The adolescent brain

Abstract

Adolescence is a developmental period characterized by suboptimal decisions and actions that are associated with an increased incidence of unintentional injuries, violence, substance 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 both childhood and adulthood. This review provides a biologically plausible model of the neural mechanisms underlying these nonlinear changes in behavior. We provide evidence from recent human brain imaging and animal studies that there is a heightened responsiveness to incentives and socioemotional contexts during this time, when impulse control is still relatively immature. These findings suggest differential development of bottom-up limbic systems, implicated in incentive and emotional processing, to top-down control systems during adolescence as compared to childhood and adulthood. This developmental pattern may be exacerbated in those adolescents prone to emotional reactivity, increasing the likelihood of poor outcomes.

FIGURE 1

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

FIGURE 2

Illustration of gray matter volume maturation over the cortical surface from 5 to 20 years of age (from Lenroot & Giedd 2006).

FIGURE 3

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

FIGURE 4

Magnitude and extent of accumbens and OFC activity to reward. Adolescents (13–17 years) showed greater percent signal change to large rewards than either children (aged 7–11 years) or adults (23–29 years) in the accumbens (A). Children had the greatest percent signal change in the OFC compared to adolescents and adults (B). Children had the greatest volume of activity in the accumbens relative to adolescents and adults (C) Children and adolescents showed greater volume of activity in the OFC than adults (D). Adapted from Galvan et al. (2006).

FIGURE 5

Bilateral amygdala activation (left). Graph depicts amygdala activity in adults, teenagers, and children. Adapted from Hare et al. (in press).

FIGURE 6

Activity in the nucleus accumbens in anticipation of reward (A). Percent change in fMRI signal in the accumbens in anticipation of reward as a function of age (B). The association between accumbens activity to reward and the likelihood of engaging in risky behavior in three age groups (C) (Adapted from Galvan et al. 2007).

FIGURE 7

Picture depicts left orbitofrontal activity. Graph illustrates correlation of activity in the OFC and in the amygdala in both adults and adolescents (adapted from Hare et al. in press).