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

By the late 1990s researchers had confirmed a range of changes associated with experience and stimulation, including brain size, gray matter volume, neuron size, dendritic branching, and the number of synapses per neuron. The more stimulation and experience, they concluded, the larger the neurons, the bushier the dendrites, the higher the number of synapses, and the thicker the gray matter.

During my senior year at Smith College in 1977–78, I wrote my first professional journal article under the tutelage of Nico Spinelli, a professor in both the psychology department and the computer and information science department at the University of Massachusetts Amherst. He was doing pioneering experiments in the plasticity of the visual cortex. Previous research had looked at the brains of mammals raised in a deprived environment. Spinelli wanted to see if plasticity was still at work in a “normal” environment. So we took kittens raised with their mothers in a standard animal facility and gave them what’s called avoidance training. In these experiments, a “safe” and an “unsafe” stimulus were associated with two different visual stimuli: vertical lines and horizontal lines. As the kittens learned to associate the safe stimulus with either the horizontal or the vertical lines, the number of neurons in those parts of the visual cortex expanded. The results, which were published in the journal Science , confirmed “that early learning produces plastic changes in the structure of the developing brain,” or, to put it more simply, young brains are shaped by experience.

Of course, adult brains can be shaped by experience as well. Researchers in neural plasticity have found that even in the last decades of life, adult brains can be remodeled, just not as easily or as constantly as during childhood and adolescence. Whereas kids’ brains will respond and change in response to virtually any stimulation, so-called adult plasticity occurs only in specific behavioral contexts. For instance, cab drivers in London(a notoriously difficult city to navigate) have been found by scientists to have an enlarged hippocampus particularly in the area responsible for spatial memory. Violinists and cellists, who must use their hands fluidly and rapidly, have been shown to have an enhanced motor cortex. And in an unusual experiment conducted several years ago, Patricia McKinley of McGill University was able to show that learning the tango, which involves both complex movement and a fine sense of balance, improved the ability of senior citizens, ages sixty-eight to ninety-one, to switch between two different cognitive tasks. “Plasticity,” then, is just another way of saying “learning.”

In the first few years of childhood there is a critical period of plasticity in which learning comes quickly and easily. Evolution experts believe this is the brain’s way of helping us adapt early to the specific environment in which we are raised. The concept is the same as that of imprinting, whereby a baby duckling develops a keen and powerful preference to follow the mother duck over any other. When I was five years old, I saw this in action, although I obviously didn’t know it at the time. It was Easter, and my baby brother had just been born. Perhaps because of that, friends of my parents gave me my own “baby”—a baby chick, that is, much to my parents’ consternation. I loved that fuzzy little animal and was absolutely fascinated that it would follow me around the house, through the swinging door between the kitchen and the dining room, even out of the house and around the yard. Because I was with the chick almost from its birth, it had determined I was its mother. Years later I would read the children’s book Are You My Mother? by P. D. Eastman to my sons. Basically, the book is really all about imprinting. A young hatchling leaves its nest too early while its mother is out foraging for food, and goes on a journey, asking every animal and object it meets—a kitten, a hen, a dog, a cow, a car, even an enormous power shovel—the question of the title. Luckily the power shovel lifts the young bird up and deposits it back in its nest beside its real mother.

Five-year-old me, of course, was the only mother my baby chick had. Unfortunately, the end of the relationship was sudden and brutal. About a week after Easter, after I’d just gotten home from kindergarten, my baby chick was once again following me all over the house, but this time, as I skipped between the kitchen and the dining room, the little hatchling failed to make it through the swinging door and was squished. I cried for days.

Thirteen years later, as a freshman at Smith, I created my own chick-imprinting experiment for a class in advanced biology. In order to imprint them to sound, I exposed my baby chicks to a specific sound or tone every day over a week. At the end of this training period, the chicks were placed on a kind of runway and were then exposed to two sounds, one of them being the familiar tone I’d played for them for seven straight days. Every one of the chicks toddled toward the familiar tone: they had imprinted to sound. I remember this so well because my mother was visiting me at the time of the experiment and she helped me type the results!

But how does learning actually happen? Young brains and old brains work much the same way, by receiving information from the senses—hearing, seeing, tasting, touching, smelling. Sensory information is transmitted by synapses through a network of neurons and is stored, temporarily, in short-term memory. This short-term memory region is highly volatile and is constantly receiving input from the nearly continuous information our senses encounter every minute of our waking life. After information is processed in the short-term memory region, it is compared with existing memories, and if the information matches, it is discarded as redundant. (Brain space is too limited and too precious to allow duplicates to take up neural real estate.) If, on the other hand, the information is new, then it is farmed out to one of several locations in the brain that store long-term memories. Although nearly instantaneous, the transmission of sensory information is not perfect. In the same way that the otherwise seamless signal coming from your TV is occasionally interrupted, briefly distorting the picture, so, too, does degradation occur as information races up and down the axons of your brain’s neurons. This explains why our memories are never perfect, but have holes or discontinuities, which we occasionally fill in, albeit unconsciously, with false information.

The brain is programmed to pay special attention to the acquisition of novel information, which is what learning really is. The more activity or excitation between a specific set of neurons, the stronger the synapse. Thus, brain growth is a result of activity. In fact, the young brain has more excitatory synapses than inhibitory synapses.

The more a piece of information is repeated or relearned, the stronger the neurons become, and the connection becomes like a well-worn path through the woods. “Frequency” and “recency” are the key words here—the more frequently and the more recently we learn something and then recall it or use it again, the more entrenched the knowledge, whether it’s remembering the route between home and work or how to add a contact to your smartphone’s directory. In both cases, the mental machinery of learning is dependent on the synapse, that minuscule space where packets of information are passed from one neuron to another by electrical or chemical messengers. For these neural connections to be made, both sides of a synapse need to be “on,” that is, in a state of excitation. When an excitatory input exceeds a certain level, the receiving neuron fires and begins the molecular process, called long-term potentiation, by which synapses and neuronal connections are strengthened. The process of long-term potentiation, or LTP, is a complex cascade of events involving molecules, proteins, and enzymes that starts and ends at the synapse.