Welcome to Your Child's Brain: How the Mind Grows From Conception to College (12 page)

In one study, researchers tested the English grammar proficiency of Chinese or Korean immigrants who had arrived in the U.S. at various ages and stayed at least five years. The test required participants to identify whether there were grammatical errors in sentences like “Tom is reading book in the bathtub” or “The man climbed the ladder up carefully.” The test was simple enough that native English speakers could ace it by the age of six, but the immigrants who began learning English after age seventeen missed many of these simple questions. Only people who came to the U.S. before age seven performed at the level of native speakers. Everyone in the group who arrived at eight to ten years of age did a bit worse, and those who arrived at eleven to fifteen were still less proficient.

Between ages eight and fifteen, researchers found a strong relationship between age of exposure and performance on the test. But in adulthood, individual variability in performance was not connected to age. No matter whether they’d started learning English at eighteen or forty, few adults learned perfectly. (Some later researchers found that language learning in adulthood also declines with age—that is, young adults learn better than older adults—but everyone agrees that young children learn better than older people.)

The take-home message for parents and schools is clear: take advantage of young children’s superior language learning abilities by beginning instruction in elementary school or earlier. When it comes to language, there’s no substitute for an early start.

There seem to be at least two sensitive periods for language learning. We already discussed the sensitive period for phonemes, in the first year or two of life, when babies’ brains become specialized for representing the sounds of their native language(s). There is also a sensitive period for learning about grammar. Children’s ability to acquire syntax rules declines gradually after age eight, and adults are worse than children at learning languages (see
Practical tip: Teach foreign languages early in life
).

Some adults manage to learn a second language to a high level of proficiency. Most of us, though, no matter how hard we study in adulthood, will always have a foreign accent and make minor grammatical errors. In contrast, there does not appear to be a sensitive period for semantic learning, as new vocabulary words can be acquired equally well at any age. The event-related potential signal for semantic violations looks the same for both native and second languages, even in people who learned their second language late in life.

Children can learn more than one native language if they are exposed to both languages early enough, but their brains appear to represent the languages at least somewhat separately. Bilingual children reach language milestones at the same age and have the same risk of language impairment as monolingual children, though the details of their language development are somewhat different. So if your household is bilingual, the research indicates that this is not a disadvantage for your child’s language learning. (Indeed, it may be an advantage for cognitive development; see
Practical tip: Learning two languages improves cognitive control
.) Learning a second language also changes the brain. A region in the left inferior parietal cortex is larger in people who speak more than one language, and it is largest in those who learned the second language when they were young or speak it fluently.

Infants quickly learn to identify different languages by their rhythms, their characteristic phonemes, and other cues. Bilingual children do sometimes mix languages in their speech, but they seem to do so for the same reasons and in the same situations as adult bilinguals, for instance, substituting a word from one language when they don’t know the word for that concept in the other one. Though bilingual children have a smaller vocabulary in a particular language than monolingual children of the same age, bilingual children know more words in total if you count both languages.

Children who hear more words while interacting with their parents in the first two years of life learn language faster than children who hear fewer words. These differences in home environments tend to fall along socioeconomic class lines. In one study, the poorest children heard 600 words an hour, working-class children heard 1,200 words, and children of professionals heard 2,100 words. These major differences in children’s language environment correlate with their later language development and IQ scores—though the finding that highly verbal parents raise highly verbal children may be partly due to genetic factors or the many other advantages of growing up in a professional household (see
chapter 30
).

Later research has shown that you can improve your children’s language skills by responding rapidly to their vocalizations, mimicking the turn-taking of conversation even before your baby is capable of forming words. Responding with a comment or a touch to your baby’s best attempts to communicate seems to encourage continued efforts to improve these skills. So talk to your baby and put up a good show of understanding what she’s saying. It’s fun for both of you, and it will help her language skills to develop more quickly.

Chapter 7
BEAUTIFUL DREAMER

AGES: BIRTH TO NINE YEARS

Sleep appears to be simple, but it is composed of many brain mechanisms working together—mostly seamlessly. In babies and young children, these brain functions mature at different times. The complex abilities involved in sleep become apparent in stages as your child grows. The intense need for sleep early in life may be related to its importance in facilitating learning.

The first function to appear, well before birth, is the internal
circadian
(Latin
circa dies
, meaning “approximately a day”)
rhythm
. This clock can run for many days without external instruction, providing our brains and bodies with cues about our daily activities even if we can’t see the sun. The brain can generate an approximately twenty-four-hour rhythm without light, using a complex signaling clockwork made of genes and proteins. This clockwork’s output is used by other brain regions and organs to set their own daily rhythms, for hunger, bowel movements, body temperature, liver activity, and stress hormone secretion.

This daily rhythm of our brains and bodies is driven by the
suprachiasmatic nucleus
, a dab of tissue containing fewer than ten thousand neurons that sits over the
optic chiasm
, where the optic nerves meet and cross on their way toward the brain. The suprachiasmatic nucleus gets its signals from ganglion cells in the retina that are dedicated to transmitting information about light levels in the world. These ganglion cells, which make a pigment protein called
melanopsin
, convert light to impulses that travel along the optic nerve to the suprachiasmatic nucleus. In this way, the brain knows when it is day and when it is night.

The fetus has a suprachiasmatic nucleus at eighteen weeks of gestation. Several weeks later, circadian cycles are found in the fetus’s heartbeat and breathing. This rhythm is probably driven by day-night signals from the mother, such as the rhythmic release of three hormones:
corticotropin-releasing hormone (CRH)
,
cortisol
, and the sleep-inducing signal
melatonin
.

Once your baby is born, that rhythm is suddenly lost. As any new parent can wearily tell you, newborns have highly irregular sleep patterns, though it is possible to drive the rhythm a bit through feeding times. Starting around three months of age, your infant’s sleep-wake patterns start to be influenced by cues such as the timing of feedings and nighttime routine. So you can make the baby’s pattern regular by providing a set daily routine. Even so, for the first few weeks after birth, there is almost no day-to-day pattern. The sleep-wake cycle in
infants typically lasts about fifty to sixty minutes, with no relationship to time of day. Later, this cycle of sleep fades into an
ultradian
(meaning “less than a day long”) cycle of greater and lesser alertness, one that has a rhythm of about an hour through age three and lengthens by age five to ninety minutes, the cycle that continues for the remainder of life.

Infants spend about two thirds of their time sleeping, about half in active sleep that is similar to adult rapid-eye-movement (REM) sleep. In adult REM sleep, muscles under voluntary control shut down except for the eyes, a phenomenon that prevents us from acting out our dreams. In contrast, infants are hardly mobile and not in a position to act out much of anything. In the face of such relative safety, your infant can move a lot during active sleep: he can make sucking motions, twitch, smile, frown, and even move his limbs. But it is unlikely that he is dreaming about movement or action, if indeed he dreams at all (see
Did you know? What children dream about
).

With or without dreaming, babies’ active sleep may serve essential functions. As we saw in
chapter 5
, early development is a period of massive growth and pruning back of neuronal connections. Neuroscientists can observe these changes most clearly in animals. In cats, kittenhood is a time when dendrites in the neo-cortex are remodeled in response to changes in visual experience—and conversely, to visual deprivation. Sleep enhances this remodeling process, a necessary part of development. Conversely, sleep loss reduces the changeability of dendrite structure, a major component of neural plasticity. Research suggests that this is true in both adult and juvenile mammals. One possible consequence of all this remodeling is that sleep may help to consolidate some kinds of learning, including the transfer of information from short-term to long-term memory (see
chapter 21
).

Over the next six months, as the baby encounters daily light and dark cycles for the first time, the circadian rhythm is gradually regained. The first sign of progress is a slight drop in core body temperature in the early morning every day. By three months, about two thirds of children sleep at least five hours at night.

The conscious, awake state depends on columns of neurons deep in the brainstem called the
reticular formation
. The reticular formation is roused to activity when we are awake by mechanisms that are not fully understood, but involve neurotransmitters that are secreted in the brain’s core, such as
acetylcholine
and
norepinephrine
(also called
noradrenaline
).

REM sleep is controlled by neurons in the
pons
. These neurons send
axons—and commands—both forward and downward. Descending connections prevent motor neurons from firing and therefore from causing muscle contraction, using several as-yet unidentified neurotransmitter pathways. The function of the forward-projecting connections is unknown, but they may drive plasticity—and perhaps dream activity. During non-REM sleep, movement is still possible, but sensory input does not get through very well, especially during deep non-REM sleep.

As babies grow, sleep changes. The amount a baby sleeps declines gradually, reaching twelve hours per day by age two. At the same time, REM sleep diminishes dramatically, as do nighttime melatonin levels. By age three, children spend just one fifth of sleep time in REM sleep, the same proportion as teens and adults. By age six, sleepers alternate between non-REM and REM sleep over a ninety-minute period, the same duration as the adult sleep cycle.

Watch for drowsiness and act quickly: put the baby down to sleep right away. Babies cycle in alertness, just as adults do, but more quickly.

The development of sleep does not always go smoothly. During sleep, many events need to be suppressed to keep the sleeper safe, whether animal or human. The sleeper doesn’t urinate. The sleeper doesn’t act out dreams by walking around or making noise, thereby attracting predators. In children, these safety mechanisms are still under construction, as you may have noticed if your child has not yet acquired them. Before the age of six, one child in three experiences interruptions called
parasomnias
, which include a suite of problems such as sleepwalking, bedwetting, and night terrors. If parasomnias do occur, they begin between three and six and are largely resolved by the onset of puberty. Parasomnias usually happen during the first few hours of the night, during the deepest sleep just before the evening’s first bout of REM sleep.

One parasomnia can be particularly upsetting to a parent: night terrors, which occur in 1 to 6 percent of children age three or older. In younger children, they can occur at least once a week. Night terrors consist of waking, typically from deep non-REM sleep, with expressions of fear, often including screaming. The child is inconsolable and takes from five minutes to half an hour to settle down. In the morning, he does not remember anything.