics the areas are disconnected so instead of knowing instinctively what a written word sounds like they have to think about each word they see and consciously translate it from one form to another," says Frith.

This translation may be easier in some languages than in others. Languages that rely on picture-like symbols, such as Japanese, can be easier to read because each symbol represents a whole word (New Scientist, Science, 20 January, p 14). But English words often need to be broken down into segments, and can only be processed normally if the insula is fully functioning.

Nevertheless, many dyslexics get round the problem by using other pathways in the brain, so the condition often only shows up under stress. But being able to cover up dyslexia may be a mixed blessing because it means that as many as half of all sufferers remain undiagnosed.

"Dyslexic children have to put much more mental effort than others into work that involves using language, and this inevitably puts them under terrific strain," says Frith. "If their dyslexia is not recognised they may be wrongly diagnosed, or marked down as being of average ability when they are in fact much, much brighter." She hopes the new study will lead to better and earlier diagnosis, and appropriate education for dyslexics.

PET scans are never likely to be used for routine diagnosis. Because they rely on people consuming small quantities of a radioactively labelled sugar, radiation from each mean is equivalent to 10 X-rays. But researchers are now developing a sensitive variant of magnetic resonance imaging, which examines the behaviour of the brain's constituent atoms in a strong magnetic field, and will also measure brain activity.

New Scientist 23 March 1996