Behavioural Inhibition in Children with ADHD: Does Stimulant Medication Eliminate Potential Deficits?

Attention-Deficit / Hyperactivity Disorder (ADHD) is characterized by higher than normal levels of inattention, hyperactivity and impulsivity. Behavioural inhibition is proposed to be a primary deficit in children with ADHD, and is included as a component of a number of models accounting for the core behavioural symptoms of ADHD. Children with ADHD often show deficits in their performance on behavioural inhibition tasks relative to typically developing children of the same age, although inconsistent findings have been observed. Stimulant medication is associated with reduction in the core symptoms of ADHD in the majority of children. The primary goal of this thesis was to examine the effects of stimulant medication on behavioural inhibition in children with ADHD. Furthermore, this thesis sought to determine whether children with ADHD who have, and have not taken stimulant medication differ from normally developing children in terms of behavioural inhibition. In order to achieve these aims, it was necessary to firstly determine which tasks provide the best measures of behavioural inhibition. While many tasks have been used to measure inhibitory control amongst children with ADHD, it was unclear from previous research which tasks measure the same constructs. Study 1 investigated relationships in task performance among seven measures of inhibitory control, including the Stop-Signal task, Go / No-go task, Sustained Attention to Response Task (SART; analogous the not-X Continuous Performance Task), Eriksen Flanker task, Stroop, Opposite Worlds task and Task-Switching task. Significant developmental changes in a variety of cognitive abilities occur across childhood and adolescence. To limit the possibility that developmental changes in task performance would be observed, the age range for children included in the study was restricted to seven to 10 years. Nevertheless, to interpret correlations among inhibition measures from the tasks, it was necessary to determine whether children showed comparable developmental trends in their performance across tasks. Study 1A investigated developmental changes in the performances of the seven tasks purported to measure inhibitory control in normally developing children aged seven to 10 years. The results of this study suggested that the tasks chosen were appropriate for use among this age group, and that there is little change in behavioural inhibition across the seven to 10 years age range. In Study 1B, inhibition measures from each of the tasks were included in an exploratory factor analysis to determine those tasks measuring the same constructs. The results of Study 1B suggested that the Stop-Signal, Go / No-go and SART tasks provided the best measures of the behavioural inhibition construct. Performance on the Eriksen Flanker task was also related to the performance on these tasks, but in a direction contrary to that predicted. The Stroop and Opposite Worlds tasks measured the same construct, which appeared to be interference control. Performance on the Task-Switching task was not related to the performance on any other task, suggesting that this task did not measure behavioural inhibition or interference control. Study 2 investigated the performance of children with ADHD on the three tasks shown to be the best measures of behavioural inhibition in Study 1B (i.e., the Stop-Signal task, Go / No-go task and Sustained Attention to Response task). Children with ADHD were aged from seven to 11 years, and were tested both when they had, and had not taken their regularly prescribed stimulant medication. The performance of children with ADHD on these tasks was compared to that of normally developing children matched in age. The results of Study 2 suggested that stimulant medication leads to significant improvement in behavioral inhibition amongst children with ADHD, such that children with ADHD do not differ from matched controls. However, this effect was not observed across all three tasks. Reasons for this, along with study limitations, and directions for future research are discussed