Phonological short-term memory and central executive processing in attention-deficit/hyperactivity disorder with/without dyslexia--evidence of cognitive overlap.

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.