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.

The literature on children with attention deficit/hyperactivity disorder, combined type (ADHD-C), is currently inconclusive as to the nature of deficits in two forms of cognitive control - interference control and response selection (Nigg, 2006). This paper examined the performance of children with ADHD-C on interference control and response selection conflict tasks that required both speed and accuracy. The data was analyzed utilizing a new efficiency method to more effectively analyze overall responses. Both interference control and response selection conditions were combined within tasks allowing for a closer comparison of how children with ADHD-C perform on these specific types of cognitive control. Computerized tasks were administered to 62 boys, ages 7 to 12 (31 controls, 31 ADHD-C). Results revealed deficits in efficient performance for children with ADHD-C on interference control tasks and response selection tasks hypothesized to involve high cognitive control demand. These results highlight the utility of analyzing efficiency data to identify deficits in performance for children with ADHD-C and to foster an increased understanding of cognitive control functioning in this clinical population.

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