Revisiting atypical language lateralization in dyslexia

Hemispheric lateralization has been central to developmental dyslexia research for over a century, yet its role in the etiology of reading and language deficits remains elusive. While altered asymmetries have long been implicated, evidence is inconsistent, with limited consideration given to individual variability in lateralization patterns. This study investigated hemispheric lateralization in 35 adults with dyslexia and 35 matched controls using functional MRI across three language tasks: word generation, rhyming decision, and lexical decision. Laterality indices (LIs) were calculated to comprehensively assess the strength, direction, and consistency of activation across global and regional task-specific brain areas. No significant group differences were found in the absolute strength of lateralization for either global or regional measures. Directional asymmetry was similar across the two groups, except in the fusiform gyrus during the reading task, where dyslexic individuals showed a higher prevalence of right-hemisphere lateralization compared to controls. Importantly, we found that dyslexic participants demonstrated greater inconsistency in regional lateralization during the reading and rhyming tasks. Among individuals with dyslexia, those with inconsistent lateralization in the reading task had lower reading performance and weaker fusiform lateralization, although fusiform LI strength itself did not predict reading outcomes. Our findings suggest that dyslexia is characterized by inconsistent, rather than universally weaker, lateralization patterns. Inconsistencies in task-related and regional lateralization may disrupt the efficiency of language networks, contributing to observed reading deficits. Our results emphasize the need to move beyond traditional group-averaged analyses, highlighting the importance of considering individual variability when investigating language disorders.