Role of Bilingualism in the Neuroanatomical Differences in Children With Reading Disability (dyslexia)

The left-hemisphere language cortex is known to be structurally aberrant in developmental dyslexia (also referred to as reading disability, RD). However, studies have not addressed the neuroanatomical bases of dyslexia in bilinguals, even though bilingualism is common, and the bilingual experience is thought to alter the language cortex. This raises the question of whether current brain-based models of dyslexia are applicable to bilinguals. We employed a factorial analysis with participants from the ABCD Study (total N = 268, aged 9-10 years), comparing Bilinguals with RD, Bilingual Controls, Monolinguals with RD, and Monolingual Controls on gray matter volume (GMV) and cortical thickness (CT). RD was determined for reading in English. We included only cultural early bilinguals of Spanish and English; these are prevalent in the United States and represent a homogeneous group of bilinguals who learned their languages early in their home environment. Both main effects analyses (RD vs. Control; Bilingual vs. Monolingual) yielded results for GMV and, to a lesser extent for CT, and the effects for bilingualism were more pronounced than those for dyslexia. Importantly, the interaction analysis revealed no exponential effect, indicating that the neuroanatomical signature of dyslexia is not compounded by experience-dependent plasticity associated with early bilingualism. Our results suggest that brain-based models of dyslexia derived from monolinguals can be generalized to early bilinguals. They also reveal no differences in left-hemisphere language cortex in dyslexia (main effects analysis of RD vs. Controls), suggesting that prior results of GMV and CT differences in these regions from smaller studies may not have been robust. SUMMARY: Neuroanatomical aberrations in dyslexia have been reported in left-hemisphere language cortex, but this work is based primarily on monolingual participants. We tested for the first time if aberrations of gray matter volume and cortical thickness in reading disability (RD) are magnified by a dual language-experience. An ANOVA of Bilinguals with and without RD and Monolinguals with and without RD resulted in no exponential effect of bilingual experience. The main effect for RD revealed no differences in left language cortex and far fewer regions than those revealed by the main effect of bilingualism.