Phonological processing, not inhibitory control, differentiates ADHD and reading disability

Objective: To test for the distinctiveness of attention-deficit hyperactivity disorder (ADHD) and reading disability (RD) and the independence of the cognitive domains, inhibition and phonological processing, which are proposed as central to ADHD and RD, respectively, using a classic double dissociation design. Method: A 2 (ADHD versus no ADHD) x 2 (RD versus no RD) model was used to examine the cognitive profile of 4 groups of children, aged 7 to 11 years. Two measures of inhibitory control and 3 phonological processing measures were used. Results: The 2 RD groups (RD, ADHD+RD) were significantly impaired relative to the 2 non-RD groups (controls, ADHD) on all phonological processing measures. The 2 ADHD groups were significantly impaired on simple go-task responding relative to the non-ADHD groups and in inhibition. Contrary to predictions, an RD effect on inhibitory control was found on one inhibition measure. The comorbid group (ADHD+RD) generally exhibited the deficits of both single groups in an additive fashion. Conclusions: These findings question the role of inhibitory control as a unique cognitive marker for ADHD and suggest true comorbidity for children with both ADHD and RD.