Structural Connectivity in Ventral Language Pathways Characterizes Nonverbal Autism

Abstract

Autism spectrum disorder (ASD) involves a large variety of language capacities, from normal scores on standardized language tests to absence of functional language in a substantial minority of 30% of individuals with ASD. Due to practical difficulties of scanning at this severe end of the autism spectrum, insights from MRI are scarce. Here we used manual deterministic tractography to investigate, for the first time, the integrity of the core white matter tracts defining the language connectivity network in nonverbal ASD (nvASD): the arcuate (AF), inferior fronto-occipital (IFOF), inferior longitudinal (ILF) and uncinate (UF) fasciculi, and the frontal aslant tract (FAT). A multiple case series of nine individuals with nvASD were compared to normative benchmarks from matched individuals with verbal ASD (vASD) and typical development (TD). Bonferroni-corrected repeated measure ANOVAs were performed separately for each tract –Hemisphere (2:Left/Right) x Group (3:TD/vASD/nvASD). Results revealed (i) a main effect of group consisting in a reduction in fractional anisotropy (FA) in the IFOF in nvASD relative to TD; (ii) a significant interaction of hemisphere and group in the UF, which showed reduced volume in the left hemisphere when compared to the right, in the vASD group only; and (iii) main effects of hemisphere in both the AF (left-lateralized in volume) and the ILF (left-lateralized in FA). These results do not replicate deficits of the dorsal language route previously observed in nvASD, and instead point to a disruption of the ventral language pathway, which is in line with semantic deficits observed behaviourally in this group.