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, children with ADHD might require different dosage in the classroom setting to optimize performance. In addition, the neuropsychological basis of performance deficits and improvement by ER and stimulants are also unclear. According to the SRD model, the underlying mechanism can be specific problems in motor activation/preparation or effort regulation. Event-related potentials (ERP), pupil size measurements and cardiac measures enable us to see objectively how motor activation/preparation and effort are affected by ER and simulants.