Dyslexia, one of the most common learning disorders, may be the result of problems with brain connectivity, according to a study published in the U.S. journal Science Thursday.
People with dyslexia, estimated to be more than 10 percent of the world's population, have difficulty in reading, processing spoken language, and ultimately, learning.
Scientists have argued why dyslexics struggle with this process. Some suggested phonetic representations are distorted in the dyslexic brain. Another theory is that phonetic representations are intact in people with dyslexia, just hard to access by other brain regions involved in language processing.
To investigate the two potential sources, Bart Boets and colleagues from the Catholic University of Leuven in Belgium scanned the brains of 22 normal and 23 dyslexic adults.
They used functional Magnetic Resonance Imaging techniques to look at patterns of nerve activity in the brain as these individuals responded to certain speech stimuli, noting how accurately sounds were mapped to their related phonetic representations.
"Quite to our surprise, and probably to the surprise of the broader dyslexia field, we found that the phonetic representations are perfectly intact in adults with dyslexia," Boets told reporters.
The researchers then performed a second analysis to explore whether connectivity in the brain differed between the two groups. They assessed how easily 13 regions involved in language processing could connect to phonetic representations, finding connectivity to be significantly hampered between certain regions in the brains of dyslexics.
The worse the connection, the more poorly the individual performed on reading, spelling and other tests, the researchers said.
According to the researchers, the research suggests that deficient access to phonetic representations, not quality of these representations, is at the heart of dyslexia.
"Our findings indicate that the speech sound representations themselves are intact, but a dysfunctional connection between frontal and temporal language areas impedes efficient access to the representations," said Boets.
However, not everyone is convinced. Michael Merzenich, a neuroscientist at the University of California, San Francisco, said that decades of "very extensive and compelling" evidence show that people with dyslexia do, indeed, process phonetic representations with lower fidelity than normal. "You can't just ignore this literature," Merzenich told Science.