Effects of methylphenidate on working memory functioning in children with attention deficit/hyperactivity disorder

Children with attention deficit/hyperactivity disorder (ADHD) often show deficits in working memory performance. Methylphenidate (MPH) is an effective medication to improve these cognitive difficulties. This study aimed to clarify which effect MPH induces on the underlying functional networks of working memory.

Fourteen boys diagnosed with ADHD and 12 healthy controls were investigated using functional magnetic resonance imaging (fMRI). Each patient was tested twice, once with medication and once without. The fMRI experiments consisted of three verbal N-back tasks with increasing difficulty. Functional images were acquired on a 3 Tesla head scanner.

On the behavioral level, medicated patients performed similar to healthy controls and significantly better than without medication. On the functional level, patients showed the expected frontal and parietal activations, which were more pronounced in the 2- and 3-back tasks. Healthy controls showed significantly more activation in these regions and additional activation in the cerebellum. Interestingly, patients showed an additional effect of laterality. Left-sided frontal and parietal activation in patients was significantly less pronounced than in controls.

Functional data indicate different activation patterns in verbal working memory tasks between healthy controls and patients with ADHD irrespective of medication condition. Intake of MPH led to a clear improvement on a behavioral level. However, this effect was not reflected by changes in functional brain organization. MPH-induced changes leading to better performance in verbal working memory tasks might be very subtle and therefore not detectable by fMRI.