Foveal crowding in children with developmental dyslexia

Excessive crowding in the visual periphery has been demonstrated in children with developmental dyslexia (DD). However, less is known about crowding in the fovea, even though foveal crowding is at least equally important, as reading is mostly accomplished through foveal vision. Here we used a special set of digit stimuli (Pelli fonts) to measure foveal crowding in DD and DD + ADHD children, and compared it to that in TD (typically developing) and ADHD children. We also used the Chinese reading acuity charts (C-READ) to assess the maximum reading speed and reading acuity, along with tests to evaluate cognitive attributes including phonological awareness, rapid automatized naming, morphological awareness, and orthographic knowledge. The results indicate significantly stronger foveal crowding in the DD and DD + ADHD groups, as well as in the ADHD group, than in the TD group. Furthermore, the DD and DD + ADHD groups exhibited poorer maximum reading speed and reading acuity compared to the ADHD and TD groups. Within the two DD groups, the slower maximum reading speed and higher reading acuity can be predicted by stronger foveal crowding. In addition, the DD and DD + ADHD groups performed the worst in four cognitive skills, with the DD group showing negative correlations between foveal crowding and performances across all these skills. Our findings thus move beyond previously well-documented peripheral crowding in dyslexia, and the easy administration of the Pelli-font-based crowding test may be useful for early diagnosis of developmental dyslexia in young children.