Event Rate and Effects of Stimulants in ADHD

Stimulants alleviate information processing and task performance deficits in Attention Deficit/Hyperactivity Disorder (ADHD). Long acting formulations of amphetamines such as lisdexamphetamine dimesylate (LDX) are especially valuable as they target the school day and improve classroom performance. Although stimulants have been widely used in treatment of ADHD, the exact mechanism action and effect on task performance is not completely known. According to the State Regulation Deficit (SRD) model, children with ADHD have difficulty regulating their levels of arousal/activation during tasks in response to the changing demands of the environment. This leads to problems with downregulating overaroused states and upregulating underaroused states. According to this view, stimulants exert their therapeutic effect (in part) by optimising arousal/activation levels - especially during states of underarousal/activation. Arousal/activation levels can also be altered by extrinsic factors such as event rate (ER), e.g., the rate at which information is presented. Multiple studies suggest that very fast and very slow events can both cause problems for individuals with ADHD, related to overarousal and underarousal state respectively. Putting these intrinsic (stimulants) and extrinsic (ERs) factors together leads to the prediction that changing the rate at which information is presented in a task may alter the efficacy of stimulants and affect the optimal stimulant dose level. More specifically, one dose of stimulant that may be optimal on slow ER tasks (as it increases arousal/activation level) may be less effective under high ER tasks because in such a setting arousal/activation level needs to be lowered and not increased further. Adding stimulants to an already overactivated state may exacerbate the associated problems. The implication of this is that a different dose of stimulant will be needed under different environmental conditions for optimal performance. For example, childre