Acute neuropsychological effects of methylphenidate in stimulant drug-naive boys with ADHD II - broader executive and non-executive domains

Background: Accumulating evidence supports methylphenidate-induced enhancement of neuropsychological functioning in attention deficit hyperactivity disorder (ADHD). The present study was designed to investigate the acute effects of the psychostimulant drug, methylphenidate (MPH), on neuropsychological performance in stimulant naive boys with ADHD. Methods: Seventy-three drug-naive boys (age 7-15) with ADHD (combined type) completed neuropsychological tasks from the CANTAB battery under randomised, placebo controlled, double-blind conditions following an acute challenge with either placebo (n = 24), .3 (n = 25) or .6 (n = 24) mg/kg oral MPH. Results: MPH did not impair performance on any task. MPH (.6 mg/kg) lengthened response latencies on a task of Spatial Recognition, shortened response times on a Reaction Time task and restored performance on a Delayed Matching to Sample visual, non-working memory task. Contrary to predictions, MPH did not enhance performance on tasks with a prominent executive component, including Go/NoGo, Spatial Working Memory, Stockings of Cambridge and Attentional Set shifting tasks. Conclusions: Acute administration of MPH to drug-naive boys with ADHD did not impair neuropsychological performance. Acute MPH enhanced performance on some aspects of non-executive functioning. MPH-induced slowing of responding on a relatively complex Spatial Recognition memory task and quickened responding on a reaction time task requiring less cognitive resources suggests that MPH may act by improving self-regulatory ability. MPH may not exert its effects on neuropsychological functioning by enhancing executive processes.