Between abstinence and dependency: understanding the brain and behavioral correlates of reward learning in occasional stimulant users.

p t s i d c i o p t d u W hen young adults leave home to pursue a career or higher education, they face a variety of new challenges. New ways of thinking and working, along with obligatory time constraints, make focus and a good work ethic essential to thrive. In today’s culture, some young adults resort to using prescription stimulants at moments when particular attentiveness is required. For instance, a majority of university students that illegally use prescription stimulants report doing so to stay awake and focus while studying (1). The perceived effectiveness of these stimulants along with their commonplace nature may lead some individuals to become occasional stimulant users (OSUs). However, this infrequent usage may also lead to more regular consumption and risk of developing stimulant dependency in the future. An important issue, then, becomes whether specific characteristics of OSUs serve as precursors to stimulant dependency. Stimulant-dependent individuals have well-characterized decision-making deficits that are related to difficulties in learning and adapting to reinforcement-based contingencies (2). For instance, there is a tendency of stimulant-dependent individuals to favor decisions resulting in immediate as opposed to delayed rewards, even if these outcomes are maladaptive in the long run (3); and to exhibit difficulty in flexibly adapting to reversal of contingencies wherein responses toward a particular stimulus are required for reward receipt (4). These decision-making patterns can be maladaptive because the consideration of long-term outcomes and the knowledge of one’s current environmental state are important for maximizing rewards. Co-occurring with these behavioral irregularities are abnormalities in the neural signature of reward processing and decision making. In healthy control populations, regions of the brain that are involved in reward learning, such as the inferior frontal gyrus (IFG) and dorsal striatum, are active early on during learning, when the consequences of various options in a response set are being learned. However as learning takes place over time, and better discrimination among potential options occurs, activation in these regions during outcome processing is attenuated (5). In contrast, stimulant-dependent individuals show aberrant, increased activation in the dorsal striatum and IFG, along with the insula, which also aids in the decision making process (6), during experimental decision-making tasks (7–9). Although these deficits are well characterized in stimulant dependency, it is unclear when exactly they manifest in particular individuals. That is, when do OSUs begin to show changes in the neural signature of reward learning and decision making that mirror deficits in stimulant-dependent individuals, and how does this reflect the transition from occasional use to dependency? These are the fundamental questions addressed in new research by Stewart et al. (10), who exam-