Structural brain correlates of poor reading comprehension

Poor comprehenders have typical word reading skill and intelligence but poorer than expected reading comprehension. While the prevalence of poor comprehenders is similar to that of poor decoders (individuals who have difficulty fluently converting written text into spoken language), less is known about the neurobiological substrates of poor comprehension. Extant studies have found small differences in grey matter volume between poor comprehenders 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 were analyzed to determine if there is a distinct neuroanatomy associated with poor reading comprehension. We computed grey matter volume, cortical thickness, and surface area, and white matter measures including mean diffusivity, fractional anisotropy, neurite orientation, and neurite density for poor comprehenders and compared to that of poor decoders and typical readers. Results revealed small but widespread white matter differences, but no grey matter differences between poor comprehenders and other readers. Poor comprehenders 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 poor comprehenders who exhibit a broad pattern of subclinical impairments in language and integrative cognitive processes.