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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 RDonly 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.One strategy for understanding cognitive development is to see how it is perturbed by different developmental disorders. In this article, we present evidence from two common developmental disorders, reading disability (RD) and attention deficit hyperactivity disorder (ADHD), which indicate the separability of two different domains of cognitive development, phonological processes and executive functions. To understand the relations between these two disorders, we had to examine the basis of their frequent overlap or comorbidity.Comorbidities are a frequently encountered phenomenon in childhood behavioral disorders (Institute of Medicine, 1989), but we know little about their causal bases. Understanding comorbidities is a fundamental issue in the study of developmental psychopathologies, because each different possible explanation has different implications not only for clinical practice but also for developmental theory.This article addresses the specific problem of the comorbidity of dyslexia (or reading disability, RD) and attention deficit hyperactivity disorder (ADHD) by means of a test for a double dissociation. This methodology addresses the basis of comorbidity (a) by testing whether similar or different domains of cognition are disrupted in the "pure" forms of these two common developmental disorders, and (b) by examining the profile

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This study explored the hypothesis that children with early-treated phenylketonuria (PKU) are selectively impaired on executive function measures, even when still on diet. The rationale for this hypothesis is that even mild elevations in phenylalanine (Phe) can lead to lower central levels of biogenic amines, including dopamine (DA). We hypothesize that this mild DA depletion causes subtle prefrontal dysfunction, which in turn affects executive functions such as set maintenance, planning, and organized search. 11 preschool early-treated PKU children (M age = 4.64) and a sample of age- and IQ-matched unaffected peers (n = 11) were evaluated on a battery of executive function (EF) measures. In addition, a "non-executive function" task, recognition memory, was administered to all subjects. Group comparisons demonstrated that PKU children were significantly impaired on an executive function composite score; there were no group differences, however, in recognition memory. These results supported the hypothesized specific deficit in executive function. Furthermore, within the PKU group the executive function composite score was significantly negatively correlated with concurrent phenylalanine levels, even after controlling for the correlation between IQ and executive function skills. This second finding provides support for the proposed biochemical mechanism underlying the specific cognitive deficits.