Welcome to Your Child's Brain: How the Mind Grows From Conception to College (19 page)

The brain pathways that process touch information are not fully formed until the fifth month of pregnancy. The brain knows which kind of sensor is activated, and where it is on the body, because each sensor has a “private line” that uses spikes to carry only one kind of information to the brain. (There is a separate brain area for painful sensations.) At birth, babies can identify touch and temperature sensations; they turn away from a cold object touching their cheek and toward a warm object. Because the axons that carry touch information are not yet myelinated, babies are eight times slower than adults in sensing most types of touch. Children’s processing speed improves in the first year but does not reach adult levels until age six. The exception is pain, which is carried by unmyelinated axons even in adults and so is processed just as quickly by babies.

Some parts of your baby’s body are more sensitive than others. As in other sensory systems, the cortical areas that process touch are arranged in a map, with nearby parts of the body represented by nearby neurons. The proportions of these maps are based on the number of receptors in each part of the body, rather than its size, so that the part of the map that receives information from the face is larger than the area that receives information from the entire chest and legs. Your face is much more sensitive. In adults, the highest density of touch receptors is found on the fingertips, with the face a close second.

PRACTICAL TIP: PROTECT YOUR CHILD FROM NOISE, STARTING BEFORE BIRTH

Another problem in our modern environment that evolution did not prepare us to handle is noise exposure, the most common cause of hearing impairment. Its victims are getting younger every year. One in eight U.S. children between the ages of six and nineteen now has some hearing loss, and it’s likely to worsen as they get older. Hearing loss is more common in boys than girls, probably because of differences in their activities. Noise can even induce hearing loss before birth, if the mother is exposed to chronic loud noise during the third trimester of pregnancy. In fact, babies’ hearing is most easily damaged by noise exposure in the last trimester and first six months of life. Premature babies are especially vulnerable to noise-induced hearing loss.

Loud noise destroys hair cells in the cochlea, starting with the most vulnerable cells, those that transduce high-frequency sounds. Doctors cannot reverse this damage, and hearing aids do not restore sound levels. The earliest symptom of hearing loss is usually difficulty understanding speech when there is background noise. By then, as many as half of the cochlear hair cells are already dead. Hearing loss is particularly damaging in children because it can impair language learning and academic accomplishment.

Tinnitus
,
a constant ringing in the ears, is another potential consequence of noise exposure that can interfere with hearing. Noise is also a chronic stressor, which means tinnitus can interfere with child development in many ways (see
chapters 26
and
30
).

Hearing loss is caused by brief exposure to very loud noises, like firecrackers, or by chronic exposure to moderate noise levels, such as city traffic. (Living near a loud highway could damage your child’s hearing as much as setting off firecrackers in her bedroom.) The most common risk factors for kids are rock concerts and portable music players like the iPod. These devices are typically played at 75 to 105 decibels, which is equivalent to the range between a loud conversation and a Harley-Davidson. Temporary hearing loss or tinnitus after listening to music is a warning sign; repeated exposure to such noise levels will cause permanent hearing loss.

The volume makes a big difference. With standard iPod ear buds, you can safely listen at 80 percent of maximum volume for ninety minutes per day, or at 70 percent of maximum for four and a half hours per day, but at full volume only for five minutes per day. Your kids should be especially careful when listening to music in a noisy environment, such as on an airplane or in the subway, which generates a temptation to turn the music up too loud. You can protect your children by downloading software that limits the music player’s volume, or by investing in noise-canceling headphones (if they don’t look too dorky for your child’s sense of style). Your kids may not appreciate it now, but at least they’ll be able to hear their own kids’ complaints someday.

DID YOU KNOW? THE NEUROSCIENCE OF SNUGGLE

Different types of touch are transmitted to your child’s brain by “private lines” that carry those particular types of information exclusively. Your child’s brain (like your own) has a special pathway dedicated to the kind of touch that leads to emotional bonding. As we’ve said, the skin contains more than a dozen anatomical types of receptors specialized for detecting particular sensations, such as temperature, pressure, and pain. One in particular is tuned to the pleasurable skin sensations that are produced by light stroking. The axons that carry information about these sensations to the brain are unmyelinated, meaning that their responses are slow. In human recordings, the electrical activity of these axons in response to gentle stroking is proportional to how pleasant the person reports the touch to be.

Damage to these pathways as they run through the spinal cord impairs emotional responses to touch, without affecting the ability to identify an object by touch. In the brain, the “pleasantness pathway” brings touch information to a region of cortex called the
anterior insula
, rather than to the somatosensory cortex where most touch-responsive fibers send their signals. The anterior insula, which receives input from a vast variety of systems, seems to be involved in monitoring a range of internal states, from thirst to maternal love.

This map develops sequentially, beginning at the head region, which is why newborn babies actively explore objects with their mouths but not with their hands. Think about the surprising number of familiar objects whose flavor you can imagine—doorknobs, grass blades, and so on. Evidently you’ve been tasting things for a long time.

The ability to discriminate objects with the hands develops slowly, and the face remains more sensitive than the hands even at age five. As the maps in the somatosensory cortex mature, babies become able to localize touch more accurately and discriminate touches that are closer together on the skin. These maps are initially established by genetic mechanisms, but their maintenance depends on experience even in adulthood. For example, the cortical space devoted to an amputated limb is eventually taken over by inputs from adjacent areas of the body (the cause of phantom limb syndrome, in which amputees imagine they can feel sensations from a missing limb).

Babies who are not touched enough in early life become developmentally delayed, demonstrating one limit to the dandelion nature of brain growth. This problem happens most often in poorly organized institutional care, such as orphanages with inadequate staffing or intensive care units in which premature infants are isolated from human contact. Cuddling is more important than food to early bonding; in experiments, baby monkeys deprived of maternal contact spent most of their time with a soft surrogate made of terry cloth, ignoring a wire surrogate that provided milk except during brief visits to feed. In most homes, though, you’re more likely to have trouble getting family members to stop playing with the baby at bedtime than you are to have a baby who’s not getting enough snuggling.

Chapter 12
EAT DESSERT FIRST: FLAVOR PREFERENCES

AGES: SECOND TRIMESTER TO TWO YEARS

Unlike many American toddlers, Sam’s daughter loved sushi. From a young age, she made it impossible for her parents to eat their raw fish in peace, without little orange fish eggs flying everywhere. Though we can’t say for sure, we suspect that a trip she took before she was even born might be responsible for this odd turn of events.

Sam and his wife traveled to Japan in the second trimester of her pregnancy. Sam’s wife adores sushi. As a physician, she knows that sushi is safe for babies in utero—and perhaps even beneficial for brain development (see
Practical tip: Eat fish during pregnancy
). So she ate a lot of it on that trip and afterward. When we started reading studies showing that children’s food preferences are influenced by what their mothers ate during pregnancy, we thought we might have found the connection.

Like vision and hearing, our basic ability to identify smells and tastes is built on sense organs and input pathways to the brain that develop largely on their own. Starting from our noses and tongues, the initial stages of input wiring connect with brain structures without much help. But much of the formation of preferences for smells and tastes depends on experience. As omnivores, people can eat such a wide range of foods that it would be difficult to genetically program innate food preferences that would apply to all possible environments. This may explain why food preferences are so idiosyncratic; for instance, many children in the U.S. like root beer, but in some parts of Europe it is considered intolerable.

The learning process starts surprisingly early, well before birth. Newborns have smell and taste systems that are well developed—and already know a thing or two. They can express an immediate preference for the flavor of milk and their mother’s nipple, and they can even distinguish between the smell of their own mother and others. (Indeed, a mother can do the same, distinguishing a shirt worn by her baby from those of other newborns.)

At birth, amniotic fluid odors serve a bridging function, helping the infant coming out of the womb by soothing him and by helping to establish preferences for Mom (and her milk). During the first week of life, your baby’s preference for the odor of amniotic fluid decreases, while a preference for natural breast odors increases. This preference for breast odor can even be helpful. A breast-fed infant attaches to the breast faster if it bears natural odor than if it has been washed.
So if nursing is an issue, you might try not washing. For similar reasons, if Mom wears perfume, switching perfumes may slightly confuse the baby at first.