Individual differences in anatomy predict reading and oral language impairments in children

Developmental dyslexia (DD) and specific language impairment (SLI) are disorders of language that differ in diagnostic criteria and outcome. DD is defined by isolated reading deficits. SLI is defined by poor receptive and expressive oral language skills. Reading deficits, although prevalent, are not necessary for the diagnosis of SLI. An enduring question is whether these two disorders are qualitatively different or simply differ quantitatively along a dimension of severity. Here we address this problem by examining neuroanatomical correlates of reading and language in children with learning disabilities. We asked whether a quantitative anatomical risk index derived from previous work could predict behavioural profiles in a heterogeneous sample of 14 boys and 8 girls (11-16 years of age) with reading and language impairments. The results confirmed our predictions that (i) children with relatively smaller and symmetrical brain structures (negative risk indices) would have the severe comprehension impairments typical of SLI; (ii) children with larger, asymmetrical brain structures (positive risk indices) would have poor word reading in the presence of relatively preserved comprehension, a profile typical of DD; and (iii) the best performance would be seen in children with anatomical risk indices near zero (normal anatomy). Also, in confirmation of previous work, rapid automatic naming was not predicted by the anatomical risk index, but by anatomical measures derived from the frontal lobes. These results highlight the key significance of comprehension deficits in distinguishing DD from SLI. Reading impaired children with and without comprehension deficits appear to occupy neuroanatomical domains on the opposite sides of normal.