For decades, scientists have proposed two theories for how dyslexia, the common reading and learning disability, forms in the brain. Do dyslexics have trouble processing phonetic sounds, like “ba” and “da”? Or is it simply an issue of being able to access those sounds, which dyslexics produce normally? New research from KU Leuven in Belgium suggests science can finally produce a definitive reply to the question: What is dyslexia?
Until now, tests of dyslexia have largely failed to isolate these two phenomena. Researchers who run behavioral tests on dyslexic subjects often find that both theories bear fruit. It’s difficult, they find, to disentangle the speech processing mechanisms of reading — when people run their eyes over a page and absorb the words, for instance — from the parts of the brain responsible for understanding that newly absorbed text. For some 700 million people worldwide, or roughly 10 percent of the world’s population, the disability is exhausting. Being able to peel back the layers of the disorder may someday reveal to scientists an even greater discovery: how dyslexia is caused.
KU Leuven researcher Dr. Bart Boets led a team of scientists to perform neuroimaging scans of 22 normal adults and 23 dyslexic adults as the subjects listened to a variety of speech sounds made up of vowels and consonants. Scans of the brain itself are superior for understanding dyslexia, Boets argued, because the two commonly proposed theories aren’t tested in a way that allows for pinpointed conclusions.
“The two hypotheses are very difficult to disentangle,” Boets, a clinical psychologist and postdoctoral research fellow at the university, said in a statement. “This is because cognitive (behavioral) tasks always tap both the representation and the access to this representation simultaneously. Therefore, we needed neuroimaging to tease the two apart and assess them in isolation.”
Publishing in the journal Science, the researchers used a technique called multi-voxel pattern analysis (MVPA) to maximize their chances of understanding the disorder. Basically, MVPA relies on provoking a subject’s brain to produce a desired nerve response after certain stimuli. If the patterns don’t match what the researchers anticipate, generally the MVPA signals a neurological disconnect. Dyslexics who read the letter “b” may process it as “d,” so the group’s MVPA would show the brain incorrectly “pegging” the letter elsewhere. But this isn’t what Boets observed.
“I was so convinced that we would observe degraded (i.e., less robust and distinct) phonetic representations in the dyslexic participants,” he said. “Yet their representations turned out to be perfectly intact.”
Dyslexics weren’t reading “b” as “d,” in other words. They were reading “b” as “b,” but accessing it as “d.” In fact, the processing side of many dyslexic subjects was sharper and more attuned to the various phonetic sounds than that of the control group. This compelled Boets and his team to run a second test in which they analyzed the specific retrieval patterns of the processed sounds.
In the second test, the team analyzed the connection quality between data storage and data extraction within the brain. Critically, they looked at 13 regions involved with language processing that were also related to the phonetic representations analyzed in the first study. Here, they found, connectivity accounted for a breakdown in comprehension.
“A finding from the functional connectivity analysis that I think is pretty striking,” Boets offered, “is that decreased connectivity is found specifically between the very same superior temporal regions found to support intact phonetic representations in the MVPA analysis.” This means that dyslexics’ brains have no difficulty mapping individual speech patterns — they place the phonemes in the correct spots — they just grab the wrong sound when seeking to turn the symbol into meaning. It’s akin to sorting utensils just out of the dishwasher, only to grab a knife later on when you meant to grab a fork. Now replicate that mistake over every meal, which must take place consistently throughout the day for undetermined lengths of time — painstaking.
The team’s findings move dyslexia research forward considerably, because in order to understand how a disorder is caused, scientists must first understand how it works. Recognizing a car won’t start because the battery is dead requires first knowing the car has a battery. Cognitive impairments, though decidedly more complex, function in much the same way. Boets argues the practical value of his study for dyslexics is in therapeutic alternatives. Rather than treat the disability through speech processing techniques, which would have little effect in light of the present research, intervention specialists should seek to improve the mechanism by which dyslexics retrieve the phonetic sounds.
“It was by combining and applying two different analysis techniques on the same data set that we were able to draw these controversial conclusions,” Boets said. "With this new knowledge, it is not unconceivable that we could design more focused and effective interventions that specifically target improving the specific connection between frontal and temporal language regions.”
Source: Boets B, Op de Beeck H, Vandermosten M. Intact But Less Accessible Phonetic Representations in Adults with Dyslexia. Science. 2013.