Neuropsychological analyses of comorbidity between reading disability and attention deficit hyperactivity disorder: in search of the common deficit.

Measures of component reading and language skills, executive functions, and processing speed were administered to groups of children with attention deficit hyperactivity disorder (ADHD; n = 113), reading disability (RD; n = 109), both RD and ADHD (n = 64), and neither RD nor ADHD (n = 151). Groups with RD exhibited pronounced deficits on all measures of component reading and language skills, as well as significant weaknesses on measures of verbal working memory, processing speed, and response inhibition. Groups with ADHD exhibited weaknesses on all response-inhibition and processing speed tasks and were impaired on some measures of component reading skills and verbal working memory. The group with comorbid RD and ADHD exhibited the combination of the deficits in the RD-only and ADHD-only groups, providing evidence against the phenocopy and cognitive subtype hypotheses as explanations for the co-occurrence of RD and ADHD. Slow and variable processing speed was characteristic of all 3 clinical groups, suggesting that measures of this domain may be useful for future studies that search for the common genes that increase susceptibility to RD and ADHD.