The adolescent brain

Abstract

Adolescence is a developmental period characterized by suboptimal decisions and actions that give rise to an increased incidence of unintentional injuries and violence, alcohol and drug abuse, unintended pregnancy and sexually transmitted diseases. Traditional neurobiological and cognitive explanations for adolescent behavior have failed to account for the nonlinear changes in behavior observed during adolescence, relative to childhood and adulthood. This review provides a biologically plausible conceptualization of the neural mechanisms underlying these nonlinear changes in behavior, as a heightened responsiveness to incentives while impulse control is still relatively immature during this period. Recent human imaging and animal studies provide a biological basis for this view, suggesting differential development of limbic reward systems relative to top-down control systems during adolescence relative to childhood and adulthood. This developmental pattern may be exacerbated in those adolescents with a predisposition toward risk-taking, increasing the risk for poor outcomes.

Fig. 1

The traditional explanation of adolescent behavior has been suggested to be due to the protracted development of the prefrontal cortex (A). Our model takes into consideration the development of the prefrontal cortex together with subcortical limbic regions (e.g., nucleus accumbens) that have been implicated in risky choices and actions (B).

Fig. 2

The most common magnetic resonance methods used in the study of human development and are illustrated above. Structural magnetic resonance imaging (MRI) to produce structural images of the brain useful for anatomical and morphometric studies (A), diffusion tensor imaging (DTI) measures myelination and directionality of fiber tracts between anatomical structures (B), and functional MRI (fMRI) measures patterns of brain activity within those structures (C). Adapted from Casey, Galvan et al., 2005; Casey, Tottenham et al., 2005.

Fig. 3

Illustration of the brain regions showing the greatest structural changes over early and late adolescence (from Sowell et al., 1999).

Fig. 4

Localization of activity in anticipation of reward outcome in the nucleus accumbens (A) and orbital frontal cortex (B). The extent of activity in these regions is plotted as a function of age, for each individual subject showing protracted development of orbital frontal cortex relative to nucleus accumbens (C; from Galvan et al., 2006).

Fig. 5

Adolescents show enhanced activity of the accumbens relative to children and adults (A). Accumbens activity is positively associated with self-ratings of the likelihood of engaging in risky behavior (B) and negatively correlated with self-ratings of the likelihood of negative consequences of such behavior (C; from Galvan et al., 2007).