Life and the Adolescent Brain
April 25, 2013
Reflection on our own adolescent years may include memories of excitingly risky activities or profound emotional vulnerability, or both. Risk and vulnerability are at the heart of two critical themes in research on adolescence. Adolescence is a period of heightened risk-taking behavior (Steinberg, 2008) and it is also the peak developmental period for the onset of psychological disturbance (Paus, Keshavan, & Giedd, 2008). However, a third theme in research on adolescence is at odds with these stereotypes of teenage emotional chaos and out-of-control behavior. This third theme highlights youth resilience and the ability to adapt and thrive in the expanding social world of the teenager (Crone & Dahl, 2012). Neuroscience unites these three themes by shedding light on the peculiarities of the adolescent brain and their impact on behavior. To understand adolescent behavior, it is helpful to look at what is happening in the adolescent brain – and this is a story that begins much earlier in life.
In the developing human brain, there is a massive early overgrowth in the number of connections or synapses between neurons (thus allowing a high degree of malleability in the brains of the very young). This early overabundance of synapses is countered by two major bouts of synaptic pruning, the first of which occurs in early childhood (around age three) and the second of which occurs during adolescence. Pruning drives a 50 to 55% decrease in the number of synapses across the entire cortex between late childhood and early adulthood. This cortical thinning is a marker of brain maturation and is associated with more adult-like cognitive abilities.
During pruning, any neural connections that have not been consistently used are eliminated. Thus, when we were adolescents, pruning served to streamline the efficiency of networks of neurons that we used most often. Our brains were sculpted to fit our own particular environment (a real life example of “you are what you do”). At the same time, increasing thickness and density of fatty white matter (myelin) served to insulate the “wiring” between neurons. This effectively boosted transmission power across the long connections in the brain that underlie the extended neural networks responsible for complex thought and behavior. Thus, during adolescence and young adulthood, pruning and myelination worked together to establish and strengthen the higher-order neural networks that we use for planning and regulating what we do.
Different brain areas develop at different rates and the prefrontal cortex (PFC) is among the last brain regions to mature. This is not surprising, since it is the most interconnected area of the brain. The PFC is referred to as the ‘C.E.O’ of the brain since it is involved in executive functioning. These executive functions include planning, decision making, and direction of working memory, i.e., the ongoing thought processes that allow us to complete tasks and plan for the future. The PFC is capable of promoting such complex human thought because of its connections with other members of the brain community. Within the adolescent PFC, pruning and myelination are creating big improvements in the fine-tuning of local connections, as well as profoundly strengthening its long-distance communications with the rest of the brain. These long-distance connections form the integrative neural networks responsible for higher level processing such as self-perception and goal-directed behavior, and so they are crucial to making rational decisions and regulating emotional drives. These “smart circuits” finish their development last because they are continuously refined and polished across adolescence and young adulthood. This is a big factor in the unique way that adolescents process their experiences and navigate their environment.
First, there may be a lag in the connections between the PFC and those regions of the brain devoted to motivated, reward-seeking behavior. A substance in the brain called dopamine is the primary chemical signaler in this network. During adolescence, there are excessive levels of dopamine in these regions, leading to increased activation of these reward systems. In turn, adolescents demonstrate elevated exploration and reward-seeking behavior. There is neuroimaging evidence that the adolescent PFC is not yet able to effectively inhibit this increased reward-seeking. These ‘rewards’ that an adolescent is after could be anything from drugs and alcohol to social acceptance. In the pursuit of the positive feelings driven by this circuitry, teens may drink, do drugs, or have unprotected sex.
The PFC has strong connections to brain regions that underlie emotion processing. These are areas of the brain that direct our survival-related behaviors (sometimes called “the four F’s” – feeding, fight, flight, and sex). These circuits also underlie the formation of social bonds, which have been key in our evolutionary history of overcoming hardships through group cooperation. These brain regions initiate pleasant emotional states of desire, as well as negative feelings when we feel fearful, ashamed, or rejected. When these ‘higher order’ social networks include a mature PFC, they allow us to behave effectively in social situations. The PFC guides our attempts to impress and comfort others, to empathize, and to having deeply meaningful exchanges with our fellow man about what we experience.
Neuroimaging technology allows scientists to measure activity within the neural circuits that underlie these behaviors. In one study, adolescents saw pictures of emotional images, such as the disapproving or angry face of a peer, while their brains were being imaged in a magnetic resonance imaging (MRI) scanner. Those adolescents who were less resistant to peer pressure showed evidence of weak connections between their prefrontal cortex and their reward areas, and those who were more socially resistant showed stronger, more mature connections between these areas. The authors of the study concluded that socially resistant adolescents were better able to recruit their PFC to help them regulate their emotions when faced with negative social information. It was suggested that this, in turn, may allow these young people to resist social pressure to engage in risky behavior (Grosbras et al., 2007).
Adolescents are more attuned to how others respond to them, picking up on subtleties of social exchanges and attributing meaning to them. They tend to be highly sociable and sensitive to acceptance and rejection from peers -- more so than children or adults. This social intensity may be due, in part, to higher adolescent levels of another chemical signaler, oxytocin, a hormone that enhances social emotions in mammals. A mature PFC is more able to modulate social highs and lows associated with social acceptance and social rejection. The still-developing adolescent PFC is less able to do so, so that a social threat is more likely to initiate the neural and hormonal cascades and negative feelings that we experience as stress. For example, an adolescent girl may angst over the details of exchanges with boys. Another teen’s self-esteem may be decimated by being picked last in gym class. Happily, this increased sensitivity to the social world may also allow social support to have an elevated beneficial effect for teens experiencing stress. For example, research on social buffering investigates how the presence of supporting and comforting others can help to decrease the intensity of the stress response and its associated negative feelings. These studies find that social buffering effects are amplified during adolescence, so that teens more readily absorb the positive effects of social support in the face of stress (Buwalda, Geerdink, Vidal, & Koolhaas, 2011). This finding suggests that interventions that enhance healthy social buffering may be particularly helpful for our stressed teens.
Adolescent brain development provides some insight into why adolescents take more risks, have increased odds of experiencing psychological distress, and rely heavily on peer approval. It has been argued, though, that study of the adolescent brain has done more to reveal that this developmental period is ripe with opportunity. Although the adolescent PFC is less efficient in inhibiting emotionally-driven impulses, a new line of research suggests that this very lack of maturity allows greater cognitive and social flexibility (Crone & Dahl, 2012). Not only is the adolescent brain is still changing and adapting to environments, it is able to more quickly switch attention to novel social features of the environment. This gives adolescents an advantage in navigating their complex social worlds, and in creatively pursuing new friendships and connections. Thus, neuroscience tells us that the adolescent brain is not merely immature, but rather it is perfectly suited to foster exploration of new environments, soak up the benefits of social closeness, and mold future adult capabilities.
Sarah Moore, graduate student, Human Development
Buwalda, B., Geerdink, M., Vidal, J., & Koolhaas, J.M. (2011). Social behavior and social stress in adolescence. Neuroscience & Biobehavioral Reviews, 35(8), 1713-1721.
Crone, E. A., & Dahl, R. E. (2012) Understanding adolescence as a period of social-affective engagement and goal flexibility. Nature Reviews Neuroscience, 13(9), 636-650.
Grosbras, M., Jansen, M., Leonard, G., McIntosh, A., Osswald, K., Poulsen, C., & Paus, T. (2007). Neural mechanisms of resistance to peer influence in early adolescence. The Journal of Neuroscience, 27(30), 8040-8045.
Paus, T., Keshavan, M., & Giedd, J. N. (2008). Why do many psychiatric disorders emerge during adolescence? Nature Reviews Neuroscience, 9(12), 947-957.
Steinberg, L. (2008). A social neuroscience perspective on adolescent risk-taking. Developmental Review, 28(1), 78-106.
BCTR Resources on adolescent neurology
Video of talks on the adolescent brain from the 2011 Bronfenbrenner Conference, The Neuroscience of Risky Decision Making, are available here. They include:
- Eveline Crone, Adolescent Brain Development: A Window of Opportunity for Learning and Social Cognition
- Jay Giedd, The Adolescent Brain: New Views from Neuroimaging
- Beatriz Luna, Adolescent Risk Taking: Immaturities in Cognitive Control and Reward Processing