The goal of this study was to evaluate the effect of methylphenidate (MPH) and clonidine in
comparison with placebo on response inhibition and state regulation in children with
Attention Deficit Hyperactivity Disorder (ADHD). The study utilised a double-blind cross-over design in which children were randomly assigned without replacement to placebo,
MPH, and clonidine following baseline assessment. The primary dependent measures were
derived from children's performance (reaction time and errors) on a GO–NO GO task under
three conditions that altered the inter-stimulus interval (ISI) for presented GO–NO GO
stimuli: ISI of 1 sec (fast condition), 4 sec (medium condition), and 8 sec (slow condition).
Findings indicated no difference in task performance between groups treated for 7 weeks
with placebo, MPH, and clonidine. We concluded that the state regulation problem in
ADHD is resistant to MPH and clonidine.