CONTRASTING COGNITIVE DEFICITS IN ATTENTION-DEFICIT HYPERACTIVITY DISORDER VERSUS READING-DISABILITY

We compared 2 common and sometimes comorbid developmental disorders, reading disability (RD) and attention deficit hyperactivity disorder (ADHD), in 2 cognitive domains, phonological processes (PP) and executive functions (EF). Subjects were 70 boys of early school age, studied by means of a 2 (RD vs. no RD) x 2 (ADHD vs. no ADHD) X 2 (domain type) mixed-model design. The 2 RD groups (RD-only and RD plus ADHD) were significantly impaired compared with both the control and ADHD-only groups on a PP composite score but performed normally on the EF composite score. The ADHD-only group had an opposite profile and was significantly different from both RD groups and from controls on the EF composite score. Thus, there was a double-dissociation between the RD-only and ADHD-only groups. The comorbid group resembled the RD-only group, consistent with the hypothesis that their ADHD symptoms are secondary to RD. These results provide evidence for the separability of PP from EF, as well as one explanation for the comorbidity between RD and ADHD.