By Merritt McKinney

NEW YORK, Mar 15 (Reuters Health) - Even though dyslexia rates seem to vary from country to country, new study findings provide more evidence that the reading disorder stems from neurological rather than cultural causes.

People with dyslexia, despite normal intelligence and vision, have difficulty reading, writing and spelling. The exact cause of the learning disorder is uncertain, although it is increasingly believed to result from genetic brain abnormalities.

But the disorder appears to be much more common in some countries than in others. For example, one study found that Italy had half the rate of dyslexia as the US.

Does this mean that Italians are less likely to have the brain abnormalities that cause dyslexia? Not likely, according to a team led by Eraldo Paulesu of the University of Milan Bicocca. Instead, the researchers suspect that dyslexia is just as common in Italy, but since Italian is an easier language to spell, many cases of dyslexia go undetected.

Italian has what is known as a shallow orthography, or spelling system. This means that, for the most part, each letter or group of letters corresponds to one particular sound. Learning to read Italian and other languages with shallow spelling systems is easier than learning to read languages with deep orthographies, such as English and French. In those languages, the connection between spelling and pronunciation is often ambiguous.

In a study involving people with and without dyslexia in Italy, France and the UK, Paulesu's team found that Italian dyslexics did tend to read better than their counterparts in the other countries. But when Italians with dyslexia were compared to Italians without the reading disorder, the difference in reading ability was comparable to the gaps seen in dyslexics in France and the UK, the researchers report in the March 16th issue of the journal Science.

Results of a brain scan, called positron emission tomography, performed as participants read were also similar in all three countries. Compared to people without the reading disorder, all dyslexics had less activity in a large region of the left side of the brain.

"Our results are clear-cut," Paulesu and colleagues write. "They show that dyslexia has a universal basis in the brain and can be characterized by the same neurocognitive deficit."

The investigators conclude that people with dyslexia have a hard time processing sounds, which makes reading more difficult for them.

In an interview with Reuters Health, Dr. Todd L. Richards, a dyslexia researcher at the University of Washington at Seattle, called the study "important," noting that it confirms previous research documenting the neurological basis of dyslexia.

Richards said that he and his colleagues have also found that people with dyslexia have difficulty processing sounds. Even during silent reading, the brain takes into account the way words sound, so this so-called phonological deficit can make reading difficult for people with dyslexia, he said. Languages with many words that are easy to sound out are less difficult for dyslexics to read, he added.

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