Abstract
ADHD is a commonly diagnosed disorder of children, and is characterized by difficulties with inattention and/or hyperactivity/impulsivity. Some theories and models of ADHD have noted that children with ADHD respond differently from controls to rewards. The two studies in this thesis used response bias (log b, a measure derived from signal detection theory) during a computerized detection task to examine the response to rewards in children with ADHD. The participants were children aged between 6 and 12 years of age. One hundred and fifty two children were recruited from Dunedin schools, and 136 children were referred by the Otago District Health Board for assessment of ADHD related difficulties. From these two groups of children 68 boys with combined type ADHD (ADHD-C) and 91 normally developing boys were identified. Both studies used a similar computer task to present stimuli and measure responses. A matrix of computerized characters was presented on a computer screen. Participants were asked to identify whether there were more red or more blue characters present. Correct responses were occasionally rewarded with an on-screen animated cartoon, verbal praise from the examiner, and tokens which were exchanged for a small gift at the end of the task. Study 1 examined sensitivity to reward frequency, and generated response bias by arranging more frequent rewards for correct responses to one stimulus than the other stimulus. Study 1 found that boys with ADHD-C developed bias towards more frequently rewarded stimuli more slowly than normally developing boys – this was particularly apparent towards the end of the task. The results were also consistent with previous literature identifying that children with ADHD show reduced bias towards frequent rewards following individual rewards to the infrequently rewarded stimulus. Study 2 examined sensitivity to reward delay, and was designed to generate response bias by arranging immediate rewards for correct responses to one stimulus and delayed rewards for correct identifications of the other stimulus. In Study 2 there were no clear differences between groups. The interpretation of both experiments was confounded by confounding variables: task version (in Study 1); and task order (in Study 2). An unintended but interesting finding is that children with ADHD-C may be more influenced by their past history of reward than control children. Children with ADHD-C who completed Study 2 after Study 1 tended to continue to show bias towards the response that had been rewarded frequently in the previous task, despite the same response being associated with delayed rewards in the current task. Children with ADHD-C may be less able than normally developing children to reverse their preference in response to altered reward contingencies. The results of the current experiments are discussed in relation to the broader literature on ADHD and rewards/reinforcement and current theories of ADHD.