These opposing findings may actually provide

These opposing findings may actually provide a useful conceptual foil, and suggest that provided the incentive task for fMRI features either: (a) very engaging visual stimuli (such as casino iconography, social cartoons or driving simulations), (b) decisions between rewarding options, or (c) peers present in the lab to potentially stoke the social reward of taking a risk, normative functional development of human mesolimbic incentive neurocircuitry features a non-linear trajectory, with peak in striatal responsiveness to rewards occurring around age 14–15. Conversely, blunted reward anticipation activation in adolescents tends to be found in more “work-like” reaction-time tasks that feature minimal visual stimuli, require intense vigilance and rapid responses, and/or have no decision-making component. It could be argued that the former class of tasks may be more naturalistically-relevant to real-world risk-taking scenarios for adolescents. Irrespective of developmental directionality, it is important to consider, however, that age-group typically accounts for only a modest portion of variance in VS responses to rewards due to substantial inter-subject variability (c.f. Fig. 2B of Somerville et al., 2010). For example, in the Bjork et al. (2010b) study, age group accounted for only 12% of the variance in right VS responses to high reward cues, despite a significant group-wise difference. This mid-adolescent peak in reward processing may be occurring against a backdrop of relatively immature or underactive frontocortical behavior control circuitry. A well-established literature indicates that with aging from adolescence to adulthood, cognitive control in rapid a01 evaluation tasks improves (Bunge and Wright, 2007), in tandem with more focal (potentially more efficient) frontocortical activation during inhibition (Durston et al., 2006). Developmental differences in frontocortical recruitment by potential penalties have also been detected. First, using a wheel-of-fortune (WoF) task, where probabilities for winning and magnitudes of potential wins were explicitly indicated by pie-chart displays, adolescents (compared to adults) showed reduced activation of posterior mFC when choosing lower-probability but more potentially rewarding (i.e. riskier) pie-slices (Eshel et al., 2007). This adolescent activation decrement occurred in a region of mFC consistently recruited by pre-decision conflict (Ridderinkhof et al., 2004a). Second, while performing a monetary game of “chicken” akin to the Balloon Analogue Risk-Taking Task (BART; Lejuez et al., 2002), accrual of risky reward (as a contrast with accruing guaranteed reward) activated posterior mFC in adults, but not in adolescents (Bjork et al., 2007). In both experiments, greater engagement in risk-taking behavior was associated with decrements in posterior mFC recruitment. This combined developmental pattern of a mid-adolescent peak in brain responsiveness to rewards, coupled with immature behavior control neurocircuitry has given rise to an influential dual-process model that ostensibly accounts in part for a proneness for engaging in illegal and risky behaviors beginning in adolescence (Somerville et al., 2010), such as experimentation with drugs and alcohol (Casey and Jones, 2010). Specifically, this opponent-process model posits a functional imbalance resulting from relatively rapid development of motivational circuits of the VS relative to more protracted development of behavior control circuits of frontal cortex, where this imbalance is most pronounced in mid-adolescence, and essentially normalizes or remits by young adulthood. This model seems very plausible in light of differing maturational trajectories among human brain structures (Brown et al., 2012) if morphometric differences with development (such as cortical thickness) reflect underlying functional differences. Moreover, preclinical research indicates greater DA responsiveness in younger relative to older animals (Luciana et al., 2012). Increased fMRI-measured activity in the ventral striatum is generally interpreted as reflecting greater phasic dopamine (DA) activity (Knutson and Gibbs, 2007), based on single unit primate studies of instrumental behavior (Schultz, 2007). A related “triadic” model of neurodevelopment (Ernst, 2014) features a more prominent role of developmental differences in fear/aversion neurocircuitry, and subdivides motivated behavior as the net output of approach and avoidance circuits, where each system of this opponent process is in turn governed by elements of a third, regulatory circuit.