Working memory in school-aged children with attention-deficit/hyperactivity disorder combined type: are deficits modality specific and are they independent of impaired inhibitory control?

This study examines differences between children with attention-deficit/ hyperactivity disorder combined type (ADHD-C) and normal controls on verbal and visuospatial working-memory (WM) tasks. The extent to which WM deficits in children with ADHD-C are independent of impaired inhibitory control was also examined. Two groups of 7- to 12-year-old boys participated in this study. The first group included 31 boys diagnosed with ADHD-C, and the second group included 34 boys without ADHD. Various verbal and visuospatial WM tasks and two inhibitory control tasks--prepotent response inhibition and interference control--were used. Overall, our results suggest impaired verbal and visuospatial WM processes in children with ADHD-C, as well as a lower level of performance on prepotent response inhibition. WM deficits in ADHD have previously been suggested to be particularly salient in the spatial domain; our results instead showed the largest effect for a verbal WM task thought to put heavy load on the executive or attentional control component of the WM system. An interpretation of this finding is that it is variation in terms of difficulty level or load on the executive WM processes, rather than variation in modality (verbal versus visuospatial), that is important in demonstrating WM deficits in ADHD-C. Finally, findings from logistic regression analyses showed that deficits in WM and inhibitory control seem to be semi-independent in children with ADHD-C, at least with regard to the elementary school age.