A multiple deficit model of reading disability and attention-deficit/hyperactivity disorder: searching for shared cognitive deficits

Reading Disability (RD) and Attention-Deficit/Hyperactivity Disorder (ADHD) are both complex neurobehavioral disorders affecting approximately 5% of children in the population (DSM-IV-TR, APA, 2000). RD and ADHD co-occur more frequently than would be expected by chance (estimates of 25% – 40% in both disorders) (Willcutt & Pennington, 2000). This study presents a multiple cognitive deficit model of RD, ADHD, and their comorbidity. Multiple deficit models (Pennington, 2006) propose a multifactorial etiology for complex developmental and psychiatric disorders where the constellation of risk and protective factors determines outcome. Comorbidity is to be expected when risk factors are shared between disorders. Despite increasing theoretical and empirical support for multiple deficit models, there has been little research directly evaluating whether such models can account for comorbidity. Neuropsychological studies of RD, ADHD, and other disorders have primarily focused on identifying cognitive deficits specific to a given disorder. For RD, the deficits identified include phonological awareness (Vellutino, Fletcher, Snowling, & Scanlon, 2004), other aspects of speech and language processing (Bishop & Adams, 1990), naming speed (Compton, Olson, DeFries, & Pennington, 2002; Purvis & Tannock, 2000; Semrud-Clikeman, Guy, Griffin, & Hynd, 2000; Wolf & Bowers, 1999), processing speed (Caravolas, Volin, & Hulme, 2005; Catts, Gillispie, Leonard, Kail, & Miller, 2002; Kail & Hall, 1994; Shanahan et al., 2006; Willcutt, Pennington, Olson, Chhabildas, & Hulslander, 2005), and verbal working memory (Rucklidge & Tannock, 2002; Swanson, Mink, & Bocian, 1999; Willcutt et al., 2001; Willcutt, Pennington et al., 2005). In the case of ADHD, there is less agreement about neuropsychological deficits. One candidate that has received considerable research attention is executive functioning, most notably response inhibition (Barkley, 1997), but also organization/planning and working memory (Willcutt, Doyle, Nigg, Faraone, & Pennington, 2005). In addition to executive functioning impairments, children with ADHD demonstrate cognitive weaknesses in domains such as processing speed (Shanahan et al., 2006) and naming speed (Rucklidge & Tannock, 2002). Taken together, these studies identify specific cognitive deficits in RD and ADHD as well as potential shared deficits in processing speed, naming speed, and verbal working memory. Of these deficits, processing speed has also emerged as a candidate shared deficit from studies analyzing combined samples of children with RD and ADHD (Shanahan et al., 2006; Willcutt, Pennington et al., 2005). The current study has two specific aims. First, we test a multiple cognitive deficit model .of RD and ADHD using structural equation modeling (SEM). Secondly, we test the multiple deficit models of RD and ADHD simultaneously to determine if any of the cognitive dimensions are predictive of both RD and ADHD dimensions. If so, we will explore the extent to which the cognitive factor(s) can account for the observed relationship between the RD and ADHD symptom dimensions. We predict that phonological awareness will be a unique predictor of RD, response inhibition will be a unique predictor of ADHD, and processing speed, naming speed and/or verbal working memory will be potential shared cognitive risk factors.