EFFECTS OF METHYLPHENIDATE AND BEHAVIORAL CONTINGENCIES ON SUSTAINED ATTENTION IN ATTENTION-DEFICIT HYPERACTIVITY DISORDER - A TEST OF THE REWARD DYSFUNCTION HYPOTHESIS

Psychostimulants and behavior therapy have been postulated to be effec tive in treating attention-deficit hyperactivity disorder (ADHD) by co mpensating for a pathologically elevated reward threshold, but no stud ies have compared reinforcement to psychostimulants in maintaining tas k performance. The separate and combined effects of methylphenidate (M PH, 0.6 mg/kg) and a behavioral intervention (reward plus response cos t) were assessed on a continuous performance test (CPT, a measure of s ustained attention) modified to deliver auditory feedback contingent u pon the subject's responses. Each of 22 children (6-10 years old) with ADHD were tested under four treatment conditions: placebo + feedback, placebo + behavioral contingencies, MPH + feedback, and MPH + conting encies. CPT performance, indexed by d' (ability to discriminate betwee n target and false targets), was significantly better with MPH than wi th placebo, showing reduced deterioration over time. Contingency treat ment improved mean d' compared to placebo + feedback but, in contrast, had no effect on the slope of performance deterioration. Addition of contingencies to MPH did not yield further improvement. The results in dicate that MPH improved sustained attention on a laboratory task (and reduced task-irrelevant and other disinhibited behaviors), whereas be havioral contingencies did not. These findings suggest that, although both interventions improved stimulus discrimination processes, only MP H enhanced processes that mediate the regulation of effort over time. In addition, the disjunction between the effects of reward and of MPH provides evidence that psychostimulant effects on attention are only p artially explained by the stimulation of brain centers associated with reward.