A study of children entering kindergarten is shedding light on the origins of dyslexia, as diffusion-weighted MRI has revealed that if certain white matter tracts in the brain are smaller and less organized, it can lead to reading difficulties.

While previous research has linked white matter deficiencies to poor reading skills, the current study, published Aug. 14 in the Journal of Neuroscience, demonstrated that limitations in structural connectivity contribute to dyslexia, not the other way around.

“The discovery that such a relation between white-matter organization and one of the strongest behavioral predictors of dyslexia, poor phonological awareness, exists prior to formal reading instruction and substantial reading experience favors the view that differences in white-matter organization are not only the consequence of dyslexia, but also may be a cause of dyslexia,” wrote Zeynep M. Saygin, PhD, of McGovern Institute for Brain Research and Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Mass., and colleagues.

Findings were based on a subset of 40 kindergarteners pulled from a larger study of reading ability in school children throughout Massachusetts and Rhode Island. In conjunction with researchers at Boston Children’s Hospital, the children underwent diffusion-weighted imaging, with results compared against a number of pre-reading tests.  

The main focus was on three white-matter tracts associated with reading skill: the arcuate fasciculus, the inferior longitudinal fasciculus and the superior longitudinal fasciculus. The left arcuate fasciculus in particular connects the Broca’s area, involved in speech production, with Wernicke’s area, which interprets written and spoken language.

Results showed that higher composite phonological awareness scores—measuring the ability to identify and manipulate the sounds of language—were significantly and positively correlated with the volume of the arcuate fasciculus. This correlation was not seen in the other tracts, and two other behavioral predictors of dyslexia, rapid naming and letter knowledge, were not correlated with volume or diffusion values in this area.

“The volume and fractional anisotropy of the left arcuate showed a particularly strong positive correlation with a phoneme blending test,” wrote Saygin and colleagues. “Whole-brain regressions of behavioral scores with diffusion measures confirmed the unique relation between phonological awareness and the left arcuate. These findings indicate that the left arcuate fasciculus, which connects anterior and posterior language regions of the human brain, and which has been previously associated with reading ability in older individuals, is already smaller and has less integrity in kindergartners who are at risk for dyslexia because of poor phonological awareness.” 

Because extra training in phonological skill has been shown to improve reading skills in some dyslexic children, the potential for early identification of those at risk of the condition could spur aggressive interventions in these children.

The next step for the researchers is to follow three waves of children as they progress to second grade to further track how well the brain measures in the study predict poor reading skills.