Attention-deficit/hyperactivity disorder (ADHD) with/without dyslexia was investigated using a double dissociation design. Neuropsychological performance representing the core deficits of the two disorders was measured in order to test the common deficit hypothesis. Phonological short-term memory, morpho-syntactical language, and central executive processing (manipulating and switching) tasks were administered to four groups of 10-14 year old children (ADHD-only n = 20, dyslexia-only n = 20, ADHD+dyslexia n = 20, and controls n = 19). Comparisons of performance on these tasks were carried out using 2 (ADHD yes/no) x 2 (dyslexia yes/no) factorial analyses of variance and covariance. Significant main effects were found for dyslexia (language processing functions) and for ADHD (EF switching). In the case of the EF manipulating a main effect for both dyslexia and ADHD was revealed. Effect sizes of mean performance indicated that all three impaired groups shared a common deficit in working memory which could reflect a cognitive overlap partly explaining the high rate of co-occurring dyslexia and ADHD.

The results indicate that motor timing ability in the millisecond range below 1000 ms in children with ADHD and/or dyslexia is intact. The performance of the comorbid group was revealed to be similar to the performance of the single disorder groups, but both the dyslexic groups were relatively worse than either the ADHD-only or the unimpaired group at reproducing complex versus simple rhythms.

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