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.


METHODS
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.
Purvis
K L
KL
Hospital for Sick Children, Toronto, Ontario, Canada.
Tannock
R
R
D001289:Attention Deficit Disorder with Hyperactivity / Q000453:epidemiology / Q000523:psychology*; D016022:Case-Control Studies; D002648:Child; D003071:Cognition; D015897:Comorbidity; D015986:Confounding Factors, Epidemiologic; D004410:Dyslexia / Q000453:epidemiology / Q000523:psychology*; D005260:Female; D006801:Humans; D007266:Inhibition, Psychological; D008297:Male; D015999:Multivariate Analysis; D009483:Neuropsychological Tests; D010700:Phonetics