The Teenage Brain: A neuroscientist’s survival guide to raising adolescents and young adults. Frances Jensen E.

shrink or grow relative to one another during development based on how much the senses are used. Structurally, the human brain is divided into four lobes: frontal (top front), parietal (top back), temporal (sides), and occipital (back). The brain sits on the brainstem, which connects to the spinal cord. In the rear of the brain, the cerebellum regulates motor patterning and coordination, and the occipital lobes house the visual cortex. The parietal lobes house association areas as well as the motor and sensory cortices (which include the homunculus in Figure 2). The temporal lobes include areas involved in the regulation of emotions and sexuality. Language is also located here, more specifically in the dominant hemisphere (the left temporal lobe for right-handed people and 85 percent of left-handed people, and the right temporal lobe for that small group of truly strong lefties). The frontal lobes sit most anteriorly and this area is concerned with executive function, judgment, insight, and impulse control. Importantly, as the brain matures from back to front in the teen years the frontal lobes are the least mature and the least connected compared with the other lobes.

      FIGURE 3. The Lobes of the Brain: A. The brain matures from the back to the front. B. The cortex of the brain can be divided into several main areas based on function.

      The brain is divided into specialized regions for each of the senses. The area for hearing, or the auditory cortex, is in the temporal lobes; the visual cortex is in the occipital lobes; and the parietal lobes house movement and feeling in the motor and sensory cortices, respectively. Other parts of the brain have nothing to do with the senses, and the best example of this is the frontal lobes, which make up more than 40 percent of the human brain’s total volume—more than in any other animal species. The frontal lobes are the seat of our ability to generate insight, judgment, abstraction, and planning. They are the source of self-awareness and our ability to assess dangers and risk, so we use this area of the brain to choose a course of action wisely.

      Hence, the frontal lobes are often said to house the “executive” function of the human brain. A chimpanzee’s frontal lobes come closest to the human’s in terms of size, but still make up only around 17 percent of its total brain volume. A dog’s frontal lobes make up just 7 percent of its brain. For other species, different brain structures are more important. Compared with humans, monkeys and chimpanzees have a much larger cerebellum, where control of physical coordination is honed. A dolphin’s auditory cortex is more advanced than a human’s, with a hearing range at least seven times that of a young adult. A dog has a billion olfactory cells in its brain compared with our measly twelve million. And the shark has special cells in its brain that help it detect electrical fields—not to navigate but to pick up electrical signals given off by the scantest of muscle movements in other fish as they try to hide from this deadly predator.

      We humans don’t have a lot else going for us other than our wile and wit. Our competitive edge is our ingenuity, brains over brawn. This edge happens to take the longest time to develop, as the connectivity to and from the frontal lobes is the most complex and is the last to fully mature. This “executive function” thus develops slowly: we certainly are not born with it!

      So in what order are these brain regions all connected to one another during childhood and adolescence? This could never have been learned before the advent of modern brain imaging. New forms of brain scans, called magnetic resonance imaging (MRI), not only can give us accurate pictures of the brain inside the skull but also can show us connections between different regions. Even better, a new kind of MRI, called the functional MRI, abbreviated fMRI, can actually show us what brain areas turn one another on. So we can actually see if areas that “fire” together are “wired” together. In the last decade, the National Institutes of Health conducted a major study to examine how brain regions activate one another over the first twenty-one years of life.

      What they found was remarkable: the connectivity of the brain slowly moves from the back of the brain to the front. The very last places to “connect” are the frontal lobes (Figure 4). In fact, the teen brain is only about 80 percent of the way to maturity. That 20 percent gap, where the wiring is thinnest, is crucial and goes a long way toward explaining why teenagers behave in such puzzling ways—their mood swings, irritability, impulsiveness, and explosiveness; their inability to focus, to follow through, and to connect with adults; and their temptations to use drugs and alcohol and to engage in other risky behavior. When we think of ourselves as civilized, intelligent adults, we really have the frontal and prefrontal parts of the cortex to thank.

      Because teens are not quite firing on all cylinders when it comes to the frontal lobes, we shouldn’t be surprised by the daily stories we hear and read about tragic mistakes and accidents involving adolescents. The process is not really done by the end of the teen years—and as a result the college years are still a vulnerable period. Recently a friend of mine told me about his son’s college classmate, Dan, an all-around great kid who’d rarely caused his parents to worry. He was popular, had been a star ice hockey player in high school, and was a finance major in college. Over the summer my friend’s son got a phone call from Dan’s mother. Dan had drowned the night before, she told him. He’d been out with friends, drinking, and sometime between three and four in the morning, on their way home, the group—there were eight of them—decided they wanted to cool off, so they stopped at the local tennis club. The club was closed, of course, but the locked gate didn’t stop them. All eight scaled the fence and jumped into the pool. It was only after they’d gotten home that someone said, “Where’s Dan?” Racing back to the club, they found their friend facedown in the water. The medical examiner listed the cause of death as accidental drowning due to “acute alcohol intoxication.” One of the news reports I read made me shake my head: “Police are asking kids and adults to think twice about potential dangers before taking any risks that could turn deadly.”

      FIGURE 4. Maturing Brain: The Brain “Connects” from Back to Front: A. A functional MRI (fMRI) scan can map connectivity in the brain. Darker areas indicate greater connectivity. B. Myelination of white matter tracks cortex maturation from back to front; this is why the frontal lobes are the last to be connected. C. Serial connectivity scans reveal that frontal lobe connectivity is delayed until age twenty or older.

      “Think twice.”

      How many times have we all said this to our teenage sons and daughters? Too many times. Still, as soon as I heard about Dan, I called my boys to tell them the story. You have to remember this, I told them. This is what happens. Drinking and swimming don’t go together. Neither does the decision to suddenly scale a fence in the middle of the night, or jump into a pool with seven friends who are also intoxicated.

      How parents deal with these tragic stories and talk about them with their own kids is critical. It shouldn’t be, “Oh, wow, I’m so glad that wasn’t my child.” Or, “My teenager would never have done that.” Because you don’t know. Instead, you have to be proactive. You have to stuff their minds with real stories, real consequences, and then you have to do it again—over dinner, after soccer practice, before music lessons, and, yes, even when they complain they’ve heard it all before. You have to remind them: These things can happen anytime, and there are many different situations that can get them into trouble and that can end badly.

      One of the reasons that repetition is so important lies in your teenager’s brain development. One of the frontal lobes’ executive functions includes something called prospective memory, which is the ability to hold in your mind the intention to perform a certain action at a future time—for instance, remembering to return a phone call when you get home from work. Researchers have found not only that prospective memory is very much associated with the frontal lobes but also that it continues to develop and become more efficient specifically between the ages of six and ten, and then again in the twenties. Between the ages of ten and fourteen, however, studies reveal no significant improvement. It’s as if that part of the brain—the ability to remember to do something—is simply not