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Teenage Head

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Why adolescence can be so baffling

By Angela Pirisi


Adolescence conjures up the opening line from Charles Dickens’s A Tale of Two Cities: “It was the best of times, it was the worst of times….” It’s a period when the sense of adventure and discovery merge messily with distraction, sensation-seeking, emotional intensity and poor decision-making.

Fresh perspectives about adolescent behaviour have emerged as new inroads are made into understanding the adolescent brain. Over the last decade, thanks largely to advances in imaging technology, science has been starting to explain the baffling behaviour that makes parents and health professionals alike scratch their heads.

Any adult would agree that it’s annoying and challenging to live through a major home renovation. For adolescents, that’s exactly what they’re going through—only it’s happening in their brain. Contrary to the long-held belief that structurally the human brain is set like cement in early childhood, recent research has shown that this isn’t the case.

“It was believed that all major changes in the brain happened up until age 6 because by then, the brain is at about 95 per cent of its adult weight,” says Dr. Jean Clinton, a child and adolescent psychiatrist and associate clinical professor in the Department of Psychiatry and Behavioural Neuroscience at McMaster University in Hamilton, Ont. But groundbreaking work begun in the late ’90s by Dr. Jay Giedd, a neuroscientist at the National Institutes of Health in Bethesda, Md., revealed otherwise. Giedd initiated a longitudinal study that involved scanning the brains of young people every couple of years. What he saw was a change in the grey-to-white–matter ratio. “Other researchers then started looking at structural changes in the teen brain, and these changes are profound,” says Clinton.

Any adult would agree that it’s annoying and challenging to live through a major home renovation. For adolescents, that’s exactly what they’re going through—only it’s happening in their brain.

Dr. Deborah Yurgelun-Todd, director of the Cognitive Neuroimaging Laboratory at the University of Utah Brain Institute, sees these profound changes. “The brain weighs the same in a 12-year-old as in an 18-year-old, but how that tissue works and how it’s sculpted is different,” she says. The 12-year-old brain may be 95 per cent of its full-grown weight, but it’s only halfway in its maturation process, which continues until about age 24.

So what’s happening in there? Key changes have to do with what is known as proliferation and pruning. The grey matter, or “thinking part of the brain,” as Giedd calls it, continues to thicken throughout childhood and peaks at about puberty (age 11 in girls and 12 in boys). By this time, the brain has sprouted thousands of new neuronal connections, much like the branches of a tree. The next stage is marked by pruning of the new connections and consequently a thinning of the grey matter—basically, connections that get used grow stronger, and less used ones get whittled away for good.

The structural changes that shape adolescent behaviour happen in many areas of the brain, but three are particularly noteworthy: the nucleus accumbens (responsible for reward-seeking behaviour), the amygdala and the prefrontal cortex. “Kids are terrific in that they’re developing new skills and abilities, but they’re very challenging because their reward-seeking and thrill-seeking systems are under construction, as is executive function (which develops later than thrill-seeking),” says Clinton. “So there’s a mismatch between reward drive and ‘stop-and-think-about-it.’” That gap between doing and thinking is at the basis of so many accidental deaths and injury among youth, adds Clinton.

Since the nucleus accumbens is still immature in adolescents, it looks for maximum excitement using minimal effort, thus explaining why video gaming is such a common pursuit among teens. The amygdala is responsible for emotional responses, including the explosive behaviour often associated with turbulent teenagehood. It goes into a sort of overdrive during adolescence—in fact, studies show that adolescents use emotional processing a lot, applying an emotional lens when adults would rely on reason. The prefrontal cortex, the judgment part of the brain, undergoes changes as well, but it’s the tortoise in the race. Although it is responsible for planning, setting goals and predicting the consequences of action, the prefrontal cortex is the last part of the brain to mature, which means that logic takes a backseat to visceral reactions.

In one study where adolescents underwent the Stroop interference task (which tests the ability to ignore interfering stimuli), results showed an age-related change in the prefrontal cortex. “As you mature from age 8 to 18, your ability to focus your attention improves. Using fMRI, we showed that activation differences in certain regions of the brain correlated with that improvement,” says Yurgelun-Todd. “These findings are more about cognitive processes than affective or emotional processing, but they’re still completely related to differences in mood and the ability to modulate affect because it is this change in frontal cortex that provides you with increasing skills to use judgment.”

The dynamic state of the adolescent brain and the changes that occur are what increases the brain’s potential, but they also render the brain vulnerable to adverse events. For example, studies have revealed that anxiety disorders, bipolar disorder, depression, eating disorders, psychosis (including schizophrenia) and substance abuse typically surface in adolescence.

The dynamic state of the adolescent brain and the changes that occur are what increases the brain’s potential, but they also render the brain vulnerable to adverse events.

Neurobiological changes that are part of normal adolescent development seem to coincide with the emergence of mental illness. For example, proliferation turns to pruning by early adolescence, which coincides with a period of higher vulnerability to schizophrenia. Dr. Martin Teicher, director of the Developmental Biopsychiatry Research Program at McLean Hospital in Belmont, Mass., suggests in his research that the pruning that is supposed to help fine-tune the brain could in fact be unmasking a congenital defect in people with schizophrenia. On the other hand, pruning in the striatum seems to coincide with the decreasing occurrence of attention-deficit/hyperactivity disorder and Tourette’s syndrome in adolescence.

However, Dr. Martin Alda, an expert in mood disorders and a professor of psychiatry at Dalhousie University in Halifax, N.S., disputes the notion that mental illness often begins in adolescence. Currently, Alda and Dr. Anne Duffy, also a psychiatry professor at Dalhousie, are conducting research that looks prospectively at children who are genetically at risk for bipolar disorder, suggests that children often develop some symptoms earlier than in adolescence, but they aren’t classic symptoms, so they may not meet diagnostic criteria. “Children may display early symptoms of bipolar disorder, such as periods of difficulty with sleep and anxiety, but it’s only in adolescence that they usually develop symptoms of depression, and only in later adolescence or early adulthood that they develop symptoms of mania or hypomania,” says Alda. “It seems as if something is already happening in the brain, but the behavioural expression of what’s going on depends on the person’s age.”

Yurgelun-Todd agrees that mental illness may often progress until symptoms are obvious enough to be diagnosed in adulthood. “Historically, we thought many of the serious mental illnesses were more adult problems; now we know they start to emerge over a number of years. For example, most individuals live seven years, on average, with bipolar disorder before seeking treatment,” she explains.

What is less understood is the role of hormones in mental illness, says Alda: “Hormones in general, including cortisol, gonadal and sex hormones, are involved in mood regulation, so it’s reasonable to assume that they play some role, but it’s not clear exactly what that is.” What we do know from data is that schizophrenia develops, on average, four years earlier in males than in females and that there is a 2:1 female to male ratio of depression rates at mid-puberty—the ratio is equal before puberty.

What we don’t know, however, and what has been lacking in the research is longitudinal data that could show exactly what is happening in the brain structurally and functionally prior to the first symptoms or episode of mental illness, says Yurgelun-Todd. Adding another level of complexity is trying to tease out the interplay between surging hormones, structural changes in the brain and the shifting psychosocial environment that seem to coincide during adolescence.

The adolescent years seem to present a unique period of vulnerability to substance use issues as well. “Because adolescence is a period of high-risk engagement and taking chances, it’s a time of opportunity, but also a time for negative, high-risk behaviour, one of which is substance abuse,” says Yurgelun-Todd.

The chances of altering structural development are greater in adolescence, and the neurotoxic events that might be associated with substance use are greater in adolescence than they appear to be in adulthood.

An immature prefrontal cortex, the seat of judgment, makes teens more impulsive and likely to try alcohol and other drugs. The reward-seeking nucleus accumbens makes them more likely to seek out activities with maximum reward and minimum effort—a quick fix—and want to repeat the experience, explains Clinton. Alcohol makes them feel more socially disinhibited, which leads to even more risk.

Substance use itself can alter brain development. The potential for brain damage is age dependent: the younger the brain, the more damage. “The chances of altering structural development are greater in adolescence,” says Yurgelun-Todd, “and the neurotoxic events that might be associated with substance use are greater in adolescence than they appear to be in adulthood.”

The good news is that the changing brain can be directed or redirected, just as a climbing vine can be coaxed along a trellis. Thanks to plasticity, that means that the brain’s final blueprint can be greatly influenced by experience. For example, Clinton says we can speed up executive function, but societal norms also have to shift from the protracted period we view as adolescence and teenagehood. “We need to rethink how we view adolescence,” she says. “In other cultures where young people are given responsibility, have intergenerational connection and have a sense of purpose within the society, there isn’t the extended period of time we call adolescence, and there’s less risky behaviour.”

For those in the mental health care sector who work with youth, whether regularly or rarely, Clinton emphasizes that they “need to understand that the teen brain is changing in ways that are very much linked to some of the challenging behaviours we see—that concept of the brain under construction is central; for example, planning and organizing are under construction and executive function may not develop until age 25.” Keeping that in mind helps when teen clients show up late for an appointment or without their health card for the fourth time, says Clinton. “What we think affects how we feel, how we act. If we see this as a skill ‘under construction,’ we can help them develop the strategies for organizing, rather than think it is non-compliant behaviour,” she adds.

Yurgelun-Todd lends some perspective to working with youth who have substance use issues. “Depending on where they are in their addiction and their age, that will impact their cognitive-emotional processing, so you can’t assume you’re working with someone who can engage with you in the same way as someone not using those substances,” she says. “The ability to read affect, to focus, to redirect energy if they’ve been engaged in drug use for a while will be limited.”

Yurgelun-Todd offers more tips for dealing with youth, particularly those with mental illness or substance use issues: “Trying to oversimplify messages and repeat them more is probably the best way to work with these individuals. If you lost the last 10 per cent of every sentence you heard, what would your take-home message be? If you have an attention problem, you’re not hearing every word or interpreting it the same way. Attention span will vary, motivation to engage in getting better will vary—and that’s true for healthy adolescents compared with adults as well.”

More than a century ago, eminent psychologist G.S. Hall described adolescence as a period of heightened “storm and stress.” The question is: What determines how well youth weather that storm? The growing body of brain research is attempting to address that question by examining what happens in the course of normal development, as well as what can go wrong and how. But let’s not get too cocky, says Yurgelun-Todd: “We have to be cautious about how much we think we know. We know that the brain is changing, that it’s not fully mature and that it has some resilience, but we still have a lot to learn.”

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