Welcome to Your Brain (16 page)

aim is to shoot as many enemies as possible before they shoot you. These games require

players to distribute attention across the screen and quickly detect and react to events.

Unfortunately, playing Tetris doesn’t have the same effect on the brain, perhaps because it

requires players to concentrate on only one object at a time, rather than multitasking.

In one study, college students who played action games regularly could count 50

percent more items in a very brief visual stimulus than students who didn’t play. The

players also processed information more quickly, could track more objects at once, and had

better task-switching abilities. You might imagine that people with naturally strong abilities

were better at the games and thus chose to play more often. But a group of nonplayers was

able to improve their attentional capacity after training one hour per day for ten days on an

action game, suggesting that these skills develop as a direct result of practice.

Does this mean that parents should encourage their kids to play shoot-’em-up action

games? We wouldn’t go out of our way to expose kids to violent images, but at least

parents can take heart that video game playing has positive effects. In the long run, we’d

love to see somebody make a lot of money by designing action-based video games that

motivate kids to practice multitasking and improve their attentional capacity without using

violence as the motivator. Kind of like Sim City … on a runaway bus!

Another possible explanation for adolescent behavior derives from work on rodents, so we don’t

yet know if it applies to humans. Neurons containing the neurotransmitter dopamine, as well as the

sensitivity of their targets, may help set individual levels of risk taking and responsiveness to

rewards, which include social experiences, novelty, and psychoactive drugs. These neurons connect

to the prefrontal cortex as well as the striatum and areas important for processing emotions, such as

the nucleus accumbens and the amygdala. The balance between these connections seems to change

during adolescence. In the early stages, cortical connections dominate and others are weaker, which

seems to favor novelty seeking; this situation reverses by late adolescence. During adolescence, the

cortical dopamine system is thought to be particularly sensitive to stress, making animals—both

human and rodent—more vulnerable to stressors.

The brain maturation process also seems to make teenagers vulnerable for the first time to a

variety of psychiatric disorders. Adolescence is marked by a gradual increase in the risk of mood

disorders and psychosis, as well the emergence of gender differences in these disorders. People who

are diagnosed with schizophrenia in their twenties often turn out to have exhibited their initial

symptoms in adolescence. Similarly, rates of depression and anxiety disorders begin to increase at

thirteen or fourteen years of age and reach adult levels by age eighteen. Twice as many women as

men suffer from these mood disorders, and this difference emerges at puberty. Why puberty increases

the risk of such brain dysfunction is not well understood at this point.

We still have only a preliminary understanding of how brain structures generate behaviors.

Although prefrontal brain structures are still developing at times when risk taking and impulsivity are

high, it isn’t clear when or how a partially mature brain structure begins to function. For instance,

prefrontal development does not appear to be very different in men and women, yet men engage in far

more risky behaviors. (Our experience follows suit: of all our scrapes, only one car accident and one

emergency room trip were Sandra’s; the rest were Sam’s.) The basis of this gender difference is not

clear, though it may be related to differences in dopaminergic systems, as male rats show a much

stronger decrease in dopamine receptors in the striatum than females during adolescence.

The idea that delayed brain maturation explains adolescent behavior is an attractive one and has

been discussed extensively by journalists. With adolescence commonly characterized by rebellion,

risk taking, and a tendency to ignore consequences, it’s no surprise that parents are excited by

research suggesting that teenage brains are not yet fully formed. It’s comforting to think that bad

behavior is a result of delayed brain maturation. That means it’s not the parents’ fault, it’s not the

kids’ fault, and most importantly, it’s a problem that will solve itself as they grow up.

Did you know? Brain growth and intelligence

You might imagine that a bigger brain would be associated with greater intelligence,

but the relationship between brain size and intelligence is weak in adults, and there’s no

measurable relationship in young children. However, research suggests that intelligence

and brain structure could be related in a more subtle way during brain development.

A key component of intelligence may depend on when in development synapses are

formed—and removed. One study found evidence that intelligence correlates with patterns

of growth and shrinkage during childhood and adolescence. Over the course of more than a

decade, the scientists used imaging methods to monitor the brain structures of more than

three hundred children, tracking their development from age seven to nineteen. They

divided the kids into three groups according to how they did on a standardized IQ test.

Higher intelligence was related to the timing of the cortical sheet’s thickening: the

higher a child’s intelligence, the later the thickness of his or her cortex peaked. The

thickness of the cortical sheet ended up the same in all three groups by the age of nineteen.

On average, the sheet thickness peaked earliest in children of average intelligence, and

latest in children with IQ scores greater than 120. Peak thickness, after which shrinkage to

adult levels occurred, typically began between age seven and nine in normal or above-

average children, but was delayed until age eleven in the highest-IQ children.

What is happening in the brain during these changes? It’s not the birth of new neurons.

The brain reaches approximately 90 percent of its adult size by age six, when nearly all the

neurons of the brain have already been born. The remaining increase in brain size has to be

caused by other forms of growth. For instance, dendrites and axons run through the

thickness of the cortex, suggesting that they might become longer or more bushy at a

prolonged, steady pace in high-achieving kids. Increases and decreases in cortical

thickness may therefore be related to the formation and loss of synaptic connections.

The growth and shrinkage of synaptic connections is interesting because it suggests that

the formation and weeding out of connections between neurons might be critical aspects of

intellectual development in children and teenagers. But even though these differences are

starting to be noticed between groups of children, it’s not time to send your child for a brain

scan. All the trends we’ve described were only apparent by averaging the results from

dozens of children. The effects are too small to predict how your child will do in school.

Although the evidence that delayed brain maturation is responsible for adolescent behavior is

largely speculative, the idea does have some support. One aspect of brain structure continues to

develop until about age twenty-one: long-distance connections. Although most neurons are present by

age two, the connections between them take much longer to mature. Axons, the wires that carry

electrical signals from one neuron to another, are covered with an insulating sheath called myelin that

allows electrical signals to move faster and more efficiently. The process of myelination is the last

stage of brain development, and it’s not complete until early adulthood. The last brain area to finish

myelinating is the prefrontal cortex, which is important for inhibiting behavior and selecting behavior

that’s appropriate for meeting goals—two abilities that many teenagers seem to lack. At the same

time, emotional areas are fully developed. This discontinuity in development may mean that emotions

aren’t regulated as well as they should be.

Even though prefrontal areas are still growing at this stage, other brain regions have developed to

adult levels of size and myelination. As a result, adolescents are mature in their reflexes and their

capacity to acquire new information. Indeed, compared to adults, they learn—and forget—new facts

more quickly.

All these signs of maturity and aptitude can make young people highly functional. Indeed, many

rural cultures across the world begin to treat young people as adults when they are twelve or thirteen

years old. To a modern reader it may seem odd, but adolescence is a relatively recent invention,

largely restricted to urban societies within the past century or so. This could be due to the increased

complexity of life in the twentieth and twenty-first centuries, which requires that education last

longer. Or perhaps growing up, like so many tasks, has expanded to fill our now longer lifespans.

Chapter 13

An Educational Tour: Learning

Imagine a dog who hangs around the front yard and chases every car that comes down the street. One

day, a red Corvette driven by a neighborhood teenager hits the dog, fracturing its leg. The dog’s

owner would like this experience to teach his pet the lesson that chasing cars is a bad idea. But that’s

not the only possibility. Instead the dog may learn that he shouldn’t chase red cars, or that he should

go to another street to chase cars, or that he should be afraid of teenagers. On the other hand, imagine

another dog who has been beaten by his first owner and now remains forever afraid of people, no

matter how kind they are. The first dog has not generalized enough from his experience, while the

second dog has generalized too much.

We all learn from experience, but figuring out exactly what we should learn can be very

complicated. We all know people who repeat the same mistake over and over, even though they’re

punished for it, or who decide from one bad relationship that no potential partner can ever be trusted.

Why does this happen?

What we learn is influenced by many factors: the biological characteristics of our species,

individual genetic factors, and personal experience. Not only do different animals have different

natural behaviors, they’re also specialized to learn certain behaviors more easily than others. Animal

trainers know that it’s easy to teach tricks that follow these natural tendencies, but very difficult to go

against them. In the wild, pigs make their living by digging up roots with their wide, flat noses. Not

only are their bodies shaped by evolution to suit this activity, but so too are their brains. For this

reason, it’s hard to teach pigs to balance a coin on their noses; instead, the pigs tend to bury the coin

and dig it up repeatedly, even if this activity is unrewarded or punished. Similarly, chickens tend to

peck at things, so it’s easy to train them to peck a key for a reward, but hard to teach them to stand on

a platform without scratching or pecking. Some behaviors can’t be conditioned at all. For instance,

rewarding a hamster for scratching herself is an exercise in futility; hamsters will scratch only when

they feel like it, no matter how hard you try to persuade them to change their ways.

Practical tip: Should you cram for an exam?

We’ve all done our share of cramming. Nearly everyone gets into a situation at some

point where they’ve fallen behind in class and there’s not enough time to catch up before

the test. Studying intensively at the last minute may allow you to pass the exam, which

certainly has some value, but it’s not the best use of your time. Why? Psychologists have

known for more than a century that your brain retains many kinds of information longer if it

has an opportunity to process what you’ve learned between training sessions.

The advantage of spread-out learning is large and reliable. Two study sessions with

time between them can result in twice as much learning as a single study session of the

same total length. Spaced training works with students of all ages and ability levels, across

a variety of topics and teaching procedures. Unsurprisingly, it also works with other

animals, so you’d do well to remember this principle when you’re trying to train your dog.

Learning also varies among individuals of the same species. Behavioral differences between

individuals are mainly due to differences in their brain anatomy, particularly in the connections

between neurons. Are you an impulsive person who reacts quickly to events, or is your behavior calm

and deliberate? Are you a talented skier? Do you know the capitals of all fifty states? Are you good at

solving mechanical problems? All these abilities are based on the way that your neurons talk to each

other, a combination of how your brain was wired up when you were a baby and the connections that

have formed or broken since then through learning.

Neural connections generally follow a rule known to your high school coach: use it or lose it.

Neurons strengthen synapses that are effective and weaken or remove synapses that stay silent when

other synapses are being used. This process occurs more easily in babies, but it continues throughout