Structural Brain Correlates of Poor Reading Comprehension.

Poor comprehenders (PCs) have typical word reading skill and intelligence but poorer than expected reading comprehension. While the prevalence of PCs is similar to that of poor decoders (individuals who struggle fluently converting written text into spoken language), less is known about the neurobiological substrates of poor comprehension. Extant studies have found small differences in gray matter volume between PCs and poor or typically reading peers. However, a detailed quantification of cortical morphometric features and white matter integrity remains unexplored. Data from 2,100 children (1,200 with imaging data), aged 8-16 years were analyzed to determine if there is a distinct neuroanatomy associated with poor reading comprehension across three common methods for classifying PCs. We computed gray matter volume, cortical thickness, and surface area, as well as white matter measures (mean diffusivity, fractional anisotropy, neurite orientation, and neurite density) for PCs and compared these measures with those of poor decoders and typical readers. Results revealed small but widespread white matter differences, but no gray matter differences between PCs and other readers. PCs showed decreased white matter integrity (increased mean diffusivity, decreased neurite density) in tracts previously associated with reading, including the superior longitudinal fasciculus and inferior longitudinal fasciculus, and in tracts that have been associated with cognitive performance such as the uncinate fasciculus. These results suggest that diffuse structural connectivity differences may underlie reading comprehension weaknesses in the face of intact decoding skills. This is consistent with the behavioral profile of PCs who exhibit a broad pattern of subclinical impairments in language and integrative cognitive processes.