Welcome to Your Child's Brain: How the Mind Grows From Conception to College (16 page)

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
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 powerfully reorganize the brain. Many of the brain changes we have described may be organized and shaped by hormone signaling.

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 signals than they did in previous years, leading to a delayed bedtime.

With puberty also comes new social pressures. Even though they need only a little less sleep than children, adolescents are expected (or want) to adopt adultlike wake and sleep times. Their schools convene earlier in the morning. At the end of the day, there is homework, after-school activities, and spending time with friends. Intellectually and socially, their world is exploding. Even after bedtime, communications such as text messaging provide a continuing source of stimulation—and sleeplessness.

The net result is the need to catch up on lost sleep. In one study, researchers surveyed sleep habits in Swiss, German, and Austrian girls for up to nine years after their first menstrual period. The girls slept almost two hours longer per day on weekends than on weekdays, compared to less than an hour of catch-up in younger children and adults. Sleep debt has serious consequences, including reduced mental performance, depressed mood, impaired health, and weight gain.

One name for this adolescent tendency,
social jetlag
, suggests that they might be able to use some tricks of long-distance travelers. Here are a few:

1. Opening the blinds in the morning will activate the melanopsin pathway in your retinas. At this time of the circadian clock, exposure to light creates a tendency to get up a little earlier the next day.

2. Evening light leads to a later bedtime the next day. Combine that with a natural tendency to stay up, and it’s a recipe for continued night-owl behavior. So even if sleep isn’t coming easily, turn down the lights. And turn off that cell phone!

3. Exercise leads to secretion of melatonin by the pineal gland. An evening soccer game or run might be just the thing to start a brain on the road to sleep.

Although sex and stress hormones rise during late childhood and adolescence, in most cases researchers have found little evidence for a direct effect of hormones on typical adolescent behavior. Hormones are a key component for organizing the neural circuitry, but by itself testosterone is not very predictive of risk taking. The combination of a poor parent-child relationship with high testosterone has somewhat more predictive power. In adolescence, a good relationship forged in your child’s early years can pay off. This principle extends to siblings too: better relationships with brothers and sisters improve adjustment during adolescence.

A degree of impulsivity and aggression is probably unavoidable in life, but in some cultures, adolescent urges play a positive role. For example, among immigrants in big-city Chinatowns, aggression by male adolescents toward potentially violent intruders can protect the community from harm. Among the Mbuti, a hunter-gatherer group in the Congo, adolescents act on behalf of the group to punish deviations in adult behavior with mockery and even vandalism.

One hallmark of adolescent behavior in people and other mammals is an increase in what behavioral scientists call
approach
, the seeking of new social contacts and situations. Combined with other changes, this tendency can lead to the making of new friends—and also, sometimes, rebellion against older family members. Some conflict is typical, though extreme emotional turmoil in relationships with
parents is experienced by only about one in ten adolescents. This happens in other species as well. For instance, adolescent rats sometimes attack their parents.

Another typically teenage behavior, the tendency to seek novelty, is likely to be driven by the brain’s reward systems.
Dopamine
is a neurotransmitter involved in initiating action and movement and in signaling rewarding events. In brain scanning data, the orbitofrontal cortex and other regions that receive dopamine-secreting inputs are still maturing during the teen years. The
serotonergic system
—involved in sensation, movement, and mood—is also changing in the adolescent years. Awkwardness and moodiness might be linked to this change.

Another change in the brains of adolescents is a proliferation of receptors for the signaling chemical
oxytocin
. (Oxytocin is a
neuropeptide
—that is, a peptide used as a neurotransmitter.) Neuroscientists found that oxytocin mediates a wide variety of bonding behaviors. In people, oxytocin is secreted during feelings of romantic and parental love. Both mothers and fathers of an infant or a small child have more oxytocin; the higher their oxytocin, the more they touch, play with, and otherwise socially engage with their child—and each other. Indeed, these signals sometimes get crossed, so that a new mother having a loving thought toward her partner might feel her milk drop. Romeo and Juliet would also have felt a newly strengthened oxytocin signal.

Adolescence is a time when the interplay between brain and environment takes on new complexity. Early adolescent brain changes increase a child’s appetite for stimulation and social contact, while self-regulatory systems continue to mature through late adolescence. In modern society, adolescence is viewed in terms of the delay between sexual maturation and true independence. Indeed, sexual, physical, and intellectual maturation are spread out over a decade or more, providing many opportunities for growth and change. What an adolescent does with this biologically defined period of transition depends on his or her culture—and the choices that come along the way. Around the world, how and when people enter society during this process varies, ranging from child workers to continuing students with children of their own. In all cases the brain has found ways to adapt to local circumstances—a testament to its flexibility.

PART THREE
START MAKING SENSE

LEARNING TO SEE

CONNECT WITH YOUR BABY THROUGH
HEARING AND TOUCH

EAT DESSERT FIRST: FLAVOR
REFERENCES

Chapter 10
LEARNING TO SEE

AGES: BIRTH TO FIVE YEARS

As you haul your kids from music lessons to swim team practice, have you ever paused to give thanks that you don’t have to take them to vision class? Learning to see is a complicated process involving the coordinated development of dozens of brain areas. It does depend on experience: children’s eye problems can affect their ability to see. However, most parents get an almost completely free ride when it comes to their children’s eyesight.

This doesn’t mean your baby arrives ready to see. Far from it. An adult who could see as well as a newborn would be legally blind, with 20/600 vision. Babies’ visual acuity starts out forty times worse than adults’ and doesn’t become equal until four to six years of age.

Like other aspects of your child’s brain development, this maturation comes about as an interplay between genes and experience. In this case, the necessary experience is visual and is widely available to any baby who can see normally. Most baby books don’t talk much about this type of development (which scientists call
experience expectant
), perhaps because it doesn’t require or reward much effort from parents. But as we outlined at the beginning of this book, such robust (self-managing) processes are more the rule than the exception in early life. Experience-expectant development is one of the key reasons that most kids end up well adapted to their environment.