Deficits in Reward Processing. Growing evidence suggests that the reward system of ADHD patients is dysfunctional in comparison to healthy controls. The behaviour of ADHD patients of all ages is frequently described to be driven by immediate reward with reduced long-term value over delayed rewards with higher long-term value (Solanto et al., 2001). Furthermore, studies suggest that children with ADHD require stronger incentives in order to modify behaviour and learn faster by direct reinforcement (Xxxxxxx, Xxxxxxx, & Xxxxxxxxxx, 1998; Xxxxxxx et al., 2008). Studies have investigated functional abnormalities in reward processing in adolescents and adults with ADHD. Xxxxxxx and colleagues investigated VS activation using the MID task in 11 adolescents diagnosed with ADHD compared to 11 healthy controls, aged 12-17 years (Xxxxxxx, Xxxxxx, Xxxxxxx, & Xxxxxxxxxxx, 2007a). The study found that adolescents with ADHD showed reduced VS activation during the reward anticipation phase of the MID task. VS activation was also negatively correlated with parent-rated hyperactivity/impulsivity scores. Xxxxxxx and colleagues had postulated that the VS would be hyperresponsive during the reward feedback phase. However, adolescents with ADHD and healthy controls did not differ in their VS activation in response to reward feedback. A later study by Xxxxxxx and colleagues used functional MRI to compare neural responses to reward anticipation and feedback in 10 male adults with ADHD and 10 male healthy controls (Strohle, et al., 2008). This study suggested that adults with ADHD show decreased activation of the VS during the anticipation phase of the MID task, but increased activation of the OFC in response to the feedback phase. Again, VS activation during reward anticipation was negatively correlated with symptoms of hyperactivity and impulsivity. These studies suggest that ADHD patients, regardless of age, show lower activation of the VS during reward anticipation. Studies by Scheres and Xxxxxxx also revealed negative correlations between measures of ADHD symptoms and VS activation, suggesting that VS-activation patterns during reward anticipation is sensitive to both categorical and continuous measures of ADHD symptoms (Scheres, et al., 2007a; Xxxxxxx, et al., 2008). Due to deficient responses to rewards, ADHD is sometimes considered a reward deficiency syndrome (see Box 1). However, it is also considered a disorder of deficient inhibitory control. It is suggested that deficiencies in reward processing an...
