Effects of methylphenidate on complex cognitive processing in attention-deficit hyperactivity disorder

Three experiments were conducted to explore the effects of methylphenidate (MPH), attention-deficit hyperactivity disorder (ADHD) diagnosis, and age o n performance on a complex visual-memory search task. Results showed that t he effects of MPH varied with information load. On low-processing loads, al l doses of MPH helped children with ADHD to improve accuracy with no cost t o reaction time (RT), whereas on high loads, higher MPH doses improved erro r rates while slowing RT. Without medication, children with ADHD showed hig h error rates and slow RTs across both low and high loads, as did younger, normal control children. Because MPH slowed performance on only the most di fficult, high-load conditions, it is argued that the drug improves self-reg ulatory ability, enabling children with ADHD to adapt differentially to hig h and low lends.