Let's imagine for a moment a family-call them the Kellys-where an argument is taking place between Shawn, 18 years old, and his father, Michael. In attendance are Shawn's six-month-old sister Katie, lying peacefully (until now) in her bassinet; Shawn's six-year-old brother Kevin, who divides his attention between the argument and a baseball game he's watching on television; and Shawn's grandfather, who look increasingly uncomfortable as the argument progresses. The argument concerns Shawn's unwillingness to return home with the family car by midnight. What "world" is each participant perceiving?
Katie doesn't perceive much of anything except loud noises coming from farther away than she can see clearly. Although her brain enables her to recognize human speech sounds, her vocabulary is zilch and she hasn't a clue about cars, curfews, or adolescent rebellion.
Kevin experiences the argument as only a distraction from the game that he's trying to watch. And distractions are very much a part of Kevin's life. He has trouble sitting in one place for more than a few minutes or keeping his mind focused. The shouting in the background only increases his difficulties; if asked, he couldn't say what the argument is about.
For Shawn, important issues are at stake. He's "sick and tired" of being told what to do, not trusted, treated "like a child." It's time Dad starting realizing that he's no longer a six-year-old like Kevin. Shawn's father also sees important issues. Is his kid going to conform to household rules that everyone understands, or is he going to consider himself "better than everybody else" and no longer obey his parents?
At age 82, Grandfather can look back on similar arguments he's seen and heard on many occasions. "Perfectly normal adolescent rebellion based on Shawn's need to establish identity and boundaries," he mutters to himself. If Shawn and his father could only see each other's point of view for just a moment, he thinks.
Each participant in this minidrama perceives the situation based on the maturational development of his or her brain. In Katie's case, brain development hasn't proceeded sufficiently for her to hold any point of view whatsoever. At the other end of the brain-maturational scheme, grandfather has a lifetime's worth of observations of the interactions of adolescents and their parents. He recognizes the futility and counterproductiveness of such spats, thanks to the wisdom provided him by the normal functioning of a part of his brain called the prefrontal cortex. That most-developed and elaborated portion of his brain provides him with the confidence that everything can be resolved if each person has the wisdom to put himself in the other's place.
In short, a "family argument" isn't the same for each participant. More important, the differences could be altered by suitable alterations in the brains of the participants. If Kevin were a few years older, he could understand why Shawn feels so strongly about asserting himself. If Shawn were his father's age he might be curious about what's happening in his father's life and whether this display of parental authority has anything to do with the problems his father has been encountering recently at work. If Shawn's father could exert firmer control over the emotional centers in his brain, he might be able to identify with how he used to feel not so many years ago when he was Shawn's age. While performing this imaginative exercise, the reasoning powers of his prefrontal cortex would be proving to him the futility of the argument while simultaneously suggesting compromises. Only grandfather sees the situation for what it is: an example of intergenerational conflict that is best handled by stepping back, seeing the other person's point of view, seeking a compromise, and most of all, placing the situation in context.
Notice that while each of the suggested behavioral modifications can be described in psychological terms ("seeing the other's point of view," looking for "compromises," et cetera), the underlying processes depend on the maturation of the brain. And this changes across the developmental spectrum from infancy to childhood to adolescence to adulthood to old age.
In The Secret Life oj the Brain we explore the five developmental stages in the human life-span and their implications for health and happiness. Development, as we will see, is a two-edged sword. While it provides opportunities for happiness and achievement, it can also exert destructive effects if the developmental sequences go wrong. For instance, the same search for identity and affirmation that nudges Shawn into adolescent disagreement with his father can also lead him into unhealthy relationships with peers involving drug use or impulsive acts like careless driving (perhaps one of the unstated fears of Shawn's father).
Each of the brain's developmental stages provides its own opportunities and perils. Each is part of a marvelous narrative that starts at the moment of conception and extends to the last breath. And as with every good story, it's best told by starting at the beginning. Here's a short overview of what we'll be exploring:
lf we were to employ a single word to characterize the human brain in all stages of development it would be plasticity: the organ's capacity to change. The brain's plasticity distinguishes it from anything else in the known universe. Without plasticity, the brain would be incapable of adjusting to changing times and conditions. Indeed, absent its plasticity, the brain would be similar to a machine, a structure with strictly limited powers of adaptation to the environment. Instead, thanks to plasticity, the brain possesses amazing powers of adaptation and recovery. Throughout this book the theme of plasticity can be discerned as a sub text to everything the brain does. Even as you read these words, your brain is changing as a consequence of your brain's plasticity.
Plasticity is most evident during infancy. From a tiny ball of cells the brain first emerges, grows, and organizes itself. As the brain increases in size and complexity inside the womb, its growing cells interact with their environment and with one another. Newly formed neurons establish connections and those connections multiply throughout infancy.
Experience provides the basis for the formation of the connections and the transformation of those connections into circuits. Change the experience and you change the brain. Deprive the baby's brain of light and sound and human contact, and it will remain stunted. The same thing will happen if the brain enters the world too soon and in its prematurity is overwhelmed with more stimuli than it's equipped to handle.
But plasticity doesn't stop at infancy. It continues across the entire life-span:
It is present in the infant's brain; the brain of the infant's five-year-old sibling who has just learned that Mother will be bringing home "a sister" from the hospital; the brain of the adolescent brother as he stares down at his new sister; the brain of the proud father in the delivery room getting his first look at his child; and the brain of grandmother as she sits proudly cuddling her new grandchild. At all of these stages and at every age the brain retains its capacity to change in response to life experiences.
And plasticity takes different forms during the five stages of human development. Although we will discuss the stages in greater detail in the following chapters, here is a thumbnail sketch of what takes place.
During gestation the brain makes its first appearance as a crest of cells from which the neurons, brain cells, will emerge. Next come the formation of the major brain regions and a migration of neurons from their original sites of generation to their final positions in the brain. As we will see, disastrous consequences can ensue whenever one of these processes goes awry as a result of disease, genetic mutation, or exposure to drugs or chemical toxins. Indeed most congenital (present at birth) brain defects result from disruption of the normal programs of neuronal growth, development, and migration.
During childhood, the second life stage, a new kind of plasticity takes precedence. With most of the neurons in place at the conclusion of infancy, a sculpting process emerges as the dominant force shaping the brain. At this point the brain contains many more neurons than it requires, and excess neurons must be pruned away according to the most fundamental tenet of brain operation: Use it or lose it. In practical terms, this means that the overabundance of neuronal connections established during gestation and infancy is thinned out in response to experience. Unused or rarely used pathways disappear, while heavily trafficked pathways flourish and elaborate. Thanks to this process of forming, re-forming, and strengthening neuronal connections, young children proceed by leaps and bounds in their abilities to pay attention, remember, and make their first efforts at mastering the universe.
In adolescence the pruning process has largely taken place in most areas of the brain, with one notable exception: the prefrontal cortex. Select any of the difficulties associated with adolescence (impulsiveness, erratic mood swings, rebellion against authority, poor judgment, et cetera) and you'll find that those difficulties are the result of immaturity in the prefrontal cortex. During adolescence pruning occurs within the immature prefrontal cortex and elsewhere. While the pruning is taking place, the adolescent remains "difficult," "unpredictable," "moody"-fill in your own favorite adjectives based on your experiences with adolescents. But when that pruning has been successfully completed we employ a different set of adjectives: The adolescent is now "more mature," "thoughtful," "likable," and even "courteous."
By adulthood all the brain areas are up and functioning. The brain contains its full repertoire of cells, although some new cells may be added, principally in discrete brain areas such as those associated with memory. Adulthood is the culmination of human brain development, the goal that nature was striving for. More than half of all the brain cells lie within the cerebral hemispheres, crowned by the wrinkled surface area called the cortex. Hidden within its folds reside our powers of thought and reason.
Old age was once thought of as principally a holding operation: retaining past gains while yielding as little as possible to the ravages of age and disease. We now know that such a view is unduly pessimistic. The healthy brain of the aged person retains a marvelous plasticity and can change for the better and the worse. Indeed, the brain of the older person can continue to function healthily and creatively, or it can decline in its powers due to abuse or simple lack of use. The older person faces a choice: accept the stereotypes about aging and sit in a corner, or remain active and vibrant, perhaps even becoming like writer Harriet Doerr, who wrote her first book, Stones for Ibarra, while in her 80's. But in order to thrive during this last stage of life, dangers must be anticipated and protected against. For instance, late-life-onset alcoholism is frighteningly common in people in their 70's and 80's who fail to keep their brain finely honed by reading and other intellectual pursuits. Again, as in the other stages of brain development, we encounter that double-edged sword of plasticity.
When things go wrong in the brain's growth or development, a smorgasbord of human afflictions and pathological conditions can result, such as drug abuse, schizophrenia, and depression, and even suicide, that most sorrowful of calamities that can befall the brain. But in the vast majority of instances, the brain develops perfectly normally. In practical terms, this means that at one or another of the five stages of development the brain can bring comfort, excitement, insights, and a host of pleasures.
While the brain is but one organ among many in the human body, it is the source and determiner of everything. Our understanding of the world changes in concert with the evolution of this delicate structure, which is unlike anything in the universe. Indeed, we understand the world the way we do at each of life's stages because of our brain. And yet, until lately, the brain jealously guarded its secrets. Only recently-with the development of powerful technologies-have we been successful in delving into the secrets of the brain.