By scanning the brains of people reading English, French and Italian, researchers for the first time have demonstrated that dyslexia can be more severe depending on which written language people learn.
Indeed, the reading disorder is twice as prevalent in the United States, where it affects an estimated 10 million children, as in Italy, where the written word more consistently matches its spoken sound.
The new research shows that dyslexia--the most common learning disability in the United States--arises from a problem in the brain that cuts across language barriers, cultural borders and writing systems, an international team led by neuropsychologist Eraldo Paulesu at the University of Milan Biocca in Italy reported today.
But the very character of certain written languages, including English and French, makes the condition worse because their spelling is so dramatically at odds with how words sound, the researchers discovered.
The findings could aid in identifying and treating dyslexia. Moreover, they help scientists understand how the brain processes written language--and why that processing sometimes goes awry.
People around the world learn to speak their native language with equal rapidity and ease. The problems arise as people learn to read and write it.
The reading difficulties arise because the human ability to communicate evolved long before written language appeared. To decode spelling, the brain must employ neural circuits that evolved to handle sound as well as those intended for vision.
The task of matching sounds that make up a word to the symbols that represent them on the page taxes some brains more than others, explained cognitive neuroscientist Elise Temple at Stanford University, who studies dyslexic children.
"Written language is something very new in the nature of humankind," said pediatric communications expert Lewis Leavitt at the University of Wisconsin in Madison. "When we think about how our brains are built and how we process things, it should not be such a surprise that something new about humankind should be a problem for a decent number of people."
To better determine how the brain handles reading, the researchers tested 72 university students in three countries using scanners that measure the amount of energy used in different parts of the brain as a person thinks. Using positron emission tomography (PET) scanners, the researchers compared the brains of people with normal reading skills to those of people who showed symptoms associated with dyslexia, such as spelling problems and impaired reading speed.
The PET scans showed that, regardless of the language, people with the common reading problems of dyslexia had less neural activity in key areas of the brain responsible for decoding sounds and matching them to the visual cues of the alphabet.
"Our results are clear-cut," Paulesu said. "They show that dyslexia has a universal basis in the brain."
Published today in Science, this newest insight into the biology of reading is among a cascade of recent findings on how the written word can make the brain stumble and stutter. They all buttress teaching methods that help readers master the sometimes elusive relationships between sounds and letters.
"It is the first time anybody has looked across cultures," said Yale University neurologist Bennett Shaywitz, who studies reading and the brain. "It shows that the same neural systems are used for reading and are distorted in a similar way in dyslexic readers whether they are reading English, French or Italian.
"This notion of the universality of dyslexia is important," he said.
Until recently, researchers disagreed widely over just how reading challenges the brain and whether dyslexia was a problem confined only to those languages like English or French that have maddening spelling systems.
For example, the sound "eee" can be written nine ways in English but only one way in Italian. The English combination of letters "ough" can be correctly pronounced at least five ways.
In all, there are 1,120 ways to represent the 44 basic sounds of English using different letter combinations. By contrast, Italian uses only 33 letter combinations to represent its 25 basic sounds.
"English is a nightmare for dyslexics," said UCLA neuropsychologist Susan Y. Bookheimer, who employs brain imaging techniques to study reading and language disorders.
Some researchers also questioned whether dyslexia, which appears to run in families, could be traced to an inherited gene. Others debated whether dyslexia should even be considered a legitimate learning disability.
"This was a way to address the mystery," Paulesu said. "Dyslexia is a major problem in English-speaking countries and is much less a problem in Italian and [Spanish-speaking] countries. We tried to understand whether it was the writing system or a biological factor."
They discovered that both were involved, combining in ways that could affect the severity of dyslexia.