Methylphenidate slows right hemisphere processing in children with attention-deficit/hyperactivity disorder

To examine the effects and particularly, any potential detrimental effects of methylphenidate on right hemisphere processing in children with attention-deficit/hyperactivity disorder (ADHD), the performance of 26 children (aged 8-15 years) with ADHD was compared under methylphenidate and placebo on various cognitive tasks in a double-blind within-subjects design. Results indicated that reaction times on a tachistoscopic task were slower on methylphenidate for stimuli that produced a left visual field advantage (reflecting better right hemisphere functioning) but not for stimuli that produced a right visual field advantage (reflecting better left hemisphere functioning) or for bilateral stimuli. It is argued that methylphenidate induces a slower response on tasks involving right hemisphere processing, resulting in slower reaction times and unchanged accuracy rates. Findings on two tasks that more closely reflect classroom-type tasks revealed an improvement in performance on methylphenidate, suggesting that any stimulant-induced benefits on learning style may have overcome detrimental effects on unilateral processing. These findings may help understand the complex, diverse, and sometimes unpredictable effects of psychostimulants on cognitive functioning that are observed clinically in individuals treated for ADHD.