The amazing mind of teenagers / J. N. Gid

A mismatch in the maturity level of different neural networks in the brains of adolescents leads them to dangerous behavior but also allows them to make mental and adaptive leaps

The phrase "adolescent brain" is often laughed at and seen as an oxymoron: an example of biology gone wrong.

Adolescents tend to take risks and behave aggressively or just confusingly. Neurobiologists saw this tendency as the result of a damaged brain in some way. But groundbreaking research in the last decade shows that this approach is wrong. Adolescent brains are not damaged. Nor is he the half-baked brain of an adult. He was designed by evolution to act differently than a child or an adult.
The main characteristic of an adolescent brain is the ability to change in response to the environment through the adjustment of the neural communication networks connecting the parts of the brain. This special ability to change, or this flexibility, is a double-edged sword. It allows the youth to make giant strides in their ability to think and socialize. But the altered neural landscape also makes them vulnerable to risky behavior and serious mental illness.

The most recent studies indicate that the most dangerous behaviors result from a mismatch between the level of maturity of the neural networks found in the limbic system, which controls emotions and receives a strong emphasis in adolescence, and between the level of maturity of neural networks found in the prefrontal cortex, which occurs later and is responsible for judgment and on impulse control. Indeed, we now know that the prefrontal cortex continues to change markedly throughout our 20s. In addition, it seems that today puberty starts earlier, and the "mismatch years" are getting longer.

The flexibility, also known as plasticity, of neural networks that connect areas of the brain, and not the growth of these areas, as previously thought, is the key to mature behavior, in the end. This understanding, and the knowledge that in young people today the gap between the development of emotional networks and between the networks of judgment is widening, can help parents, teachers, counselors and the youth themselves. People will be better able to understand that behaviors such as taking risks, seeking thrills and turning their backs on parents in favor of friends, are not evidence of cognitive or emotional problems. They are a natural result of the development of the brain, a natural stage in which adolescents learn how to deal with a complex world.

This understanding will also help adults decide when to intervene. When a 15-year-old girl breaks away from her parents in dress, music or politics, it may be a source of anxiety for mom and dad, but it does not indicate mental illness. The tendency of a 16-year-old boy to skateboard without a helmet or to respond to dangerous challenges posed to him by his friends may not be without meaning, but it must be assumed that it indicates short-term thinking and social pressure more than a desire for self-harm. But other behaviors, such as seeking new experiences and aggression, may be a red flag. A better understanding of the adolescent brain will help us all learn how to distinguish between unusual age-appropriate behavior and behavior that may indicate illness. Such awareness can help society reduce the rate of addictions, sexually transmitted diseases, traffic accidents, unwanted pregnancies, murders, depressions and suicides among teenagers.

Higher connectivity

Few parents of teenagers will be surprised to hear that the minds of 16-year-olds are different from those of eight-year-olds. But studies have struggled to identify the differences scientifically. Wrapped in a hard, skin-like membrane, surrounded by a protective channel of fluid, and covered entirely by a bony sheath, the brain is well protected from falls, attacks by predators, and the curiosity of scientists.

The development of imaging methods such as computed tomography (CT) and positron emission tomography (PET) has opened the door to some progress, but because these methods emit ionizing radiation, it is not ethical to use them for extensive studies in teenagers. The invention of magnetic resonance imaging (MRI) has finally provided an opportunity to remove the veil, being a safe and accurate way to study the anatomy and physiology of the brain in people of all ages. Studies currently being conducted follow thousands of twin pairs and single subjects throughout their lives. The consistent finding obtained is that the adolescent brain does not mature through an increase in volume, but through an increase in connectivity between the parts of the brain and an increase in the degree of specialization of the various brain areas.

The increase in connectivity between the parts of the brain is expressed in MRI scans as larger volumes of white matter. The "white" in the white matter comes from a fatty substance called myelin, which wraps and isolates the long wire, the axon, that extends from the nerve cell body. The process of myelination, i.e. the creation of the fatty sheath, occurs from childhood to adulthood and significantly accelerates the speed of transmission of nerve signals in the nerve cells. Myelinated axons conduct signals 100 times faster than unmyelinated ones.
Myelination also speeds up the speed of information processing in the brain by helping axons recover quickly after firing signals, so they are ready and willing to send another signal. Faster recovery time allows up to a 30-fold increase in the frequency at which a given neuron can transmit information. The combination of a higher speed of signal transmission and a shorter recovery time provide, between infancy and adulthood, a 3,000-fold increase in the computational bandwidth of the brain, an increase that enables the creation of branched neural networks between the various brain regions.

Recent studies reveal another, more sophisticated role of myelin. Neurons integrate information from other neurons, but fire an electrical signal to pass the information on only if the intensity of the input exceeds a certain electrical threshold. If the neuron fires, this action triggers a series of molecular changes that strengthen the synapses, or connections, between the firing neuron and neighboring neurons.
This strengthening of the neural connections is the basis of learning. What the researchers themselves are now learning is that for input from nearby and distant neurons to reach a given neuron at the same time, nerve conduction must be precisely timed, and myelin is intimately involved in setting the timing. As children grow up and become teenagers, the rapid increase in the amount of myelin helps to connect and coordinate the activity between different areas of the brain in a variety of cognitive tasks.

Scientists can now measure the changes in connectivity using graph theory, a branch of mathematics that quantifies the relationships between nodes and edges in a network. Vertices can be any identifiable object or entity, such as a neuron, a brain structure such as the hippocampus, or a larger region such as the prefrontal cortex. Arcs can be any connection between vertices, starting with a physical connection like a synapse between nerve cells and ending with a statistical correlation, for example when two parts of the brain work in a similar way during a cognitive task.

Graph theory has helped me and others to measure how different areas of the brain develop and communicate with each other, and to find correlations between these processes and between changes in behavior and cognition. Brain changes are not limited to adolescence. Most brain circuits develop in the womb, and many continue to change throughout life, many years after the teenage years. But it turns out that during adolescence there is a dramatic increase in connectivity between areas of the brain involved in judgment, dealing with others and long-term planning, abilities that profoundly affect the rest of a person's life.

Time to specialize

While the white matter along the nerve cells develops with age, in teenagers another change occurs. Brain development, like other complex processes in nature, proceeds through excess production followed by selective dilution. Like Michelangelo's uncle emerging from a block of marble, many cognitive developments are created through a sculpting process where unnecessary or unwanted connections between brain cells are pruned. On the other hand, connections that are used with high frequency are strengthened.

Although pruning and strengthening connections occur throughout our lives, during puberty the balance moves towards pruning, as the brain adapts itself to the demands of the environment. Specialization of brain areas results from the fact that useless connections between nerve cells are eliminated and the amount of gray matter in the brain decreases. The gray matter consists mainly of unmyelinated structures, such as the cell bodies of nerve cells, dendrites (antenna-like extensions that extend from the cell body and receive information from other nerve cells) and certain axons. Overall, the amount of gray matter increases during childhood, peaks around age 10, and decreases during puberty. It stabilizes in adulthood and decreases again to some extent in old age. This pattern is also true for the density of receptors on nerve cells, which respond to nerve messengers (neurotransmitters): molecules such as dopamine, serotonin and glutamate that regulate communication between brain cells.

Although the raw amount of gray matter peaks around the onset of puberty, full development of different parts of the brain occurs at different times. The first areas where gray matter peaks are the primary sensory-motor cortices, specialized for sensing and responding to light, sound, smell, taste and touch. Only at the last stage does it reach its peak in the prefrontal cortex, which is essential for executive functions, a concept that encompasses a wide array of abilities, including organization, decision-making and planning, as well as emotional regulation.

An important feature of the prefrontal cortex is the ability to imagine, through mental time travel, what might happen under certain conditions. That is, to consider results obtained in the past and present and possible results in the future by running simulations in the brain instead of actually being exposed to a potentially dangerous reality. As the philosopher Karl Popper put it, instead of endangering ourselves, "theories die in our place". As we mature cognitively, our executive functions make us choose big, long-term rewards instead of small, immediate gratifications.

The prefrontal cortex is also an essential component of the neural circuitry associated with social cognition: the ability to navigate the world of complex social relationships, differentiate between friends and enemies, gain the protection of the group, and accomplish the main goal in adolescence: to attract a spouse.

Adolescence is therefore a period in which there are changes in the gray and white matter, which together change the level of connectivity between areas of the brain, while the adult brain takes its final form. The prefrontal cortex is active in teenagers, but its operations are not as successful as they will be later on. And because these abilities don't mature until a person's twenties, teens may have trouble controlling impulses or assessing risk and reward.

uncoordinated ripening

Unlike the prefrontal cortex, the hormone-driven limbic system undergoes dramatic changes during puberty, which usually begins between the ages of 10 and 12. The system controls emotions and feelings of reward. It also reacts with the prefrontal cortex during puberty and increases the search for thrills and risks and the tendency for reciprocal relationships with friends. These behaviors, which are well rooted in biology and exist in all social mammals, encourage teenagers to say goodbye to the comfort and security of the family and look for external connections. These behaviors reduce the chance of inbreeding and create a genetically healthier population, but they are also a source of danger, especially when they involve modern temptations such as easy access to drugs, weapons and fast vehicles, without the control of a firm judgement.

What therefore determines the behavior of teenagers to the greatest extent is not the late development of executive functions or the early development of emotional behavior but a mismatch between the two processes. If young ten-year-olds are emotionally driven by the limbic system, but prefrontal control is not as tight as it would be at, say, age 25, then there is a whole decade left where an imbalance between emotions and thinking may dominate. Furthermore, puberty today begins at a younger age worldwide, extending the time between the onset of risk-taking and thrill-seeking and the strengthening and stabilization of the prefrontal cortex.

The lingering discrepancy supports the growing sense that the teenage years are no longer synonymous with adolescence. Puberty, which society defines as the transition from childhood to adulthood, begins biologically with the onset of sexual puberty, but ends socially when we reach independence and behave as adults. In the US, adult behavior, often characterized by events such as marriage, having children and buying a house, is happening about five years later than in the 70s.

The great influence of social factors on the definition of an adult has led some psychologists to hypothesize that puberty is not necessarily a biological reality but a product of changes that have taken place since the industrial revolution in the way we raise children. But studies of twin pairs, which examine the relative effects of genes versus environment by following twins who go through different experiences, negate the view that social influences can significantly override biology. They show that the rate of biological maturation of white and gray matter is to some extent influenced by the environment, but the underlying timing depends on biological control. Sociologists also see the same phenomenon. Taking risks, chasing thrills, and getting close to peers occurs in all cultures, albeit to varying degrees.

Vulnerability and opportunity

The development of the gray and white matter and the level of connectivity, which can be seen on MRI, highlights the fact that the most striking feature of brain development in adolescence is the occurrence of dramatic changes. In general, this flexibility is reduced during adulthood, and yet humans retain brain flexibility for a longer period of time than any other biological species.

Maturation and longer flexibility allow us to "keep all options open" during the development of the individual and during the evolution of the entire human species. We can thrive anywhere from the frozen north to warm equatorial islands. With the help of technologies developed by our brains, we can even live in spaceships orbiting the Earth. 10,000 years ago, a blink of an eye in evolutionary terms, we spent most of our time searching for food and shelter. Today many of us spend most of our waking hours engaged in words and signs, a remarkable development since reading is only 5,000 years old.

Longer flexibility has served us well but it not only creates opportunities but also makes us more vulnerable. During adolescence, the risk of developing certain mental illnesses peaks, including anxiety disorders, bipolar disorder, depression, eating disorders, psychosis, and the use of illicit substances. Amazingly, 50% of mental illnesses appear before the age of 14, and 75% begin before the age of 24.

The connection between the typical changes in the minds of teenagers and the outbreak of mental illness is a complex connection, but one possible reason is the idea that can be called: "moving parts may break". That is, the extensive changes that occur in the white matter, gray matter and connectivity increase the chance that problems will arise. For example, almost all of the abnormal findings in the brains of adults with schizophrenia resemble an exaggeration of typical adolescent brain changes.

In many other ways, adolescence is the healthiest period of life. The immune system, cancer resistance, heat and cold tolerance, and other traits are all at their peak. But despite the physical strength, the rates of serious illness and death are three to four times higher in teenagers compared to children. Car accidents, the number one cause, account for about half of teen deaths. Murder and suicide are in second and third place. Unwanted pregnancies, sexually transmitted diseases and behavior leading to imprisonment are also common and have serious consequences for the rest of life.

So what can doctors, parents, teachers and the youth themselves do about these dangers? Due to the lack of new psychiatric drugs and the tendency of the adolescent brain to respond to environmental challenges, doctors tend to think that non-pharmacological interventions may be more effective, especially in early adolescence, when white matter, gray matter and connectivity are rapidly changing. One example is the treatment of obsessive compulsive disorder. Behavioral therapy that stimulates the compulsive urge but gradually changes the response to it may be highly effective and prevent lifelong disability. The understanding that the brain can be changed during the teenage years negates the feeling that certain adolescents are "lost cases". She instills optimism that treatments can change the trajectory of teenagers' lives.

Further studies may also help. The infrastructure for research in adolescents is not well developed, the funding for this type of research is modest and only a few neurobiologists specialize in this age group. The good news is that as researchers elucidate the mechanisms that influence adolescent brain development, more resources and researchers are drawn to the field in a desire to minimize risks to youth and harness the incredible plasticity of their brains.

Understanding that the adolescent brain is unique and rapidly changing may help parents, society, and the youth themselves better deal with the risks and take advantage of the opportunities of the teenage years. For example, the knowledge that the executive functions of the prefrontal cortex are still emerging, may help parents not overreact when their daughter suddenly dyes her hair orange and take comfort in the feeling that there is hope for better judgment in the future. Flexibility also suggests that constructive dialogue between parents and adolescents about issues such as freedom and commitment can influence development.

Adolescents' inherent ability to adapt raises questions about the impact of one of the biggest environmental changes in history: the digital revolution. Computers, video games, cell phones and apps have in the last 20 years had a significant impact on the way teenagers learn, play and communicate with each other. There are huge amounts of information on the subject, but their quality is not uniform. The important future skills are not memorization of facts but critical evaluation of vast amounts of data, understanding between signal and noise, merging content and applying this merging to solve real-world problems. Educators need to challenge the adolescent brain with these tasks and train its flexibility for the demands of the digital age.

The company as a whole also has some opportunities that should be taken advantage of. First, it can focus more on harnessing the passion, creativity and skills of the unique period of adolescence. Society should also understand that the teenage years are a turning point for a life that can be peaceful or can be aggressive or in rare cases, extreme. In all cultures, it is easiest to recruit teenagers as soldiers or terrorists, and it is also easiest to influence them to become teachers or engineers. A better understanding of the adolescent mind can also help judges and juries make decisions in criminal trials.

For the teenagers themselves, the new insights into adolescent neurobiology should encourage them to challenge their brains in the kinds of skills they want to excel in later in life. They have a wonderful opportunity to shape their identity and their minds according to their personal choice, and prepare themselves for a knowledge-rich future that will be dramatically different from their parents' lives now.

About the author

J. N. Giedd is the chairman of the division of child and adolescent psychiatry at the University of California, San Diego, and a professor at the Bloomberg School of Public Health at Johns Hopkins University. He is also the editor-in-chief of the scientific journal Mind, Brain and Education.

in brief

MRI studies show that adolescent brains are not the older brain of a child or the half-baked brain of an adult. He is a unique entity characterized by the ability to change and an increase in connectivity between areas of the brain.
The limbic system, which controls emotions, gets stronger with puberty, but the prefrontal cortex, which controls impulses, doesn't mature until the 20s. This mismatch makes teenagers take risks but also allows them to adapt quickly to their environment.
Earlier puberty in children worldwide extends the years of incompatibility.
A better understanding of the adolescent brain will help parents and society better differentiate between typical behavior and mental illness and at the same time help adolescents achieve their aspirations.

More on the subject

The Primal Teen: What the New Discoveries about the Teenage Brain Tell Us about Our Kids. Barbara Strauch. Doubleday, 2003.
Development of Brain Structural Connectivity between Ages 12 and 30: A 4-Tesla Diffusion Imaging Study in 439 Adolescents and Adults. Emily L. Dennis et al. in NeuroImage, Vol. 64, pages 671-684; January 1, 2013.
Age of Opportunity: Lessons from the New Science of Adolescence. Laurence Steinberg. Houghton Mifflin Harcourt, 2014.

The article was published with the permission of Scientific American Israel