Dissociation between cognitive deficits and reading mechanisms in Chinese children with comorbid dyslexia and ADHD

Developmental dyslexia (DD) and attention-deficit/hyperactivity disorder (ADHD) often co-occur, but the cognitive profile of their comorbidity and the mechanisms supporting reading performance remain unclear. This study aimed to characterize cognitive profiles associated with DD, ADHD, and their comorbidity, and to determine whether different cognitive mechanisms support reading performance across diagnostic groups.

A matched sample of 200 Mandarin-speaking children was selected from a screened cohort of 2003 students, including four groups: typically developing (TD), DD-only, ADHD-only, and comorbid DD and ADHD (n = 50 per group). Participants completed assessments of reading fluency and accuracy, orthographic knowledge, phonological awareness, morphological awareness, rapid automatized naming (RAN), visual attention, and processing speed.

Children with DD showed broad impairments in reading-related measures compared with TD and ADHD-only children. Discriminant analyses revealed that phonological and morphological awareness primarily differentiated DD, whereas non-linguistic measures distinguished ADHD. Notably, RAN uniquely differentiated comorbid DD and ADHD from DD-only, with the comorbid group showing the greatest impairment. However, regression analyses revealed a dissociation between core cognitive deficits and reading mechanisms: the contribution of RAN to reading fluency was reduced in the comorbid group, whereas morphological awareness was a stronger predictor of character reading accuracy in the comorbid group.

These findings suggest a distinct cognitive profile in comorbid DD and ADHD, with altered RAN-reading associations and greater reliance on morphological awareness. This pattern highlights the need for assessment and intervention approaches that address both core deficits and compensatory resources.