Phonological processing, not inhibitory control, differentiates ADHD and reading disability.J Am Acad Child Adolesc Psychiatry. 2000 Apr; 39(4):485-94.JA
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.
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.
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.
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.