Welcome to Your Child's Brain: How the Mind Grows From Conception to College (25 page)

Though the habit is best started young, the brain benefits of regular aerobic exercise are especially noticeable in old age. Problems with an aging circulatory system can reduce the blood supply that brings oxygen and glucose to your brain. Elderly people who have been athletic all their lives are better at planning and organizing their behavior and dealing with ambiguity or conflicting instructions than sedentary people of the same age. When inactive people get more exercise, even in their seventies, their brain function improves in just a few months. Exercise is also strongly associated with reduced risk of dementia, including Alzheimer’s disease, late in life.

Physical activity leads to growth and increased activity in the frontal and parietal cortex in adults. In brain imaging studies, these areas have been observed
to be active during tests of reading comprehension and mathematical calculation, as well as during many tests of self-control. Fit adults, young or old, show reduced activity (suggesting increased efficiency) in the anterior cingulate cortex, which is involved in detecting errors and resolving conflict between alternatives.

Brain differences related to exercise have also been observed in children. In physically fit nine- and ten-year-olds, the hippocampus and the
dorsal
striatum
(a region important for response selection, cognitive flexibility, and the performance of learned behaviors) are larger than in sedentary children. These differences are related to improved performance on behavioral tasks that rely on these areas. The role of exercise in maturation of the prefrontal cortex of children has not yet been examined, but the behavioral effects of exercise suggest that this brain region might be affected as well.

There are several possible ways that exercise might change the brain, and they are not mutually exclusive. Neuroscientists found that in people and lab animals, exercise causes the release of
growth factors
, proteins that support the growth of dendrites and synapses and increase synaptic plasticity. Exercise also increases the survival of newly formed brain cells in the hippocampus of young adult and old lab animals—and even in rat pups whose mothers exercised during the pregnancy. Proteins that are released during exercise in juvenile and old lab animals stimulate blood vessel growth, helping nutrients from the circulation to reach the brain. Most of these effects have been studied in adult animals, but they probably also occur in children’s brains. The question of how exercise interacts with brain development remains to be studied.

PRACTICAL TIP: PROTECT YOUR CHILD FROM HEAD INJURIES

There’s one big exception to the rule that exercise is good for your brain: the risk of head injuries from contact sports. Consider the case of a twenty-one-year-old college (and high school) football player named Owen Thomas, who killed himself in 2010. He is the youngest person known to have had chronic traumatic encephalopathy, a neurodegenerative disease that was found at autopsy after his suicide. (
Neurodegeneration
is a general term referring to progressive loss of structure and function of neurons.) He had never been diagnosed with a concussion and had no previous history of depression, but he was known as a hard hitter on the field.

Multiple studies have shown that repeated blows to the head increase the likelihood of developing neurological symptoms later in life, including dementia, movement disorders, and depression. As 1.6 to 3.8 million sports-related concussions occur each year in the U.S. alone, this is potentially a big problem. Girls are more likely to be diagnosed with concussions than boys in the same sports, although the difference may be simply that boys are less likely to report symptoms.

Chronic traumatic encephalopathy has a variety of symptoms, including depression, memory loss, and impulse control problems. Its symptoms are easily confused with those of depression, Alzheimer’s disease, Lou Gehrig’s disease, or Parkinson’s disease, so its prevalence remains uncertain. (Some researchers now believe that Gehrig himself may have had chronic traumatic encephalopathy, misdiagnosed as the disease that was named for him.) Individuals who carry the ApoE4 gene variant are at higher risk of Alzheimer’s disease, and this variant also seems to increase the risk of neurodegeneration associated with head injury.

Many popular childhood sports may be risky. Football, wrestling, rugby, hockey, karate, lacrosse, soccer, basketball, and skiing are all associated with repetitive head injury. It is not yet clear how the number, timing, and severity of head injuries relate to these long-term risks. Experiencing three concussions significantly increases risk in most studies, and some studies find increased risk after a single concussion (or undiagnosed head injuries, as in Thomas’s case).

Compared to adults, children take longer to recover from a concussion. This may be because there is less space between the brain and the skull in childhood, making brain swelling after injury a more serious problem for children than adults. Children are also especially susceptible to complications from reinjury during the recovery period. Over 90 percent of deaths due to sports-related head injury since 1945 in the U.S. have been athletes who had not yet finished high school.

Though longitudinal research is needed to sort out these questions, caution seems well justified already. Children whose family history puts them at risk of Alzheimer’s disease should be especially careful to avoid head injury. No child should return to play with any lingering symptoms of a concussion. (Not all concussions lead to loss of consciousness; confusion and memory loss after a head injury are also signs of concussion.) During recovery, children should rest their brains as well as their bodies, avoiding schoolwork and even reading. Finally, parents should seriously consider a nonimpact sport for children who have suffered more than one head injury in their current sport. For more information, see
http://www.cdc.gov/concussion
.

To gain the benefits of exercise on the brain, children of all ages should have fun moving their bodies for at least an hour a day. Young children rarely need much encouragement to run around, so most kids meet or exceed the activity requirement through elementary school. In addition to letting kids play tag and climb trees, which are unlikely to continue into adulthood, parents should take advantage of these active years to introduce their children to sports that can become lifelong hobbies and social activities, such as martial arts, dance, softball, or hiking. Parents can also help by limiting children’s exposure to sedentary activities like TV and computer time (see
chapter 16
) and by demonstrating to their children that exercise is a regular feature of adult life.

In our fat-obsessed culture, it is tempting to focus on weight control as a key benefit of exercise, but this approach is likely to be counterproductive (see
Practical tip: Worried about your child’s weight?
). Thinking of exercise as a part of dieting defines it as an occasional activity, not something you do every day for fun—and makes it feel like a punishment for bad behavior, especially to young girls. No one looks forward to activities associated with shame and guilt, so using this type of motivation is likely to be actively harmful to your attempts to make exercise a regular and enjoyable part of your child’s life. Exercise is beneficial for everyone, fat or thin, whether or not it leads to weight loss.

There are also social reasons for children to master motor skills when they’re young. A swing and a miss, which is cute in a preschooler playing T-ball, may be mortifying to an adolescent boy in gym class. The embarrassment associated with attempts to catch up with the better-practiced skills of peers often leads to a lifelong distaste for athletics. In longitudinal studies, an inactive childhood is a strong predictor of an inactive adult life.

Parents’ efforts in laying this sort of groundwork during middle childhood are likely to pay off in adolescence and adulthood. Physical activity typically decreases in the early teen years, on average at age thirteen for girls and fifteen for boys, as organized activities and socializing squeeze out competitors like exercise and sleep (see
chapter 9
). For many kids, this stage of development marks the divide between an active childhood and a sedentary adulthood, with substantial consequences for adult health.

Through most of our evolutionary history, people were much more active than they are today. Our bodies and our brains are adapted to regular exercise, and they do not function well without it. There are exercises for every child’s taste, from solitary distance running to competitive team sports to sociable yoga classes. Helping your children find a few that suit their personalities and talents will go a long way toward giving them a good start in life.

Chapter 16
ELECTRONIC ENTERTAINMENT AND THE MULTITASKING MYTH

AGES: BIRTH TO EIGHTEEN YEARS

Both of us are young enough to be distracted frequently by e-mail or the Web when we’re trying to work, but we’re old enough that our brains didn’t develop under those conditions. In contrast, many of today’s children are growing up with continuous access to electronic media, from the TV in the bedroom to video games for the road. In the U.S., the average baby starts watching TV at five months of age, before he can sit up by himself. By seventh grade, 82 percent of children are online.

Your child’s brain is wired to seek out and pay attention to new information because our ancestors’ survival often depended on detecting changes in the environment—from the arrival of a lion to the new expression on a mate’s face. But what happens to our brains when getting new information becomes too easy? Our society seems to be in the process of finding out, as the Internet brings an avalanche of facts and ideas (along with daredevil stunts and keyboard-playing cats) into our lives.

Researchers do not yet know the full effects of exposing children to a constant stream of highly salient stimuli, as the new media environment is a recent occurrence. Let’s start with some basic conceptual principles, established by decades of neuroscience research. Throughout this book, we have emphasized that your child’s brain depends on commonly available sensory experiences to help
determine which neural connections should be maintained or lost. If the character of these experiences changes dramatically, we would expect to see effects on brain development. In the case of new media, the few experiments that have been done support this idea, though not with certainty. One thing we do know is that the effects of media experience on the brain depend on the details—whether your child is passively watching TV or actively playing video games, at what age the experience occurs, as well as which activities your child is neglecting in order to find time for it all.

At any given moment in everyday life, deciding which information should have your attention is far from simple. Your brain needs to combine the ability to concentrate deeply on a particular problem with the ability to reorient itself quickly if important changes happen. To achieve this balance, top-down attention, the deliberate direction of your brain’s focus by the cortex, must compete with bottom-up attention, the automatic capture of your brain’s focus by salient events, such as someone screaming—or the buzz of your cell phone reporting an incoming text message. You’ve probably experienced moments when one system dominated the other: either your concentration was so deep that you didn’t hear your baby cry, or you were getting interrupted so often that you couldn’t make any progress on your work assignment.

The bottom-up system is functional at birth, earlier than the top-down system, which can take control for brief periods by the end of the first year and continues to improve at least to age ten and perhaps into adolescence. This discrepancy in the maturity of the two systems is why younger children are easier to distract than older children. As we discussed in
chapter 13
, the ability to use executive function is a key ingredient of self-control and improves with practice.