No Support for White Matter Alterations in Adults With Dyslexia: A Fixel-Based Diffusion MRI Study

Developmental dyslexia is a common learning disability marked by reading and spelling difficulties. While previous imaging studies aiming to elucidate the neurobiological mechanisms of this disorder have reported white matter alterations, they are inconsistent with regards to which specific tracts are implicated and in what way. These inconsistencies might partially stem from methodological limitations such as small sample sizes and the use of outdated diffusion models. To address these issues, we used fixel-based analyses, an advanced diffusion framework, to compare structural white matter organization between 35 adults with dyslexia and 34 controls across three levels of analysis (whole-brain, tract-specific, and tract-averaged). Contrary to expectations, none of the analyses yielded significant group differences. However, within the dyslexic group only, poorer word reading proficiency was linked to greater fiber density and cross-section of the bilateral inferior longitudinal fasciculus. Taken together with the existing literature, our results suggest that white matter structure might not be altered in (adulthood) dyslexia or might be idiosyncratically impacted to such an extent that group-average studies are unable to detect these changes. Future large-scale research and efforts to pool datasets across studies will prove essential for understanding the white matter correlates of dyslexia.