Adolescents are notorious for risky behavior, such as drug and alcohol use initiation. A popular theory posits that adolescent risk-taking is partly attributable to robust functioning of earlier-developing mesolimbic motivational neurocircuitry that is poorly controlled by immature frontocortical executive-control neurocircuitry. Functional magnetic resonance imaging (fMRI) findings of increased mesolimbic recruitment (including ventral striatum (VS)) by rewards in adolescents compared to adults support this theory, in that activation of VS reflects covert emotional responses to incentives. Other studies, however, have shown blunted VS recruitment by reward-predictive cues in adolescents compared to adults. Task features may explain this discrepancy, but have never been systematically explored within-subject in age-group comparison. For example, tasks that elicited relatively increased VS activation in adolescents have featured colorful imagery, payoffs of uncertain magnitude, making choices, and cue-contingency learning. Conversely, studies have shown reduced VS activation by reward prospects in adolescents in tasks with explicit payoffs for vigilance-intensive motor performance. Adolescent motivational neurocircuitry may be especially activated by the affective components of rewards, whereas the fully-developed adult brain may more robustly recruit VS as part of a fully-developed executive-control feedback loop (that includes VS to incorporate incentive information) to sustain attention. Notably, the VS is also recruited by vigilance in adults even in non-incentivized contexts. During instrumental behavior, VS recruitment in adolescents may be primarily driven by the affective components of reward, whereas VS recruitment in adults may primarily reflect optimal attention allocation. This R21 project will launch a research program aimed at providing critical contextual nuance to the development of human reward processing. Neurotypical adolescents and adults will perform a novel fMRI reward task that holds constant the expected value of all rewards, but varies whether rewards are 1): explicit, 2) dependent on vigilance-intensive responding, and 3) dependent on a successful choice versus a single response option. Finally, dynamic causal modeling will be employed to measure corticostriatal connectivity as a function of age and reward demands. If adolescents show greater VS activation by choosing responses for rewards or for uncertain payoffs, and adults show greater VS responses to reward in executive control-demanding trials, it will help resolve controversial literature, and clarify reward contexts that best engage motivational circuitry in adolescents. Discovering the incentive contexts that most strongly engage adolescent motivational neurocircuitry can inform each of: a) policies to prevent risky behavior in teens, b) contingency management and other incentive-based therapies for behavior change in adolescence, and c) components of longitudinal developmental projects, by identifying reward task features that yield the most robust mesolimbic signal, and are most germane to drug abuse risk.
Increased risk for drug use and other risky behavior in adolescence has been attributed to increased function of motivational neurocircuitry relative to immature executive control. Some neuroimaging studies show greater mesolimbic activation by rewards in adolescents in support of this theory, but other studies show the reverse. This project introduces a novel fMRI task that manipulates vigilance demand, availability of reward choice, and certainty of reward magnitude, to begin a program aimed at identifying reward contexts uniquely engaging to adolescents. The data could inform behavioral interventions and large developmental neuroimaging initiatives.
