The neural basis of dyslexia may finally have been nailed – showing that the problem lies with faulty communication between different areas of the brain’s language network. The discovery may solve a long-running debate as to why people with dyslexia struggle to read and write.
Dyslexia affects about 5 to 10 per cent of the population in the UK and US, and those affected have difficulties in reading accurately and fluently, and sometimes in writing and spelling words. One theory proposes that people with dyslexia have a subtle difficulty in hearing, particularly involving timing of speech, which in turn leads to faulty representations of the basic units of speech sounds, or phonemes, in the brain.
But if speech isn’t represented faithfully in the brain, why doesn’t this lead to problems comprehending what is being said? “One of the things that’s slightly mystifying about that [theory] is that people with dyslexia don’t have a problem understanding speech,” says Sophie Scott at University College London, one of the co-authors of the study. “Why don’t they find that difficult?”
To investigate further, Scott, together with Bart Boets at the Catholic University of Leuven in Belgium, and their colleagues, scanned the brains of 23 adults with dyslexia and 22 without, looking at patterns of nerve activity as they responded to certain speech sounds. In all the participants patterns of nerve activity in the auditory cortex, which processes incoming sound, were equally reliable in their response to different speech sounds. This suggests that the dyslexic brain represents sounds in the same way as the non-dyslexic brain.
This doesn’t rule out the possibility that these dyslexic adults had less distinct representations when they were young, which then developed differently, but it does show that phonetic representations can be intact in adults, despite persistent reading difficulties.
Another possibility is that other parts of the brain’s language network that need to access those sound representations have trouble doing so. To test this, the team explored the connections between 13 brain regions involved in language processing. They investigated both the functionality across these regions and the structure of the actual nerves that connect the regions. In people with dyslexia, both these measures revealed faulty connections between the superior temporal gyrus – the region that contains the auditory cortex – and the inferior frontal gyrus – a region involved in language processing and speech production.
Furthermore, the worse the functional connectivity between these regions, the worse participants performed on reading and other phonological tasks. Taken together, the measures predicted whether the participant was dyslexic or not with 73 per cent accuracy. The authors claim these findings add to the growing recognition of dyslexia as a “disconnection syndrome”.
“What we’re learning from functional imaging is that it’s not enough to talk about brain areas; you’re always dealing with a network, and when you’ve got a network you’ve got patterns of connectivity,” says Scott. “So is there some way you could restructure that network that might be beneficial?”
“This work is helpful in thinking about intervention approaches,” says Guinevere Eden, director of the Center for the Study of Learning at Georgetown University in Washington DC. At the very least, knowing that sound is well represented in the cortex, researchers may want to look elsewhere for a way of intervening with dyslexia.
Boets thinks it may be possible to help people with dyslexia by targeting the faulty connections. “It might be possible with behavioural interventions, but it might also be possible using non-invasive brain stimulation techniques, like transcranial magnetic stimulation,” he says.
A follow-up study is already underway, which will scan a group of children at hereditary risk of dyslexia at different stages of learning to read, to try to answer questions about when these differences appear, and how they change over the course of learning to read.