WELCOME TO YOUR CHILD’S BRAIN, Cont’d is only 85 percent of adult size and continues to grow even into the forties. As white matter grows, axonal fibers are likely to be widening, and fatter axons transmit signals at higher speeds. Because white-matter axons mediate communication between distant brain regions, this change is likely to have strong functional implications—though at present we don’t know what they are. The tempo of developmental change varies from child to child. In a study of children whose brains were imaged repeatedly as they passed through late childhood and adolescence, children of higher intelligence had gray matter thickness that rose to peak more steeply—and declined more quickly as well. This result suggests the possibility that a key to intelligence is not brain size but capacity for change, though these differences are too variable for evaluating individuals. Indeed, increases and decreases in gray matter thickness also appear in childhood-onset schizophrenia and ADHD, so these structural changes may reflect a variety of underlying processes in different children. What do all these brain changes mean for your adolescent’s thought—or lack thereof, as the case may be? The relatively late maturation of the frontal cortex has received a lot of media attention recently as a way of explaining adolescent impulsivity. Even one car insurance advertisement points out that this brain region is not done growing. This area participates in executive function tasks, such as self-control, planning, and resisting temptation (see chapter 13). It becomes more active with age, an exception to the general trend of decreasing activity. In anterior and superior regions of the frontal cortex, activity rises from ages twelve to thirty. In combination with the earlier maturation of subcortical areas participating in emotion and reward, adolescence is a time when the balance between impulse and restraint may be quite different from either childhood or adulthood. Adolescents seek novel experiences more often and weigh positive and negative outcomes differently from adults. Such judgments can be probed using the Iowa Gambling Task, a game in which people can pick cards from several decks to win play money. Without the player’s knowledge, some decks are stacked, leading to more losses overall, but large occasional gains. In a version of the game in which participants can play or pass, adolescents learn to prefer winning decks but are less prone to avoid losing decks. Only in their late teens do players show full To superficial appearances, the adolescent brain appears to be nearly finished. In fact, it is undergoing considerable reorganization, avoidance of bad outcomes. In this game, then, adolescents make decisions that recognize the possibility of a lucky win but give little weight to losing. This laboratory finding is reminiscent of the real-life observation that teenagers tend to underestimate the consequences of their actions. This tendency, noted since ancient Roman times, is seen in areas as diverse as unprotected sex, experimenting with drugs, and impulsive speech. Even though adolescents are physically healthy, this risk taking makes the mortality rate of this life phase high. Sam is fortunate to have survived his own youth, during which he habitually returned very late at night from social outings—the reward. Once he got into a bad car crash—a risk of staying up to the point of drowsiness unforeseen by his adolescent brain, which was focused on the short term. These forms of impulsivity come at a time when white-matter connections between the frontal cortex and other parts of the brain that handle reward and emotion are not yet complete. Teenaged laboratory subjects are more likely to take a risk in games where there is a possible reward (money or the display of a happy face). When placed in an fMRI scanner, the teenagers showed more activity than adults do in the ventral striatum, a region that can signal anticipation of a reward. Another late-maturing participant in impulse control, the orbitofrontal cortex, appears to orchestrate the connection between emotion and good judgment. In general, decisions are often informed by the brain’s evaluation of whether an outcome is desirable or undesirable. Such a decision carries some emotional weight—even when it’s as simple as picking an outfit to wear. People with damage to their orbitofrontal cortex are unable to sensibly manage their lives, making bad investments and unsuitable life choices. One patient (known by his initials EVR) had a benign tumor pushing on his orbitofrontal cortex. He lost his job, left his wife, married a prostitute, and divorced again in a matter of months. Removal of the tumor reduced these unadaptive behaviors. Adolescent changes in mood, aggressiveness, and social behavior are driven by other aspects of brain development. These changes may be linked to increases in size and activity of the amygdala, a part of the forebrain that processes strong emotions, both positive and negative. Even puberty itself is ultimately driven by the brain, because the hypothalamus, a grape-sized structure that sits under the brain just in front of the brainstem, secretes gonadotropin-releasing hormone as the first step in a chain reaction that ultimately leads to the release of estrogens and testosterone to drive sexual maturation. Together, these hormones powerAdolescence: MYTH: ADOLESCENTS HAVE A LONGER DAY-NIGHT CYCLE The eight-year-old who got up early every morning has turned into a sluggish teenager. Although his body is in front of you, his brain is at least one time zone to the west. Everyone else is getting up, but he still wants to sleep—a kind of Adolescent Savings Time. What is going on? Our brain’s circadian rhythm sets the times that we want to wake and sleep (see chapter 7). Individuals vary, so that larks have peaks and troughs earlier in the day than night owls. Adolescence is accompanied by a shift toward evening wakefulness— and not just in people. At puberty, a shift of one to four hours has also been seen in monkeys and a variety of rodents. One popular view is that adolescents have a longer day-night cycle. This impression is false; if you take away normal light-dark signals or suddenly shift the signals, a teenager’s internal clock will react the same way as everyone else’s. But a real difference in adolescent circadian rhythms is a decrease in melatonin levels, as well as a shift in the time when melatonin rises and falls. Melatonin helps trigger the onset of sleep. When puberty hits, nocturnal melatonin levels decline sharply, continuing a general decreasing trend that started back in infancy. So it’s possible that adolescents are simply experiencing smaller and later sleep
Missoula, Montana
June 23-26, 2011