Dopamine, learning, and impulsivity: A biological account of attention-deficit/hyperactivity disorder

Background: Attention-deficit/hyperactivity disorder (ADHD) affects up to 10% of school-age children. The impulsivity which is seen as its core feature persists over years, yet experimental measures of impulsivity can be altered in a single session. In this study, we tested the theory that both the persistence and the variability of impulsivity could be the result of abnormalities in learning mechanisms and environment. Method: We extended an existing model of the role of dopamine in operant conditioning to address the delayed response time task, which is one of the standard tests for impulsivity in ADHD. In this task, subjects choose between immediate responding for a small reinforcer and later responding for a larger one. We studied the influence on impulsivity of four key parameters of the model: The learning rate, discount factor, brittleness, and action bias. Results: The behavior of the model is broadly comparable with electrophysiological (monkey) and behavioral (ADHD and normal) data. Variations in any of the parameters can cause impulsivity. All parameters except the discount factor show inverted U-shaped curves for their effects on impulsivity, suggesting, for example, how either hyper- or hypofunctioning of dopamine can cause impulsivity. The model suggests how decision making can be affected by environmental unpredictability, and thus offers an account of one aspect of the natural history of ADHD. Conclusions: Some types of ADHD may be caused by specific deficits in reinforcement learning and in the use of learned lessons. Environmental factors can interact with these deficits to delay maturation.