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

infants are better able to process basic information from their senses—their eyes, ears, mouth, skin, and nose. Within the first year, the neural tracts that support brain regions involved in vision and other primary senses, as well as those involved in gross motor activity, are completed. This is, in part, why it takes about a year for a baby to become coordinated enough to walk. Much of the brain becomes insulated by age two, and high-level areas involved in language and fine motor coordination follow over the next few years when children are particularly primed to learn to talk and improve their fine motor skills. The more complex areas of the brain, especially the frontal lobes, take much, much longer and are not finished until a person is well into his or her twenties.

      All of this learning is dependent on excitation, the driving force in our brains. Excitatory signals between neurons build brain connections and are required for brain development. Excitation can come from outside or inside your brain, but regardless, if a particular pathway of cells and their synapses are activated repeatedly, the synapses between them strengthen. Thus, cells that “fire” together “wire” together.

      In the developing brain, especially in early childhood, as groups and pathways of neurons and their synapses get activated, the process of excitation “turns on” the molecular machinery in the cell. This actually results in the building of more synapses, a process we term synaptogenesis (birth of synapses). Synapses are increased in infancy through adolescence, peaking in early childhood. Because synaptogenesis is so dependent upon brain cells being activated by one another, a child’s brain has more excitatory than inhibitory neurotransmitters and synapses compared with an adult’s brain, where there is more balance between the two.

      Excitation is a key element of learning. The period in early life in which excitation is so prominent is also called the “critical period,” when learning and memory are more robust than in later life. This allows the brain to be very sensitive to excitation and grow. Unfortunately, the abundant excitation in the developing brain carries a price: the risk for overexcitation. This explains why diseases that are a result of overexcitation, like epilepsy, are more common in childhood than adulthood. Seizures are the main symptom in epilepsy, and they are caused by too many brain cells turning on together without enough inhibition to balance them.

      FIGURE 8. The Young Brain Has More Excitatory Synapses Than Inhibitory Synapses: The number of synapses increases from infancy through adolescence, peaking in early childhood.

      Arborization, or the branching out of neurons, peaks in the first few years of life but continues, as we’ve seen, into adolescence. Gray matter density peaks in girls at age eleven and in boys at age fourteen, and waxes and wanes throughout adolescence.

      White matter, or myelin, however, has only one trajectory in adolescence: up. Jay Giedd and colleagues at the National Institute of Mental Health scanned the brains of nearly one thousand healthy children, ages three to eighteen, and discovered this pattern of wiring. As we saw in Figure 4, researchers at the University of California, Los Angeles, built on those findings and compared the scans of young adults, ages twenty-three to thirty, with those of teenagers, ages twelve to sixteen. They found that myelin continues to be produced well past adolescence and even into a person’s thirties, making the communication between brain areas ever more efficient.

      FIGURE 9. Gender Differences in Rate of Cortical Gray Matter Growth: Like the body, the male brain is on average larger than the female brain. Rates of growth in male and female brains also are different. In females, the growth rate of two areas important for cognitive maturity—the frontal lobes and the parietal lobes—peaks in the early teen years, but in males the peak does not occur until the late teens.

      Without those insulated connections, a signal from one area of the brain, say fear and stress coming from the amygdala, has trouble linking up with another part of the brain, for instance the frontal cortex’s sense of judgment. For adolescents whose brains are still being wired, this means they sometimes find themselves in dangerous situations, not knowing what they should do next. This was confirmed scientifically in a 2010 study conducted by the British Red Cross into how teenagers react to emergencies involving a friend drinking too much alcohol. More than 10 percent of all children and young teens between the ages of eleven and sixteen have had to cope at one time or another with a friend who was sick, injured, or unconscious owing to excessive alcohol consumption. Half of those had to deal with a friend who passed out. More broadly, the survey found that nine out of ten adolescents have had to deal with some kind of crisis involving another person during their teenage years—a head injury, choking, an asthma attack, an epileptic seizure, etc. Forty-four percent of the teens surveyed admitted to panicking in that emergency situation, and nearly half (46 percent) acknowledged they didn’t know how to respond to the crisis at all.

      Dan Gordon, a fifteen-year-old boy from Hampshire, England, who was interviewed by the Guardian for a story about the study, spoke about a house party he attended at which there was widespread underage drinking. After one girl passed out on the floor, facedown, she began to vomit, and the others in the room, all teenagers, panicked. Thinking only that they needed to prevent her from choking, they stood her up and, with effort, walked her outside for fresh air and waited for her to wake up. Dan admitted to the reporter that neither he nor anyone else at the party had thought to call for an ambulance. In other words, the teenagers’ amygdalae had signaled danger, but their frontal lobes didn’t respond. Instead, the teens acted in the moment.

      My son Andrew witnessed something similar during college. He was visiting his then-girlfriend at a college in Boston. The girlfriend’s roommate also had an out-of-town visitor, a shy freshman girl from the South who quickly became intoxicated at a party in another student’s room. When Andrew and his girlfriend returned to her dorm, they found the young girl passed out, and just as in Dan Gordon’s story, they all panicked. Instead of calling 911 or campus security, or driving her to an emergency room, they found a couple of friends to help, and then drove all the way out to our house, about ten miles away.

      “We didn’t want to call campus security,” Andrew’s girlfriend explained, as I observed the young girl, whom they had helped into the house and who was now almost unresponsive. “She’s a freshman. If we brought her to the health center, me and my roommate could get in trouble.”

      Andrew and his former girlfriend were both twenty-one at the time, but the visiting student was just eighteen.

      “What about taking her to the hospital?” I said.

      “We didn’t know how drunk she was,” the other friend said. “She was talking when we put her in the car, and now she’s completely out of it.”

      None of them in fact knew the girl—they had met her briefly for the first time earlier that day, when she had arrived to visit the roommate. She had her wallet and an ID from her South Carolina college with her, but no other information. The roommate who had invited her to Boston was nowhere to be found. Already drowsy, she was rapidly becoming more sedated, and then she vomited on the floor. At that point, I insisted they get her to a local community hospital just a mile from our house. It took three of them to half-carry her back to the car. About fifteen minutes later, I got a call from Andrew’s girlfriend, who said the hospital was going to admit the girl for observation. The poor thing spent an unhappy night in the hospital, and the college crew picked her up the next afternoon. On their way back to Boston, they stopped by my house to gather things they had left there the night before. The young freshman looked pale and very tired, but otherwise was fine. Apparently her blood alcohol level had peaked at 0.34, which was more than four times the legal driving limit, and life threatening. Had she not been taken to the hospital, where her stomach was pumped and charcoal administered to prevent her body from absorbing any more alcohol, I shudder to think of what might have happened. As I had a captive audience, I sat them all down