The Teenage Brain: A neuroscientist’s survival guide to raising adolescents and young adults. Frances Jensen E.
just a small adult brain, and brain growth, unlike the growth of most other organs in the body, is not simply a process of getting larger. The brain changes as it grows, going through special stages that take advantage of the childhood years and the protection of the family, then, toward the end of the teen years, the surge toward independence. Childhood and teen brains are “impressionable,” and for good reason, too. Just as baby chicks can imprint on the mother hen, human children and teens can “imprint” on experiences they have, and these can influence what they choose to do as adults.
Such was the case with me. I “imprinted” on neuroscience and medicine pretty early on. My experiences cultivated in me a curiosity that I found irresistible, sustaining me from my high school years through medical school and graduate research, and to this very day. I grew up the oldest of three children in a comfortable family home in Connecticut, just forty minutes from Manhattan. I happened to live in Greenwich, which even back then was the home of actors, authors, musicians, politicians, bankers, and the independently wealthy. The actress Glenn Close was born there, President George H. W. Bush grew up there, and the great bandleader Tommy Dorsey died there.
My parents were from England; they had immigrated after World War II, and my dad came over after medical school in London to do his urological surgery residency at Columbia. To them, Greenwich seemed a great place to settle within commuting distance of New York City. It was a matter of convenience, and they were pretty oblivious to the celebrity status of the town. Perhaps because of my father, I was open-minded about learning math and science. For me a major “imprinting” moment that propelled me in the direction of medicine was a ninth-grade biology class at Greenwich Academy. The best part to me, memorable in fact, was when we each got a fetal pig to dissect. While many of my classmates slumped in their seats at the proposition of slicing up these small mammals, some rushing to the girls’ washrooms with waves of nausea, a few of us jumped into the task at hand. It was one of those defining moments. The scientists had separated from those destined to be the writers, lawyers, and businesspeople of the future.
Injected with latex, the pigs’ veins and arteries visibly popped out with their colorful hues of blue and red. I’m a very visual person; I also like thinking in three dimensions. That visual-spatial ability comes in handy with neurology and neuroscience. The brain is a three-dimensional structure with connections between brain areas going in every direction. It helps to be able to mentally map these connections when one is trying to determine where a stroke or brain injury is located in a patient presenting with a combination of neurological problems—definitely a plus for a neurologist. Actually, that’s how the minds of most neurologists and neuroscientists work. We’re a breed that tends to love to look for patterns in things. I’ve never met a jigsaw puzzle, in fact, that I didn’t like. My attraction to neuroscience in high school and college began at a time before CT scans and MRIs, when a doctor had to imagine where the problem was inside the brain of a patient by picturing the organ three-dimensionally. I’m good at that. I like being a neurological detective, and as far as I’m concerned, neuroscience and neurology turned out to be the perfect profession for me to make use of those visuospatial skills.
If the human brain is very much a puzzle, then the teenage brain is a puzzle awaiting completion. Being able to see where those brain pieces fit is part of my job as a neurologist, and I decided to apply this to a better understanding of the teen brain. That’s also why I’m writing this book: to help you understand not only what the teen brain is but also what it is not, and what it is still in the process of becoming. Among all the organs of the human body, the brain is the most incomplete structure at birth, just about 40 percent the size it will be in adulthood. Size is not the only thing that changes; all the internal wiring changes during development. Brain growth, it turns out, takes a lot of time.
And yet the brain of an adolescent is nothing short of a paradox. It has an overabundance of gray matter (the neurons that form the basic building blocks of the brain) and an undersupply of white matter (the connective wiring that helps information flow efficiently from one part of the brain to the other)—which is why the teenage brain is almost like a brand-new Ferrari: it’s primed and pumped, but it hasn’t been road tested yet. In other words, it’s all revved up but doesn’t quite know where to go. This paradox has led to a kind of cultural mixed message. We assume when someone looks like an adult that he or she must be one mentally as well. Adolescent boys shave and teenage girls can get pregnant, and yet neurologically neither one has a brain ready for prime time: the adult world.
The brain was essentially built by nature from the ground up: from the cellar to the attic, from back to front. Remarkably, the brain also wires itself starting in the back with the structures that mediate our interaction with the environment and regulate our sensory processes—vision, hearing, balance, touch, and sense of space. These mediating brain structures include the cerebellum, which aids balance and coordination; the thalamus, which is the relay station for sensory signals; and the hypothalamus, a central command center for the maintenance of body functions, including hunger, thirst, sex, and aggression.
I have to admit that the brain is not very exciting to look at. Sitting atop the spinal cord, it is light gray in color (hence the term “gray matter”) and has a consistency somewhere between overcooked pasta and Jell-O. At three pounds, this wet, wrinkled tissue is about the size of two fists held next to each other and weighs no more than a large acorn squash. The “gray matter” houses most of the principal brain cells, called neurons: these are the cells responsible for thought, perception, motion, and control of bodily functions. These cells also need to connect to one another, as well as to the spinal cord, for the brain to control our bodies, behavior, thoughts, and emotions. Neurons send most of their connections to other neurons through the “white matter” in the brain. The commonly used brain imaging tool, magnetic resonance imaging, or MRI, shows the distinction between gray and white matter beautifully. On the outside surface, the brain has a rippled structure. The valleys or creases are referred to as sulci and the hills are referred to as gyri. Figure 1 shows an image from a brain’s MRI scan, like those done on patients. There are two sides to the brain, each called a hemisphere. (When an MRI image shows a cut across the middle in one direction or the other [slice angles A and B], it is easier to see the two sides.) The most superficial layer of the brain is called the cortex and it is made up of the gray matter closest to the surface, with the white matter located beneath it. The gray matter is where most of the brain cells (neurons) are located. The neurons connect directly to those close by, but in order to connect to neurons in other parts of the brain, in the other hemisphere, or in the spinal cord to activate muscles and nerves in our face or body, the neurons send processes down through the white matter. The white matter is called “white” because in real life and also in the MRI scans its color is light, owing to the fact that the neuron processes running through here are coated with a fatty insulator-like substance called myelin, which truly is white in color.
As I said before, sheer size—or even weight, for that matter—doesn’t mean everything. A whale brain weighs about twenty-two pounds; an elephant brain about eleven. If intellect were determined by the ratio of brain weight to body weight, we’d be losers. Dwarf monkeys have one gram of brain matter for every twenty-seven grams of body matter, and yet the ratio for humans is one gram of brain weight to forty-four grams of body weight. So we actually have less brain per gram of body weight than some of our primate cousins. It is the complexity of the way neurons are hooked up to one another that matters. Another example of how little the weight of the brain has to do with its functioning, at least in terms of intelligence, is that the human female brain is physically smaller in size than the male brain but IQ ranges are the same for the two sexes. At only 2.71 pounds, the brain of Albert Einstein, indisputably one of the greatest thinkers of the twentieth century, was slightly underweight. But recent studies also show that Einstein had more connections per gram of brain matter than the average person.
FIGURE 1. The Basics of Brain Structure: A magnetic resonance imaging (MRI) scan of a brain. The horizontal and vertical cross sections (slice angles A and B) show the cortex (gray matter) on the surface and the white matter underneath.
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