METHYLPHENIDATE AND COGNITIVE FLEXIBILITY - DISSOCIATED DOSE EFFECTS IN HYPERACTIVE-CHILDREN

A randomized, double-blind, placebo-controlled trial was conducted to assess the acute effects of placebo and three doses of methylphenidate (MPH) (0.3, 0.6, 0.9 mg/kg) on cognitive flexibility and overt behavi or in 28 children with a confirmed diagnosis of attention deficit-hype ractivity disorder Two underlying cognitive processes (response inhibi tion and response reengagement) were assessed by measuring the probabi lity and speed with which subjects could inhibit responses to a primar y task (forced-choice letter discrimination) and immediately execute a response to a secondary task (simple reaction time) when given a sign al to do so. Results indicated that MPH enhanced cognitive flexibility although the high dose was less effective than lower doses in enhanci ng response inhibition. Dissociations of dose effects on cognitive fun ction and behavior were demonstrated: Dose-response functions for chan ges in behavior were linear whereas the function for response inhibiti on was U-shaped. Findings argue against the typical clinical practice of determining the response to stimulant treatment from a single measu re such as parent report of child behavior.