Dyslexia's roots traced to faulty brain connections
For decades, dyslexia experts maintained that phonetic representations – the unique neural patterns effected in the brain by different speech sounds – were distorted in people with dyslexia. But new findings by KU Leuven researchers led by Dr Bart Boets suggests that these representations are perfectly intact in dyslexics, while the brain’s ability to access them is impaired.
People with dyslexia – estimated to be more than ten percent of the world’s population – have difficulty reading and processing spoken language. This is because they struggle with the process by which real-world sounds are mapped onto internal phonemes in the brain, the little neural pegs that categorise sounds and help make them interpretable.
Boets and his colleagues set out to study exactly what goes wrong in the dyslexic brain. What they found may settle a longstanding point of contention in the field. Scientists have argued for 40 years over why dyslexics struggle with this mapping process. Some suggest phonetic representations are distorted in the dyslexic brain. Another theory is that phonetic representations are intact in people with dyslexia, but hard to access by other brain regions involved in language processing.
“The two hypotheses are very difficult to disentangle,” said Bart Boets, a clinical psychologist and postdoctoral research fellow at KU Leuven. “This is because cognitive (behavioural) 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.”
In research published in the 6 December issue of the journal Science, Boets and his colleagues did just that.
"I was so convinced that we would observe degraded phonetic representations but they were perfectly intact in our dyslexic participants."
Boets and colleagues used functional magnetic resonance imaging (fMRI) to scan the brains of 22 normal and 23 dyslexic adults as these individuals listened to specific speech sounds made of vowels and consonants.
They used a technique known as multi-voxel activity analysis, or MVPA, 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.
“I was so convinced that we would observe degraded (i.e., less robust and distinct) phonetic representations in the dyslexic participants,” Boets said. “Yet, their representations turned out to be perfectly intact.”
Indeed, to the researchers’ surprise, phonetic representations in dyslexic readers were just as sharp, perhaps even more so, than those in normal readers.
The researchers then performed a second analysis to explore whether connectivity in the brain differed in the two groups. They assessed how easily 13 regions involved in language processing could connect to the same phonetic representations they had analysed before, finding connectivity to be significantly hampered between certain regions in the brains of dyslexics. The worse the connection, the more poorly a dyslexic individual performed on reading, spelling and other tests.
Access vs. quality
“A finding from the functional connectivity analysis that I think is pretty striking,” said Boets (pictured left), “is that “decreased connectivity is found specifically between the very same superior temporal regions found to support intact phonetic representations in the MVPA analysis.”
Taken together, these findings suggest that deficient access to phonetic representations is at the heart of dyslexia, not the quality of these representations.
“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, explaining the leap forward that he and his group could make.
Boets cautioned that further studies would be needed to confirm this finding holds true in other samples (for example, in much younger dyslexic children), or when using techniques more fine-grained than MVPA.
Even so, he says, the significant differences he and colleagues found in connectivity and access should be taken into account when designing the most appropriate intervention techniques for people with dyslexia. (He notes that traditional interventions were mainly designed to improve the quality of phonetic representations, not access to them -- though, by their nature, they also engage the access component.)
"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," Boets said.
Photos: © Michael De Lausnay