Your teenage daughter gets top marks in school, captains the debate team, and volunteers at a shelter for homeless people. But while driving the family car, she text-messages her best friend and rear-ends another vehicle.
How can teens be so clever, accomplished, and responsible—and reckless at the same time? Easily, according to two physicians at Children’s Hospital Boston and Harvard Medical School (HMS) who have been exploring the unique structure and chemistry of the adolescent brain. “The teenage brain is not just an adult brain with fewer miles on it,” says Frances E. Jensen, a professor of neurology. “It’s a paradoxical time of development. These are people with very sharp brains, but they’re not quite sure what to do with them.”
Research during the past 10 years, powered by technology such as functional magnetic resonance imaging, has revealed that young brains have both fast-growing synapses and sections that remain unconnected. This leaves teens easily influenced by their environment and more prone to impulsive behavior, even without the impact of souped-up hormones and any genetic or family predispositions.
Most teenagers don’t understand their mental hardwiring, so Jensen, whose laboratory research focuses on newborn-brain injury, and David K. Urion, an associate professor of neurology who treats children with cognitive impairments like autism and attention deficit disorder, are giving lectures at secondary schools and other likely places. They hope to inform students, parents, educators, and even fellow scientists about these new data, which have wide-ranging implications for how we teach, punish, and medically treat this age group. As Jensen told some 50 workshop attendees at Boston’s Museum of Science in April, “This is the first generation of teenagers that has access to this information, and they need to understand some of their vulnerabilities.”
Human and animal studies, Jensen and Urion note, have shown that the brain grows and changes continually in young people—and that it is only about 80 percent developed in adolescents. The largest part, the cortex, is divided into lobes that mature from back to front. The last section to connect is the frontal lobe, responsible for cognitive processes such as reasoning, planning, and judgment. Normally this mental merger is not completed until somewhere between ages 25 and 30—much later than these two neurologists were taught in medical school.
There are also gender differences in brain development. As Urion and Jensen explain, the part of our brain that processes information expands during childhood and then begins to thin, peaking in girls at roughly 12 to 14 years old and in boys about two years later. This suggests that girls and boys may be ready to absorb challenging material at different stages, and that schools may be missing opportunities to reach them.
Meanwhile, the neural networks that help brain cells (neurons) communicate through chemical signals are enlarging in teen brains. Learning takes place at the synapses between neurons, as cells excite or inhibit one another and develop more robust synapses with repeated stimulation. This cellular excitement, or “long-term potentiation,” enables children and teenagers to learn languages or musical instruments more easily than adults.
On the flip side, this plasticity also makes adolescent brains more vulnerable to external stressors, as Jensen and Urion point out.
Teen brains, for example, are more susceptible than their adult counterparts to alcohol-induced toxicity. Jensen highlights an experiment in which rat brain cells were exposed to alcohol, which blocks certain synaptic activity. When the alcohol was washed out, the adult cells recovered while the adolescent cells remained “disabled.” And because studies show that marijuana (cannabinoid) use blocks cell signaling in the brain, according to Jensen, “We make the point that what you did on the weekend is still with you during that test on Thursday. You’ve been trying to study with a self-induced learning disability.”
Similarly, even though there is evidence that sleep is important for learning and memory, teenagers are notoriously sleep-deprived. Studying right before bedtime can help cement the information under review, Jensen notes. So can aerobic exercise, says Urion, bemoaning the current lack of physical-education opportunities for many American youths.
Teens are also bombarded by information in this electronic age, and multitasking is as routine as chatting with friends on line. But Jensen highlights a recent study showing how sensory overload can hinder undergraduates’ ability to recall words. “It’s truly a brave new world. Our brains, evolutionarily, have never been subjected to the amount of cognitive input that’s coming at us,” she says. “You can’t close down the world. All you can do is educate kids to help them manage this.” For his part, Urion believes programs aimed at preventing risky adolescent behaviors would be more effective if they offered practical strategies for making in-the-moment decisions, rather than merely lecturing teens about the behaviors themselves. (“I have yet to meet a pregnant teenager who didn’t know biologically how this transpired,” he says.)
By raising awareness of this paradoxical period in brain development, the neurologists hope to help young people cope with their challenges, as well as recognize their considerable strengths.
It’s not easy being a teenager.
The teen years can play out like a choose-your-own-adventure novel, where everyday temptations lead to tough decisions. What if I took that big jump on my bike? What’s the worst thing that could happen if I snuck out after curfew? Should I try smoking?
Teenagers must act on an endless parade of choices. Some choices, including smoking, come with serious consequences. As a result, adolescents often find themselves trapped between their impulsive tendencies (Just try it!) and their newfound ability to make well-informed and logical choices (Wait, maybe that’s not such a good idea!).
So what makes the teenager’s brain so complex? What drives adolescents — more than any other age group — to sometimes make rash or questionable decisions? By peering into the brains of teenagers, scientists who study brain development have begun finding answers.
The evolved teenager
If you have ever thought that the choices teenagers make are all about exploring and pushing limits, you are on to something. Experts believe that this tendency marks a necessary phase in teen development. The process helps prepare teenagers to confront the world on their own. It is something all humans have evolved to experience — yes, teens everywhere go through this exploratory period. Nor is it unique to people: Even laboratory mice experience a similar phase during their development.
For example, laboratory experiments show that young mice stay close by their mothers for safety. As mice grow, their behavior does too. “When they reach puberty, they’re like, ‘I’m gonna start checking out how this environment looks without my mom,’” explains Beatriz Luna, of the University of Pittsburgh.
As a developmental cognitive neuroscientist, Luna studies those changes that occur in the brain as children develop into adults. She and other researchers are showing how the teen experience can lead to powerful advantages later in life. Take mice again: Young mice that explore most tend to live longest — that is, unless a cat eats them, Luna adds.
What really goes on in a teenager’s brain? Of course, neuroscientists can’t actually peer inside the brains of living teenagers. So they do the next best thing; Researchers scan teen brains while their owners are thinking, learning and making critical decisions.
Eveline Crone is a psychologist at Leiden University in the Netherlands who studies how the brain develops. To do so, Crone uses a huge, high-tech instrument called a magnetic resonance imaging (MRI) scanner. The scanner relies on a powerful magnet and radio waves to create detailed images of the brains of Crone’s young volunteers. It is painless and safe. All that Crone’s adolescent subjects have to do is lie back — and play a few games.
As Crone’s volunteers look up, they see a mirror that reflects a computer screen on which they can play casino-like computer games. Press a button and a slot machine appears, allowing teens to gamble — and win. Three bananas in a row? You win a dollar! “Kids love it. They always want to come back,” laughs Crone.
Teens also can play games that require them to make choices, such as whether to pull a trigger, smile at an attractive face or accept a tempting offer. Some choices earn them rewards, such as coins or food.
While her subjects play away, Crone and her coworkers are hard at work observing and measuring which parts of the teens’ brains are most active. The researchers can pinpoint activity by observing how much oxygen various brain regions are using. Very active parts of the brain use a lot of oxygen.
During the risk-taking and rewards-based tests, one region deep inside the brain shows more activity in adolescents than it does in children or adults, Crone says. This region, known as the ventral striatum, is often referred to as the “reward center” of the brain. The region can drive us to repeat behaviors that provide a reward, such as money and treats.
Concludes Crone: This physical difference in adolescent brain activity “shows that adolescence is a unique phase in development.”
Adolescents are particularly sensitive and responsive to influence by friends, desires and emotions, researchers say. It’s one of the hallmarks of this stage in life.
A major reason why teenagers often respond to those influences with irrational decisions is the presence of a brain chemical known as dopamine. The brain releases dopamine when something makes us feel good, whether it’s receiving a teacher’s compliment or finding a $20 bill. Dopamine levels in general peak during adolescence. In teenagers, the strength of this “feel good” response helps explain why they often give in to impulsive desires.
B.J. Casey of Cornell University tries to understand these biological patterns in teenagers. In laboratory experiments, this brain scientist and her coworkers have seen increased activity in the ventral striatum whenever someone at any age is confronted by a risky decision or the offer of a reward. However, this brain region seems “to be shouting louder” between the ages of 13 and 17 than at any other time during human development.
Crucially, the ventral striatum also communicates with another brain region, this one located just behind the forehead. Called the prefrontal cortex, it’s the brain’s master planner.
Another way to think of the prefrontal cortex is as the conductor of an orchestra. It gives instructions and enables chatter among other brain regions. It guides how we think and learn step-by-step procedures, such as tying our shoelaces. Even preschoolers rely on the prefrontal cortex to make decisions. Overall, the prefrontal cortex’s ability to boss the brain increases with age.
Casey’s research shows how the adolescent brain is locked in a tug-of-war between the logical pull of the prefrontal cortex and the impulsive pull of the ventral striatum. Although teens can make good decisions, “in the heat of the moment — even when they know better,” the reward system can outmuscle the master planner. That can lead to poor decisions, Casey says.
In fact, teenagers almost cannot help but respond to the promise of a reward, Casey says. “It’s like they’re pulled toward it.” It even happens if the choice appears illogical.
While this would appear to push teenagers toward years of serious risk-taking, it is no mistake of evolution. Casey and other researchers believe the adolescent brain specifically evolved to respond to rewards so teens would leave behind the protection provided by their parents and start exploring their environment — a necessary step toward the independence they will need in adulthood.
While all of this is going on during adolescence, the prefrontal cortex seems to lag in developing. It turns out this delay serves an important evolutionary function, says Michael Frank of Brown University. Frank studies the brain processes that occur during learning and decision making.
The prefrontal cortex is important because it teaches the rest of the brain the rules about how the world works. So it’s important that the master planner not be too rigid or restrictive during adolescence. Instead, it stays open to learning. Only later on in development can the brain disregard less useful information, Frank says.
Prior to adolescence, the master planner isn’t quite advanced enough to guide allthe other brain regions. That’s because it still doesn’t know the rules of the game. “So that’s why you have parents to act as your prefrontal cortex,” Frank jokes. Then, all too often, he says, “you reach adolescence and you don’t listen to your parents anymore.”
Pruned, not shriveled
During adolescence, two key processes appear to play an important role in the maturing of our brains. One of the processes involves axons, or fibers that connect nerve cells. From infancy, these fibers allow one nerve cell to talk to another. Throughout the teen years, fatty tissue starts to insulate the axons from interfering signals — it is a bit like the plastic that coats electrical cables.
In axons, the insulating tissue allows information to zip back and forth between brain cells much more quickly. It also helps build networks that link the prefrontal cortex with other brain regions, allowing them to work together more efficiently. Eventually, the master planner can send messages throughout the brain with speed and precision.
The second key process involves synapses. A synapse is like a dock between nerve cells. Nerve cells communicate by transmitting chemical and electrical signals. Those signals move through the synapses.
In their first three years of life, children develop seemingly endless connections in their brain circuitry. Then, beginning in adolescence, the brain starts discarding many of these connections. Luna, the developmental cognitive neuroscientist, compares it to an artist who begins with a block of granite and carves away any unneeded stone to create a sculpture. In this case, the brain acts as the sculptor and chops away excess synapses. Scientists refer to this process as synaptic pruning.
By this stage, the brain has learned which synapses are most useful, Luna explains. So the brain strengthens the synapses it really needs and eliminates those that either slow things down or aren’t useful. For example, as people grow older, they become more proficient in their native tongue but find it harder to learn a language they have never spoken before. They may lack some of their earlier language-learning synapses.
Synaptic pruning and other changes that occur in the adolescent brain give teenagers the tools to start making decisions on their own — even if they’re bad decisions, says Luna.
“Now you have a brain that says, ‘I can make my own decisions. I can skateboard down those steps,’” says Luna. “When you’re a kid, you’d check with Mom. But now you have the prefrontal system that gives you the ability to make decisions.”
Combined, all of these processes help explain the sometimes logical — but often impulsive or unpredictable — decisions that the teenage brain can make. So next time you are torn over whether a reward is worth a certain risk, remember the tug-of-war that’s taking place in your brain — and that somewhere in there, you have the tools to make the best decision.