Functional Brain Development
The most influential method for studying human brain development is that of functional magnetic resonance imaging (fMRI). This method allows for seeing what areas of the brain are active when an individual is behaving by indexing changes in blood oxygen levels in the brain. In the last decade, there has been an explosion of fMRI studies examining adolescent brain development (Casey et al., 2008). This work challenges the traditional view that changes in behavior during adolescence are due simply to immature cognitive control capacities and the underlying neural substrates (e.g., prefrontal cortex). Instead, the latest studies suggest that much of what distinguishes adolescents from children and adults is an imbalance among developing brain systems (Casey, Getz, and Galvin, 2008; Steinberg et al., 2008). This imbalance model implies dual systems: one that is involved in cognitive and behavioral control and one that is involved in socioemotional processes. Accordingly, adolescents lack mature capacity for self-regulation because the brain system that influences pleasure-seeking and emotional reactivity develops more rapidly than the brain system that supports self-control.
Empirical evidence to support this view comes from three areas of work. First, prefrontal circuitry implicated in self-regulation and planning behavior continues to develop into young adulthood (Casey et al., 1997, 2002; Luna et al., 2001; Bunge et al., 2002; Klingberg, Forssberg, and Westerberg, 2002; Bitan et al., 2006). This development is slow and linear in nature. Specifically, adolescents tend to recruit prefrontal regions less efficiently than adults, and these areas become more fine-tuned with age and experience (Casey et al., 1995; Brown et al., 2005; Durston et al., 2006). For example, imaging studies using tasks in which children and adolescents are asked to suppress a compelling response or to look away from a target have shown less focal prefrontal recruitment than in adults (Casey et al., 1995; Luna et al., 2001; Durston et al., 2006). These studies provide insights into the role of prefrontal circuitry in behavior regulation across development, but they do not speak to the heightened sensitivity of adolescents to rewards and emotional cues.
Several research teams (May et al., 2004; Ernst et al., 2005; Galvan et al., 2006; Geier et al., 2010; Van Leijenhorst et al., 2010) have examined brain systems involved in reward to address this issue. Their studies (Bjork et al., 2004) have shown enhanced sensitivity to rewards in adolescents, relative to children and adults. For example, Van Leijenhorst and colleagues (2010) showed exaggerated ventral striatal responses in adolescents during the anticipation and receipt of a monetary reward. The magnitude of activity in this region is associated with real-world behavior. Specifically, greater ventral striatal activity to rewards is predictive of risk-taking tendencies (Galvan et al., 2007).