White matter differences in college-educated dyslexic and control adults: A diffusion MRI study in Brazilian Portuguese speakers.

The brain white matter microstructure of adult dyslexic readers is rarely studied, particularly in non-English speaking populations. This study investigated structural differences in white matter between Dyslexic and Control adults, all native Brazilian Portuguese speakers with a college-level education. We analyzed diffusion MRI data from 62 volunteers (26 with dyslexia and 36 controls), using deterministic and probabilistic approaches. Groups were matched for age, sex, handedness, and level of formal education. Deterministic analysis revealed significantly lower mean diffusivity and radial diffusivity in individuals with dyslexia in the posterior right hemisphere, particularly in core reading-related tracts such as the superior longitudinal fasciculus, inferior longitudinal fasciculus, and inferior fronto-occipital fasciculus. Probabilistic analysis showed widespread group differences in all diffusion measures investigated, including tracts not typically linked to dyslexia, and revealed alterations mainly in fractional anisotropy and radial diffusivity in posterior portions of tracts within the right hemisphere. Correlations between reading fluency and diffusion metrics revealed distinct neural behavioral patterns between groups, suggesting different neural mechanisms underlying reading abilities. These findings provide novel insights into white matter microstructure in dyslexia among Portuguese speakers and underscore the importance of cross-linguistic studies in reading disorders.