Methylphenidate improves response inhibition in adults with attention-deficit/hyperactivity disorder

Background: Response inhibition is an executive function that requires voluntary control over responses when there is a change of context. The right inferior frontal cortex is necessary for response inhibition, and a deficit in right frontostriatal circuitry might underlie attention-deficit/hyperactivity disorder (ADHD). Many studies of childhood ADHD have demonstrated impaired response inhibition and its amelioration by methylphenidate (MPH). The current study tested response inhibition and the effect of MPH in adult ADHD. Methods: Response inhibition was assessed with the "tracking" stop-signal test in 13 adults with a diagnosis of ADHD, both while taking and while not taking medication, and 13 healthy, unmedicated, age- and intelligence quotient-matched control subjects. Results: Stop-signal reaction time was significantly slower in unmedicated adults with ADHD relative to healthy control subjects, and this deficit was significantly ameliorated by medication. Conclusions: Adult ADHD patients had a response inhibition profile similar to that produced by lesions to the right inferior frontal cortex, which was remedied by stimulant medication.