A second form of support for the imbalance model of adolescent development comes from studies that directly examine how brain systems interact when self-control is required in a motivational or emotional context. Incentives can both motivate (Hardin et al., 2009) and interfere with (Somerville, Fani, and McClure-Tone, 2011) cognitive functioning in adolescents. Geier and colleagues (2010) have shown enhancement of behavioral control by adolescents as compared with adults when a financial reward was promised for accurate performance relative to when it was not. Relative to adults, adolescents had exaggerated activation in the ventral striatum when preparing and executing a response that would be reinforced and an increase in prefrontal activity important for controlling the movements, suggesting a reward-related up-regulation in control regions. In contrast, Somerville and colleagues (2011) have shown that adolescents’ performance is worse than both children and adults when having to suppress a response to an alluring social cue relative to a neutral one. This inverted-U pattern of performance is paralleled by a similar inflection in ventral striatal activity and heightened prefrontal activity.
Perhaps the most compelling imaging findings supportive of the imbalance model are those by Chein and colleagues (2011). They examined the neural basis of riskier driving decisions by adolescents relative to adults in the presence of peers during a simulated driving task. Adolescents, but not adults, showed heightened activity in reward-related circuitry, including the ventral striatum, in the presence of peers. This activity was inversely correlated with subjective ratings on resistance to peer influences. Individuals rating themselves low on this scale showed more reward-related brain activity in the presence of peers. Not only are peers influential but also positive exchanges with others may be powerful motivators (Baumeister and Leary, 1995; Steinberg et al., 2008). Asynchronous development of brain systems appears to correspond with a shift from thinking about self to thinking about others from early adolescence to young adulthood (van den Bos et al., 2011). Together these studies suggest that in the heat of the moment, as in the presence of peers or rewards, functionally mature reward centers of the brain may hijack less mature control systems in adolescents.
Although regional changes in brain structure and function are important in understanding how behavior changes during adolescence, development in the connections between brain regions with age and experience are equally important (Casey et al., 2005). There are two relatively new approaches to indexing human brain connectivity. The first is that of diffusion tensor imaging (DTI). DTI detects changes in white matter tracts related to myelination, the process through which nerve fibers become