The researchers showed that the functional and structural connection (blue arrow) between frontal (green) and temporal (red-yellow) language areas is impaired in individuals with dyslexia.
Credit: Photo courtesy of Bart Boets
Dyslexia, the learning disability that makes reading and processing speech a challenge, may result from problems with brain connectivity, a new study suggests.
Scientists estimate that dyslexia affects more than 10 percent of the world's population. Some hypothesize that in people with dyslexia, the way that speech sounds are represented in the brain is impaired, while others contend that the brains of people with dyslexia represent the sounds correctly, but have trouble accessing them because of faulty brain connections.
Ultimately, understanding the roots of dyslexia could lead to better ways to help people with the disability, the researchers said. [Top 10 Mysteries of the Mind]
Normally, when people read words or hear spoken language, the brain creates a map to represent the basic sounds in speech, called phonemes. These brain representations have to be robust, for instance, all "b" sounds must map to the same category. But they must also be distinct, in order to distinguish between similar sounds such as "b" and "d."
In the new study, Bart Boets, a clinical psychologist at KU Leuven, in Belgium, and his colleagues used brain imaging to test which hypothesis — flawed sound representations or flawed wiring — best explains dyslexia.
The researchers scanned the brains of 23 adults with dyslexia and 22 adults without the condition as they responded to various speech stimuli. The scientists looked at how accurately the participants' brains mapped sounds to their phonetic representations.
People with dyslexia had intact representations of basic sounds, just as non-dyslexic people did, the scans revealed.
"To our surprise, and I think to the surprise of a large part of the dyslexia research society, we found out that phonetic representations were perfectly intact. They were just as robust and distinct in individuals with dyslexia as they were in typical readers," Boets told reporters today (Dec. 5).
The researchers then investigated whether brain connectivity differed between the dyslexic and normal participants. In particular, they examined how well 13 brain areas involved in language processing were connected to phonetic representations.
The participants with dyslexia had notably worse connectivity between Broca's area, a region in the brain's frontal lobe linked to speech production, and the left and right auditory cortexes, the researchers reported online today in the journal Science. In addition, the people with the weakest connections performed the worst on reading and spelling tests.
The findings suggest dyslexia stems from a failure to connect to fundamental sound representations, rather than problems with those representations themselves, the researchers said.
Boets compared the dyslexic brain to data stored on a computer server. "The data [itself is] perfectly intact, but the connection to access this data is somehow degraded — maybe too slow or somehow distorted," Boets said.
Frank Ramus, a cognitive scientist at the École Normale Supérieure, in France, who was not involved in the study, called it the most conclusive study of dyslexia's causes in the last five years, adding that, if the results hold, they would change scientists' understanding of dyslexia.
However, Usha Goswami, a neuroscientist at the University of Cambridge, in England, who supports the view that dyslexia is a problem of faulty representations, interpreted the results differently.
"The data in the study do not show that dyslexia is caused by a difficulty in accessing phonemes, as the participants are adults," Goswami told LiveScience. "The reduced connectivity shown could be a result of a lifetime of poor reading, rather than evidence for a reduced access to phonemes which has caused dyslexia."