From drag racing to abusing drugs, teenagers sometimes act with scant apparent regard for consequences. Scientists have linked this impulsiveness and risk-taking to immaturity of the brain region called the orbitofrontal cortex (OFC), which helps us control impulses to seek gratification when they are out of line with our overall goals. New NIDA-funded research suggests that, in addition to having an underdeveloped restraint system, the teenage brain generates more intense impulses than a child's or an adult's.
Drs. B.J. Casey and Adriana Galvan and colleagues at Cornell and Stanford Universities undertook the research because they doubted that the protracted development of the OFC fully explained age-related patterns of risk-taking. "Children and adolescents both have an immature prefrontal area, but only adolescents make risky decisions," says Dr. Galvan. "We speculated that the adolescent brain must be unique in some way that promotes risk-taking."
The researchers hypothesized that the nucleus accumbens (NAc) might play a complementary role to the OFC's in adolescent risk-taking. The NAc—whose functions include alerting and motivating us when we have an opportunity to obtain something desirable—generates the very impulses to act that the OFC moderates in the interests of safety and longer-term goals. As a result, if the NAc activity were highly sensitized at the same time the OFC response was weak, the drive to act could more markedly overbalance the inclination to caution—and youths would take more chances. Drs. Casey and Galvan's experiment confirmed their hypothesis, and also produced insights into the interplay between the NAc and OFC during reward learning.
Pirates and Payoffs
The investigators gave 13 children, 12 adolescents, and 12 adults an opportunity to win up to $25 playing a video game. First, a picture of a pirate would flash on the video screen, followed by a brief pause, then a picture of treasure chests. The goal was to remember which side the pirate appeared on and indicate it by pressing a button as quickly as possible after the chests appeared. After each correct response, the screen showed a picture of coins, signaling either a small, medium, or large payoff.
The researchers did not tell the participants about one element of the game: The pirate picture had three variants, each linked to one of the payoff levels. A correct response after a picture of the pirate holding a cup produced a picture of a single coin; the pirate with his sword brought two piles of coins; and the pirate with a telescope yielded four piles.
The participants played 90 rounds each and responded accurately on at least 96 percent of tries. The researchers registered the participants' reaction times and recorded brain activity in the NAc and OFC with functional magnetic resonance imaging.
Responding to Large Treasure
The neural responses during the large payoff confirmed previous findings that the OFC is underdeveloped in childhood and adolescence, relative to adulthood. Specifically, OFC activity was more diffuse—spread through a larger volume of tissue—among the two younger groups in the study, relative to the adults. Greater activity diffusion generally is a sign that a brain region is less mature and lacking tight organization. Children's OFC activity remained the same no matter which coin picture appeared and was the most intense of all the groups, suggesting that the process of evaluating the consequences of responding required greater effort.
Consistent with the researchers' hypothesis, the adolescent study participants exhibited twice as much NAc activity when they saw the large payoff, compared to the adults and children. "Our findings suggest that a normally developing adolescent's NAc and associated subcortical brain circuits dopaminerich areas that generate emotion, motivation, and reward—mature earlier than the prefrontal brain region," Dr. Galvan explains. As a result, until OFC development catches up in a person's early twenties, NAc-generated motivational drive overbalances OFC-instituted caution and forethought.
Learning and Motivation
The data on brain activity, together with observations of the study participants' reaction times, illuminated the dynamics of learning at each of the three developmental stages. Overall, the adults' performance and brain scans pointed to an integrated and—by the end of the trial—complete process of learning and responding. The adolescents demonstrated learning powers and processes intermediate between adults and children, and the children exhibited no obvious signs of learning.
Adults: Although only one adult could articulate the connections between the three pirate pictures and their associated payoff levels, every adult's reaction times indicated that his or her brain registered these relationships. At the start of the game, the adults responded equally quickly to all three pictures; by the end, they were responding fastest to the pirate linked to the big payoff, slowest to the small-payoff pirate, and with intermediate speed to the moderate-payoff pirate.
Like their reaction times, the adults' NAc activity levels quickly settled into a pattern of high, moderate, and low responses corresponding to the sizes of the rewards in play in each round. Their OFC activity levels declined from the early to the later rounds. The researchers suggest that the high initial OFC levels reflected the region's involvement in early learning, as it received information on the sizes of the payoffs and fed it back to the NAc. Once the NAc attuned its responses appropriately to the significance of each pirate, the need for OFC participation lessened, and OFC activity accordingly declined.
Adolescents: Instead of reacting with rapidity proportional to the reward size associated with each pirate, the teens responded quickest to the pirate that predicted the maximum payoff, and with similar slower speeds to the medium and small payoff pirates. It is likely that the adolescents felt highly motivated to obtain the highest reward, but failing that, desired the medium reward no more than the low one.
Like the adults, the teens ultimately evolved a trilevel pattern of NAc responsiveness, one for each of the three rewards. The teens' NAc activity levels also suggested that they experienced an all-or-nothing response to the rewards: by the later trials, they were higher than adults' responses to the large treasure, but lower in response to the low payoff at the end of the trials (see chart).
Children: Children reacted to all the pirate pictures with similar speed from the first to the last rounds of the game. Consistent with their lack of learning, their NAc intensity remained constant throughout the game, no matter the size of the reward on offer.
"The children's lack of learning may reflect the immaturity of the OFC in this early developmental stage or that the task was too complex for them to learn in the few trials given," says Dr. Galvan. It is also possible, she says, that children perceived the reward values differently than adolescents or adults. "The kids were happy to play the game and have an opportunity to win money, regardless of the amount," she observes.
"Our behavioral and imaging results suggest that the adolescent dopamine system is 'turned up' relative to adults', exaggerating rewards for teens," says Dr. Casey. Although people who care for teens may worry about risk-taking, the researchers say the power imbalance between impulse and behavior control during adolescence relative to adulthood makes sense. "Adolescence is when people strike out on their own, explore new opportunities, and seek social status—all risks worth taking in healthy contexts," says Dr. Casey.
"The results are impressive and consistent with previous findings of refinement and specialization of the brain's cerebral cortex as people mature," says Dr. Laurence Stanford of NIDA's Division of Clinical Neuroscience and Behavioral Research. "With age and maturation of the frontal cortex, the ability to control impulses generally improves," he says. Extrinsic influences, including drug abuse, however, may alter the normal course of maturation.
"To understand how drugs might change the motivational system and goal-oriented behavior, we first must understand how young people process rewards and the normal development of the brain circuits involved. Such information may help prevention specialists develop targeted interventions that are more meaningful to young people," says Dr. Stanford.
Galvan, A., et al. Earlier development of the accumbens relative to the orbitofrontal cortex might underlie risk-taking behavior in adolescents. The Journal of Neuroscience 26(25):6885-6892, 2006. [Full Text, PDF, 489 KB]
Galvan, A., et al. The role of ventral frontostriatal circuitry in reward-based learning in humans. The Journal of Neuroscience 25(38):8650-8656, 2005. [Full Text, PDF, 335 KB]