Behave: The Biology of Humans at Our Best and Worst
Page 17
Thus, as adolescence dawns, frontal cortical efficiency is diluted with extraneous synapses failing to make the grade, sluggish communication thanks to undermyelination, and a jumble of uncoordinated subregions working at cross-purposes; moreover, while the striatum is trying to help, a pinch hitter for the frontal cortex gets you only so far. Finally, the frontal cortex is being pickled in that ebb and flow of gonadal hormones. No wonder they act adolescent.
Frontal Cortical Changes in Cognition in Adolescence
To appreciate what frontal cortical maturation has to do with our best and worst behaviors, it’s helpful to first see how such maturation plays out in cognitive realms.
During adolescence there’s steady improvement in working memory, flexible rule use, executive organization, and frontal inhibitory regulation (e.g., task shifting). In general, these improvements are accompanied by increasing activity in frontal regions during tasks, with the extent of the increase predicting accuracy.10
Adolescents also improve at mentalization tasks (understanding someone else’s perspective). By this I don’t mean emotional perspective (stay tuned) but purer cognitive challenges, like understanding what objects look like from someone else’s perspective. The improvement in detecting irony reflects improvement in abstract cognitive perspective taking.
Frontal Cortical Changes in Emotional Regulation
Older teenagers experience emotions more intensely than do children or adults, something obvious to anyone who ever spent time as a teenager. For example, they are more reactive to faces expressing strong emotions.*11 In adults, looking at an “affective facial display” activates the amygdala, followed by activation of the emotion-regulating vmPFC as they habituate to the emotional content. In adolescence, though, the vmPFC response is less; thus the amygdaloid response keeps growing.
Chapter 2 introduced “reappraisal,” in which responses to strong emotional stimuli are regulated by thinking about them differently.12 Get a bad grade on an exam, and there’s an emotional pull toward “I’m stupid”; reappraisal might lead you instead to focus on your not having studied or having had a cold, to decide that the outcome was situational, rather than a function of your unchangeable constitution.
Reappraisal strategies get better during adolescence, with logical neurobiological underpinnings. Recall how in early adolescence, the ventral striatum, trying to be helpful, takes on some frontal tasks (fairly ineffectively, as it’s working above its pay grade). At that age reappraisal engages the ventral striatum; more activation predicts less amygdaloid activation and better emotional regulation. As the adolescent matures, the prefrontal cortex takes over the task, and emotions get steadier.*13
Bringing the striatum into the picture brings up dopamine and reward, thus bringing up the predilection of adolescents for bungee jumping.
ADOLESCENT RISK TAKING
In the foothills of the Sierras are California Caverns, a cave system that leads, after an initial narrow, twisting 30-foot descent down a hole, to an abrupt 180-foot drop (now navigable by rappelling). The Park Service has found skeletons at the bottom dating back centuries, explorers who took one step too far in the gloom. And the skeletons are always those of adolescents.
As shown experimentally, during risky decision making, adolescents activate the prefrontal cortex less than do adults; the less activity, the poorer the risk assessment. This poor assessment takes a particular form, as shown by Sarah-Jayne Blakemore of University College London.14 Have subjects estimate the likelihood of some event occurring (winning the lottery, dying in a plane crash); then tell them the actual likelihood. Such feedback can constitute good news (i.e., something good is actually more likely than the person estimated, or something bad is less likely). Conversely, the feedback can constitute bad news. Ask subjects to estimate the likelihood of the same events again. Adults incorporate the feedback into the new estimates. Adolescents update their estimates as adults do for good news, but feedback about bad news barely makes a dent. (Researcher: “How likely are you to have a car accident if you’re driving while drunk?” Adolescent: “One chance in a gazillion.” Researcher: “Actually, the risk is about 50 percent; what do you think your own chances are now?” Adolescent: “Hey, we’re talking about me; one chance in a gazillion.”) We’ve just explained why adolescents have two to four times the rate of pathological gambling as do adults.15
So adolescents take more risks and stink at risk assessment. But it’s not just that teenagers are more willing to take risks. After all, adolescents and adults don’t equally desire to do something risky and the adults simply don’t do it because of their frontal cortical maturity. There is an age difference in the sensations sought—adolescents are tempted to bungee jump; adults are tempted to cheat on their low-salt diet. Adolescence is characterized not only by more risking but by more novelty seeking as well.*16
Novelty craving permeates adolescence; it is when we usually develop our stable tastes in music, food, and fashion, with openness to novelty declining thereafter.17 And it’s not just a human phenomenon. Across the rodent life span, it’s adolescents who are most willing to eat a new food. Adolescent novelty seeking is particularly strong in other primates. Among many social mammals, adolescents of one sex leave their natal group, emigrating into another population, a classic means to avoid inbreeding. Among impalas there are groups of related females and offspring with one breeding male; the other males knock around disconsolately in “bachelor herds,” each scheming to usurp the breeding male. When a young male hits puberty, he is driven from the group by the breeding male (and to avoid some Oedipus nonsense, this is unlikely to be his father, who reigned many breeding males ago).
But not among primates. Take baboons. Suppose two troops encounter each other at some natural boundary—say, a stream. The males threaten each other for a while, eventually get bored, and resume whatever they were doing. Except there’s an adolescent, standing at the stream’s edge, riveted. New baboons, a whole bunch of ’em! He runs five steps toward them, runs back four, nervous, agitated. He gingerly crosses and sits on the other bank, scampering back should any new baboon glance at him.
So begins the slow process of transferring, spending more time each day with the new troop until he breaks the umbilical cord and spends the night. He wasn’t pushed out. Instead, if he has to spend one more day with the same monotonous baboons he’s known his whole life, he’ll scream. Among adolescent chimps it’s females who can’t get off the farm fast enough. We primates aren’t driven out at adolescence. Instead we desperately crave novelty.*
Thus, adolescence is about risk taking and novelty seeking. Where does the dopamine reward system fit in?
Recall from chapter 2 how the ventral tegmentum is the source of the mesolimbic dopamine projection to the nucleus accumbens, and of the mesocortical dopamine projection to the frontal cortex. During adolescence, dopamine projection density and signaling steadily increase in both pathways (although novelty seeking itself peaks at midadolescence, probably reflecting the emerging frontal regulation after that).18
Changes in the amount of dopaminergic activity in the “reward center” of the brain following different magnitudes of reward. For the adolescents, the highs are higher, the lows lower.
Visit bit.ly/2o3TBI8 for a larger version of this graph.
It’s unclear how much dopamine is released in anticipation of reward. Some studies show more anticipatory activation of reward pathways in adolescents than in adults, while others show the opposite, with the least dopaminergic responsiveness in adolescents who are most risk taking.19
Age differences in absolute levels of dopamine are less interesting than differences in patterns of release. In a great study, children, adolescents, and adults in brain scanners did some task where correct responses produced monetary rewards of varying sizes (see figure above).20 During this, prefrontal activation in both children and adolescents was diffuse and unfocused. However, act
ivation in the nucleus accumbens in adolescents was distinctive. In children, a correct answer produced roughly the same increase in activity regardless of size of reward. In adults, small, medium, and large rewards caused small, medium, and large increases in accumbens activity. And adolescents? After a medium reward things looked the same as in kids and adults. A large reward produced a humongous increase, much bigger than in adults. And the small reward? Accumbens activity declined. In other words, adolescents experience bigger-than-expected rewards more positively than do adults and smaller-than-expected rewards as aversive. A gyrating top, nearly skittering out of control.
This suggests that in adolescents strong rewards produce exaggerated dopaminergic signaling, and nice sensible rewards for prudent actions feel lousy. The immature frontal cortex hasn’t a prayer to counteract a dopamine system like this. But there is something puzzling.
Amid their crazy, unrestrained dopamine neurons, adolescents have reasoning skills that, in many domains of perceiving risk, match those of adults. Yet despite that, logic and reasoning are often jettisoned, and adolescents act adolescent. Work by Laurence Steinberg of Temple University has identified a key juncture where adolescents are particularly likely to leap before looking: when around peers.
PEERS, SOCIAL ACCEPTANCE, AND SOCIAL EXCLUSION
Adolescent vulnerability to peer pressure from friends, especially peers they want to accept them as friends, is storied. It can also be demonstrated experimentally. In one Steinberg study adolescents and adults took risks at the same rate in a video driving game. Adding two peers to egg them on had no effect on adults but tripled risk taking in adolescents. Moreover, in neuroimaging studies, peers egging subjects on (by intercom) lessens vmPFC activity and enhances ventral striatal activity in adolescents but not adults.21
Why do adolescents’ peers have such social power? For starters, adolescents are more social and more complexly social than children or adults. For example, a 2013 study showed that teens average more than four hundred Facebook friends, far more than do adults.22 Moreover, teen sociality is particularly about affect, and responsiveness to emotional signaling—recall the greater limbic and lesser frontal cortical response to emotional faces in adolescents. And teens don’t rack up four hundred Facebook friends for data for their sociology doctorates. Instead there is the frantic need to belong.
This produces teen vulnerability to peer pressure and emotional contagion. Moreover, such pressure is typically “deviance training,” increasing the odds of violence, substance abuse, crime, unsafe sex, and poor health habits (few teen gangs pressure kids to join them in tooth flossing followed by random acts of kindness). For example, in college dorms the excessive drinker is more likely to influence the teetotaling roommate than the reverse. The incidence of eating disorders in adolescents spreads among peers with a pattern resembling viral contagion. The same occurs with depression among female adolescents, reflecting their tendency to “co-ruminate” on problems, reinforcing one another’s negative affect.
Neuroimaging studies show the dramatic sensitivity of adolescents to peers. Ask adults to think about what they imagine others think of them, then about what they think of themselves. Two different, partially overlapping networks of frontal and limbic structures activate for the two tasks. But with adolescents the two profiles are the same. “What do you think about yourself?” is neurally answered with “Whatever everyone else thinks about me.”23
The frantic adolescent need to belong is shown beautifully in studies of the neurobiology of social exclusion. Naomi Eisenberger of UCLA developed the fiendishly clever “Cyberball” paradigm to make people feel snubbed.24 The subject lies in a brain scanner, believing she is playing an online game with two other people (naturally, they don’t exist—it’s a computer program). Each player occupies a spot on the screen, forming a triangle. The players toss a virtual ball among themselves; the subject is picking whom to throw to and believes the other two are doing the same. The ball is tossed for a while; then, unbeknownst to the subject, the experiment begins—the other two players stop throwing the ball to her. She’s being excluded by those creeps. In adults there is activation of the periaqueductal gray, anterior cingulate, amygdala, and insular cortex. Perfect—these regions are central to pain perception, anger, and disgust.* And then, after a delay, the ventrolateral PFC activates; the more activation, the more the cingulate and insula are silenced and the less subjects report being upset afterward. What’s this delayed vlPFC activation about? “Why am I getting upset? This is just a stupid game of catch.” The frontal cortex comes to the rescue with perspective, rationalization, and emotion regulation.
Now do the study with teenagers. Some show the adult neuroimaging profiles; these are ones who rate themselves as least sensitive to rejection and who spend the most time with friends. But for most teenagers, when social exclusion occurs, the vlPFC barely activates; the other changes are bigger than in adults, and the subjects report feeling lousier—adolescents lack sufficient frontal forcefulness to effectively hand-wave about why it doesn’t matter. Rejection hurts adolescents more, producing that stronger need to fit in.25
One neuroimaging study examined a neural building block of conformity.26 Watch a hand moving, and neurons in premotor regions that contribute to moving your own hand become a bit active—your brain is on the edge of imitating the movement. In the study, ten-year-olds watched film clips of hand movements or facial expressions; those most vulnerable to peer influence (assessed on a scale developed by Steinberg)* had the most premotor activation—but only for emotional facial expressions. In other words, kids who are more sensitive to peer pressure are more prepared to imitate someone else’s emotionality. (Given the age of the subjects, the authors framed their findings as potentially predictive of later teen behavior.)*
This atomistic level of explaining conformity might predict something about which teens are likely to join in a riot. But it doesn’t tell much about who chooses not to invite someone to a party because the cool kids think she’s a loser.
Another study showed neurobiological correlates of more abstract peer conformity. Recall how the adolescent ventral striatum helps the frontal cortex reappraise social exclusion. In this study, young adolescents most resistant to peer influence had the strongest such ventral striatal responses. And where might a stronger ventral striatum come from? You know the answer by now: you’ll see in the remaining chapters.
EMPATHY, SYMPATHY, AND MORAL REASONING
By adolescence, people are typically pretty good at perspective taking, seeing the world as someone else would. That’s usually when you’ll first hear the likes of “Well, I still disagree, but I can see how he feels that way, given his experience.”
Nonetheless, adolescents are not yet adults. Unlike adults, they are still better at first- than third-person perspective taking (“How would you feel in her situation?” versus “How does she feel in her situation?”).27 Adolescent moral judgments, while growing in sophistication, are still not at adult levels. Adolescents have left behind children’s egalitarian tendency to split resources evenly. Instead, adolescents mostly make meritocratic decisions (with a smattering of utilitarian and libertarian viewpoints thrown in); meritocratic thinking is more sophisticated than egalitarian, since the latter is solely about outcomes, while the former incorporates thinking about causes. Nonetheless, adolescents’ meritocratic thinking is less complex than adults’—for example, adolescents are as adept as adults at understanding how individual circumstances impact behavior, but not at understanding systemic circumstances.
As adolescents mature, they increasingly distinguish between intentional and accidental harm, viewing the former as worse.28 When contemplating the latter, there is now less activation of three brain regions related to pain processing, namely the amygdala, the insula, and the premotor areas (the last reflecting the tendency to cringe when hearing about pain being inflicted). Meanwhile, there is increasing dlPFC and vmPFC
activation when contemplating intentional harm. In other words, it is a frontal task to appreciate the painfulness of someone’s being harmed intentionally.
As adolescents mature, they also increasingly distinguish between harm to people and harm to objects (with the former viewed as worse); harm to people increasingly activates the amygdala, while the opposite occurs for harm to objects. Interestingly, as adolescents age, there is less differentiation between recommended punishment for intentional and unintentional damage to objects. In other words, the salient point about the damage becomes that, accidental or otherwise, the damn thing needs to be fixed—even if there is less crying over spilled milk, there is no less cleaning required.*
What about one of the greatest things about adolescents, with respect to this book’s concerns—their frenzied, agitated, incandescent ability to feel someone else’s pain, to feel everyone’s pain, to try to make everything right? A later chapter distinguishes between sympathy and empathy—between feeling for someone in pain and feeling as that someone. Adolescents are specialists at the latter, where the intensity of feeling as the other can border on being the other.
This intensity is no surprise, being at the intersection of many facets of adolescence. There are the abundant emotions and limbic gyrations. The highs are higher, the lows lower, empathic pain scalds, and the glow of doing the right thing makes it seem plausible that we are here for a purpose. Another contributing factor is the openness to novelty. An open mind is a prerequisite for an open heart, and the adolescent hunger for new experiences makes possible walking miles in lots of other people’s shoes. And there is the egoism of adolescence. During my late adolescence I hung out with Quakers, and they’d occasionally use the aphorism “All God has is thee.” This is the God of limited means, not just needing the help of humans to right a wrong, but needing you, you only, to do so. The appeal to egoism is tailor-made for adolescents. Throw in inexhaustible adolescent energy plus a feeling of omnipotence, and it seems possible to make the world whole, so why not?