By Carl Sherman
Among adolescents, unintentional injury (primarily motor vehicle accidents) is the leading cause of death, homicide is next1, and excessive drinking, unprotected sex, and assorted misadventures leave a trail of turmoil in their wake. Why are otherwise smart, savvy young people notoriously prone to unwise actions that put their own and others’ health and safety at risk? The question has engaged a broad range of research from which an increasingly complex, nuanced picture is beginning to emerge.
Beyond an understanding of the biological and environmental interactions that characterize adolescent brain development in general, researchers are teasing apart the details behind individual differences: why only some teens are risk takers, only some of the time. Their answers could lead to strategies that buffer this vital and vulnerable stage of development against serious harm.
A Question of Connection
“A decade ago, you’d read articles suggesting that adolescents engage in high-risk behavior because the prefrontal cortex [a key brain area for judgment and self-control] was not fully developed,” says B.J. Casey, Ph.D., director of Sackler Institute for Developmental Psychobiology at Weill Medical College of Cornell University, and a Dana Alliance for Brain Initiatives member. “But [that area] is even less developed in children who don’t engage in such behavior. We now think more in terms of neural circuitry; how regions of the brain talk to one another.”
Recent research at Casey’s lab and elsewhere has begun to tell a story in which the evolution of physical and functional connections across the brain can help us understand the perils on the road from adolescence to adulthood.
“We came up with a model of imbalance within a widespread circuit: various regions are activated and the one that screams the loudest wins,” Casey says. Prominent within this decision-making circuit, in her view, are the emotionally reactive ventral striatum/nucleus accumbens, which responds to reward and the anticipation of reward, and cortical areas that inhibit impulses and regulate behavior.
The reward system, as Casey and others have shown, reaches maturity by adolescence and in fact appears to be highly reactive in that period. “It’s really banging away, while the prefrontal cortex is not quite fully developed,” she says. Problems arise primarily in emotionally charged situations. “Adolescents are quite capable of making rational decisions… they just have more difficulty in the heat of the moment.” A measured appreciation of severe long-term consequences is no match for the immediate gratification promised by fast driving, heavy drinking, or unprotected sex, when the brain is in this mode, she suggests.2
Beatriz Luna, Ph.D., professor of psychiatry and director of the Laboratory of Neurocognitive Development at the University of Pittsburgh, agrees that the cortical brain apparatus needed to regulate behavior on an adult level comes more or less on line in the course of adolescence, but “it’s a bit fragile and untested, and can be taxed by other demands.”
Her research suggests that what separates the teens from the adults is the strength of connections, structural and functional, between parts of the brain that make integrated activity efficient and reliable. “It’s the ability of the prefrontal cortex to engage in networks with regions across the brain that allows it to support complex processes we need for inhibition and emotional and social processing,” Luna says.
She and her colleagues analyzed fMRI data to determine effective connectivity—the degree to which areas of the brain fire together while performing a task, and the direction of control: which region regulates the other. Much of their research used an elegantly simple test of the ability to inhibit a response. When a flash of light appeared on a screen, participants were instructed to look in the opposite direction, rather than following the reflexive tendency to look toward it.
Children performed much worse than adults, with adolescents in between. The fMRI data showed a corresponding increase, with age, in the degree to which frontal regions and lower sensorimotor brain areas worked in synchrony to perform the task. What’s more, the conversation between brain regions was apparently “top-down”—improved ability to inhibit a response reflected greater strength in the signals that allowed the higher brain to direct behavior.3
“Looking at a light isn’t the same as risk-taking in the real world,” Luna says. “But if this simple system isn’t in place—the top-down control that makes it possible to say, ‘I want to do this, but I won’t do it’—you can imagine how more complex behaviors are handicapped.”
At least part of the explanation for less effective top-down control in adolescents than adults is anatomical. As neurons develop the insulating sheath of myelination (reflected by a relative increase in white matter) they carry messages faster and more efficiently, and it is well established that white matter volume in the brain rises from adolescence to adulthood.4 Researchers in Luna’s lab have shown that much of this development takes place in tracts connecting frontal and subcortical brain regions—the same circuits involved in inhibitory control.5
Who Takes Risks When
“A lot of risk taking occurs during adolescence, but not all adolescents are risk-takers,” observes Adriana Galvan, Ph.D., director of the GalvanLab for Developmental Neuroscience at UCLA. “It’s important not to lump all teenagers together.”
While a doctoral candidate at B.J. Casey’s lab, Galvan was part of a team that analyzed child, adolescent, and adult brain activity and the propensity for risk. “But when we looked at the data, there was a lot of variability within groups. People who reported more risk-taking behavior—at any age—showed a neurobiological correlate in the reward response.” In particular, nucleus accumbens-frontal cortex activity rose more, during a money-winning game, in teens who said they were more likely to engage in risky sex, heavy drinking, high-impact sports and the like.
Galvan is now exploring how individual differences may play out in the real world. There’s abundant evidence that stress can disrupt decision-making in general and, in adolescents in particular, to amplify the tendency toward risk-taking. “But there are great individual differences in stress response and perception,” she says.
An ongoing study that tracks daily stress levels, Galvan says, confirms that adolescents’ scores on a risk taking exercise do go up on high-stress days.7 But preliminary fMRI findings suggest that here, too, not all teens are equal: risk-taking only rises in those who show the most activation in the emotion-regulating limbic system on such days.
In dissecting the complexities of risk-taking, Laurence Steinberg, Ph.D., Distinguished University Professor of psychology at Temple University, is taking a closer look at another factor of well-recognized importance in teen life: peer influence.
In one series of experiments, adolescents and adults performed similarly in a simulated driving exercise. When teens took the test in the presence of two friends, however, risk-taking and its consequences—they ran more lights and had more crashes— rose dramatically, while adult performance was unaffected.
The difference, according to fMRI data collected during the driving exercise, was once more in brain areas associated with reward—in teens, but not adults, the nucleus accumbens-prefrontal cortex circuit became significantly more activated in the presence of peers.8
“We hope that parents and teens themselves will recognize from some of this work that they need to take into account the fact that teen judgment is not the same when they’re with their friends, that they do riskier things,” Steinberg says.
If the presence of peers increases teens’ risk-taking by heightening the reward response, the same circuit may potentially be enlisted to curb risky tendencies. A study in Beatriz Luna’s lab found that monetary incentives enhanced performance on the demanding eye-movement inhibition task for adolescents (but not adults). Not only did activity increase in the reward circuit, but also in the brain regions regulating eye movement itself.9
“It was as if the adolescent brain was saying, ‘since there’s a reward, let’s go full throttle,’” Luna says. “What incentive does is enhance the brain’s ability to do whatever it needs to do to get the reward… here that means pumping up inhibitory control.”
More generally, adaptability is an emerging theme in the study of adolescent risk-taking. “It’s not right if the story gets told as if this is some biologically driven maturational process not affected by context and environment,” Steinberg says. “We know that experience matters, what we’re just beginning to study as a field is how it plays out in the brain.”
He is exploring whether the perils of peer influence may be lessened by training individuals in ways shown to improve cognitive control. “We’re going to look at patterns of brain activity in the presence of peers among those who have and haven’t had the training,” he says.
To Dana Alliance member Abigail Baird, Ph.D., professor of psychology at Vassar College, the role of experience in a very broad sense—culture—in adolescent behavior should not be underestimated. She has described adolescence as “the social and emotional expression of the biological event known as puberty.”10
“There’s nothing in human behavior that’s simply biological or environmental,” Baird says. To understand adolescent risk-taking, she suggests, demands an appreciation of both.
In one experiment, she compared brain activity in adults and adolescents when asked to rate whether various scenarios were a good or bad idea. Both groups wisely rejected such notions as “swimming with sharks,” “biting a light bulb” or “jumping off a roof”—although the adults did so significantly faster. The difference was in the mental processes apparently involved: adults showed greater activation in the visual cortex and the insula (a brain area that translates thoughts into visceral sensations), while the prefrontal cortex worked harder in teens. In simple terms, the adults could visualize the prospect and respond immediately, while the teens had to mull it over, Baird suggests.11
“What I think happens is that teens just haven’t had enough experience to develop that gut system, those physical feelings of right and wrong that grown-ups can use in making decisions that they don’t have to think about.” With mature cognition, she adds, comes the ability to generalize from one’s own experience and that of others. An adult who has cut her hand on a glass might well be able to visualize and physically respond to the idea of “biting a light bulb” in a way a teenager can’t.
Her findings don’t contradict research that emphasizes the role of powerful drives toward reward in risk-taking, she says. The slow pace of thinking, versus an immediate gut response, could further disadvantage cognitive control in competing with emotional gratification.
Nor do her findings imply something lacking in the teen brain. A central task of this developmental stage is learning the rules of adult life within a particular culture, and “an inexperienced adolescent is a healthy adolescent,” Baird says. Some risk-taking comes with the teen territory. “The trick is to help them train up their system with informative, not lethal experience… One of my greatest concerns is that many teens aren’t getting that experience—that people are trying to protect them too much. I’d much rather a kid fall off her bike than crash the car.”
Adolescence with its attendant pitfalls, she points out, lasts far longer in the U.S. than elsewhere; there are some cultures where kids assume adult responsibilities by the time they’re 14 or 15. Do their brains look and act more “adult” than Americans’ of the same age, particularly in risk-taking situations? “I would give my left arm for that data,” Baird says.
Published October 2012
1 U.S. Department of Health and Human Services, Health Resources and Services Administration, Maternal and Child Health Bureau. Child Health USA 2011. Rockville, Maryland: U.S. Department of Health and Human Services, 2011: http://mchb.hrsa.gov/chusa11/hstat/hsa/pages/229am.html
2Casey, BJ et al. Braking and accelerating of the adolescent brain. J Res Adolesc. 2011 March 1; 21(1): 21–33.
3Hwang K, Velanova K, & Luna, B. Strengthening of top-down frontal cognitive control networks underlying the development of inhibitory control: a functional magnetic resonance imaging effective connectivity study. J. Neuroscience. 17 November, 2010; 30(46):15535-15545.
4 Giedd, JN. The teen brain: primed to learn, primed to take risks. Cerebrum 26 February 2009: http://www.dana.org/news/cerebrum/detail.aspx?id=19620
5 Asato MR et al. White Matter Development in Adolescence: A DTI Study. Cerebral Cortex September 2010; 20:2122–2131
6 Galvan, A et al. Risk-taking and the adolescent brain: who is at risk? Developmental Science 10:2 (2007), pp F8–F14
7Galván A & McGlennen KM. Daily stress increases risky decision-making in adolescents: a preliminary study. Dev Psychobiol. 2012 May; 54(4):433-40.
8Chein, J. et al. Peers increase adolescent risk taking by enhancing activity in the brain’s reward circuitry. Developmental Science 14 (2011): F1–F10.
9Geier CF. et al. Immaturities in reward processing and its influence on inhibitory control in adolescence. Cereb Cortex. 2010 Jul; 20(7):1613-29.
10Baird, A.A., Silver, S.H. (2011) The Teen Species: Why gender matters. (in press) Mercer Law Review Lead Articles Edition, 62(3): http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CDgQFjAE&url=http%3A%2F%2Ffaculty.vassar.edu%2Fabbaird%2Fabout%2Fpublications%2Fpdfs%2FBaird_Mercer_easyread.doc&ei=tF9PUKCRK-P00gG644C4Cg&usg=AFQjCNGQQ0iZwmioUfI3C6tC-TovQvGAhQ
11Baird AA, et al, “What were you thinking?” A neural signature associated with reasoning in adolescence. Poster presentation: 12th annual Cognitive Neuroscience Society meeting 2005: http://faculty.vassar.edu/abbaird//research/presentations/pdfs/CNS_05_ab.pdf