Methylphenidate restores visual memory, but not working memory function in attention deficit-hyperkinetic disorder

Rationale: Dysfunction of executive neuropsychological performance, mediated by the prefrontal cortex, has been the central focus of recent attention deficit/hyperkinetic disorder (AD-HKD) research. The role of other potential neuropsychological "risk factors", such as recognition memory, remains understudied. Further, the impact of methylphenidate (MPH) on key neuropsychological processes in AD-HKD remains poorly understood. Objectives: To compare the performance of boys with AD-HKD on a spatial working memory (SWM) task and on two non-working memory tasks [a simultaneous and delayed matching-to-sample task (DMtS) and a pattern-recognition task] with that of healthy boys, and to investigate the impact of acute and chronic MPH on performance of these tasks. Methods: Baseline performance of 75 stimulant-naive boys with AD-HKD was compared with that of 70 healthy boys. The AD-HKD boys were then re-tested following the administration of acute and chronic challenges with MPH (0.3 mg/kg and 0.6 mg/kg) under randomised double-blind placebo controlled conditions. Results: Compared with healthy boys, the AD-HKD boys demonstrated performance deficits on all neuropsychological tasks. A single dose of MPH restored performance on the DMtS task but had no impact on the SWM or pattern-recognition tasks. Chronic MPH administration did not alter performance on the SWM task but did improve performance on both the pattern-recognition and DMtS tasks. However, the acute restorative effect of MPH on DMtS diminished with repeated administration. Conclusions: Our results suggest that current conceptualisations of the neuropsychological basis of AD-HKD and the proposed therapeutic mechanisms of MPH require broadening.