Inhibition in Attention Deficit Hyperactivity Disorder

Attention-deficit hyperactivity disorder (ADHD) is a developmental condition estimated to affect approximately 5.9 to 7.1 per cent of children and 5.29 per cent of adults worldwide (Willcutt, 2012). Whilst the aetiology of the disorder is unknown, ADHD is characterised by attention deficits, hyperactivity, and impulsivity. These symptoms often significantly impair an individual’s functioning in the domains of school, employment, home life, and general social settings. In this thesis, I investigated the effect of dysfunctional inhibition in individuals with ADHD. Specifically, bottom-up processes, such as motor inhibition, motor control, and attentional inertia, were investigated to see to what extent these processes are affected by inhibitory dysfunction. This essential process is impaired in individuals with ADHD. This was accomplished by a battery of tasks, including a motor inhibition task, two motor skills tasks, and an attentional inertia task. These tasks were indexed to a Stroop task to investigate the possibility of a general inhibitory dysfunction in both top-down and bottom-up processes. In addition, the motor inhibition and Stroop task were indexed to self-inventories commonly used to identify individuals with ADHD. Across these eight experiments, adolescent ADHD individuals, age-matched controls, and adults undertook various tasks designed to index automatic bottom-up motor inhibition. Results showed that, compared with controls, ADHD individuals did not exhibit usual levels of inhibition. However, higher-level cognitive inhibition, as measured with Stroop, was comparable with controls. Results also revealed a positive association, but not a statistically significant one, between the degree to which a person exhibits ADHD-like behaviour and the degree to which they lack automatic motor inhibition. I will later show that it is due to a specific diagnostic construct of ADHD that does not include motor difficulties. These data suggest that bottom-up inhibitory motor processes are an essential component of ADHD. I will argue that including dysfunctional motor inhibition complements current ADHD models, particularly those developed by Barkley (1997) and Nigg (2001).