Welcome to Your Child's Brain: How the Mind Grows From Conception to College (11 page)

Even infants are not blank slates whose brains and behaviors are infinitely modifiable. Before sensory experience can act on a child’s brain, the neurons need to be able to talk with each other via synaptic connections. Developmental programs specify particular patterns of connectivity, which are standard for all individuals. Unless there is a genetic error or a developmental accident, the output cells of the eyes will send their axons to the visual areas of the thalamus, which will pass the information along to the primary visual cortex. Axons that carry signals from the touch-sensitive receptors in the fingertips will occupy more space in the somatosensory cortex than axons carrying signals from the less sensitive elbow, and so on.

Under most circumstances, these connection patterns are adaptive, but in unusual cases, this may not be true. In people who cannot see, parts of the visual cortex can be taken over by adjacent regions and used for other functions. Similar types of plasticity allow people to recover from impairments due to strokes by using another part of the brain to compensate for the damaged region. But if the damage is extensive, recovery is likely to be incomplete.

Plasticity outside a sensitive period, if it is possible at all, usually requires more than simple exposure to stimuli. For instance, adults whose vision was impaired by
amblyopia
(also known as lazy eye) can improve their sight after extensive practice on a challenging task, a far cry from the effortless development of the same abilities in normal children. You can change the floor plan of your house after it is complete, but it is much easier to change it during construction.

Retraining the brain in adulthood is possible in some cases, but it is slow and difficult—as it should be. Neural plasticity has costs as well as benefits. Perhaps most important, if routine experience could easily change your brain, you would risk losing hard-won knowledge, abilities, and memories that you acquired earlier in life.

In some cases, higher-level brain areas can compensate for poor development at lower levels, so that adult behavior is relatively unaffected. For example, depth perception can be determined from a variety of visual cues, so people who lack binocular vision due to abnormal visual experience (see
chapter 10
) often can use other strategies to determine depth accurately.

As we have already said, learning language requires experience during a sensitive period. In extreme cases, children who grow up in a poor-quality language environment can fall progressively further behind as development continues. But in normal circumstances, babies are sponges for language. You don’t need to train your baby to imitate your voice instead of the sounds of the family car because her brain areas for language are best activated by speech sounds and because language acquisition, like so many other types of learning, is most effectively driven by social interactions. In the next chapter, we will consider language further as a well-studied example of a sensitive period.

Chapter 6
BORN LINGUISTS

AGES: BIRTH TO EIGHT YEARS

Complex skills require deep foundations. Babies start to learn language a long time before they are able to speak, preferentially focusing their attention on speech from birth—or even earlier, as hearing becomes functional during the third trimester of pregnancy (see
chapter 11
). Because babies do not have the motor abilities to express all the knowledge that they have obtained, though, you may not realize how much language they understand at a given age.

Newborn babies already prefer their mother’s voice over other female voices, their native language over other languages, and speech over other sounds that have the same acoustic properties, including speech played backward. They can also detect a variety of vocal cues, including acoustic characteristics, stress patterns, and the rhythms of different languages. From early in life, your infant absorbs the huge amounts of information that will make him an expert in his native language, learning about its cadences, its sounds, the structures of its words, and the grammar of its sentences. As we discussed in
chapter 3
, most adults instinctively speak to infants in motherese, which is slower than normal language and contains exaggerated versions of consonant and vowel sounds.

Young infants can distinguish and categorize the sounds of all languages of the world, though adults often confuse the sounds of a foreign language. For example, the
r
and
l
of English sound the same to Japanese adults, but different to Japanese infants. As they acquire experience with speech, babies begin to specialize in the sounds (called
phonemes
) of their own language (or languages). By six months of age (for vowels) or ten months (for consonants), babies become better at identifying the phonemes of their native language and worse at identifying the phonemes of other languages. In other words, experience with language shapes
the categories into which babies place sounds, determining which variations in sound characteristics are meaningful (reflecting different phonemes) and which should be ignored (reflecting different speakers or other unimportant variations).

As we would expect, their neural activity reflects this phoneme learning. In older infants, the patterns of electrical signals in the brain recorded from electrodes on the scalp, termed
event-related potentials
, show that babies distinguish between a pair of sounds from the native language, while failing to distinguish two confusable foreign sounds. In younger infants, event-related potential patterns distinguish both foreign- and native-language sound pairs. This brain specialization is important for future language learning. Babies whose brains discriminate native sounds well (and foreign sounds poorly) at seven and a half months go on to learn language earlier than babies who show the less mature pattern of distinguishing all sounds equally well. The more discriminating babies learn words more quickly, produce more words and more complex sentences at twenty-four months, and produce longer phrases at thirty months than the less discriminating babies. So even though your baby isn’t talking back, he is absorbing the patterns of your talk.

Social interaction is one cue that babies use to determine which sounds they should be learning. Nine-month-old infants who hear a brief tape recording or video of someone speaking a new language do not learn its sounds, but the same amount of speech from a live person is sufficient to allow the babies to discriminate phonemes in the new language. (Under some circumstances, babies can learn from tape or video, but it takes longer than learning from a live person.) Indeed, certain measures of social interaction with a language teacher (including a parent) predict how well individual infants will remember the sounds of the new language. The preference for social interaction may be part of the reason that autistic children (see
chapter 27
), who do not interact well with other people (and do not prefer the sounds of motherese), have difficulty learning language.

Responding with a comment or a touch to your baby’s best attempts to communicate seems to encourage continued efforts to improve these skills.

The timing of speech production is determined by maturation of the brain regions that control movement. Forming understandable sounds requires considerable fine motor control and apparently a lot of practice. Babies first attempt to talk at around two months, when they begin cooing vowels, the least complicated speech sounds to produce. Some consonant sounds follow around five months, when babbling begins. Early babbling sounds the same in all babies, regardless of their native language. Around the end of the first year, babbling starts to include language-specific phonemes.

Word learning also starts long before babies can produce words of their own. Six-month-old infants know their own names and will look at a picture of their
mommy
or
daddy
when they hear the word. As we discussed in
chapter 1
, infants can listen to a string of nonsense syllables and determine which of them are most commonly heard together as “words.” They apply this talent to identifying words in normal speech, where words tend to run together without pauses. (To understand this phenomenon, think of the way a foreign language sounds; you can’t guess where one word ends and the next begins.) Later, their brains learn about the regularities of sentence structure that constitute the rules of grammar in their native language. By nine months, familiar and unfamiliar words trigger noticeably different event-related potentials. By the first half of baby’s second year, these potentials are different for words whose meaning
the child does or doesn’t understand. Babies’ brains also respond differently to made-up words depending on whether or not they obey the rules for which syllable should be stressed in the baby’s native language. Stress patterns appear to be another tool that babies use to determine which groups of sounds are words.

In the second year, as children learn more words and become able to say many of them, they become better at distinguishing similar words, like
bear
and
pear
. Babies at fourteen months will direct their gaze toward an object even when its name is mispronounced, suggesting that their brain does not yet represent the sounds in known words with complete accuracy. Similarly, at this age, brain activity does not distinguish between familiar words and similar-sounding nonsense words. This changes at around twenty months. The relationship between learning words and learning sounds seems to be bidirectional, so that learning sounds makes it easier to learn words, but learning more words also helps babies improve their ability to distinguish sounds.

Sentences add new layers of complexity to language learning. Again, children can comprehend sentences and grammatical connecting words before they’re able to use them in speech. To understand a sentence, your child must know not only the meanings of the individual words (called
semantic information
) but also how they relate to each other within the sentence (
syntactic information
). The brain represents these two types of information separately.

For almost everyone (excepting some left-handers), the left hemisphere is dominant for language production. Similar regions in the right hemisphere are responsible for
prosody
, the tone and rhythm of speech that conveys much of its emotional content. (For example, prosody tells you when someone is being sarcastic or making a joke.) Laterality of language representations seems to be part of the basic pattern of brain connections laid down by genes before sensory experience becomes effective (see
chapter 2
) because it is apparent by two or three months of age and even occurs in deaf infants. If the dominant speech regions are damaged in childhood, though, especially before the age of five, the other side of the brain can take over their function, leaving language skills relatively normal. If the same damage occurs after puberty, it severely impairs communication abilities.

When we hear something that sounds “wrong,” event-potentials in our brains reveal whether we’re reacting to syntactic or semantic violations. “The boy walked down the flower” is an example of a semantic violation, while “The boy walk down the road” is syntactic. In small children, these mistake-detection responses develop slowly, starting as children transition from two-word phrases to their first full sentences, around thirty months of age. Brain responses gradually become faster and more precisely localized through childhood and into the early teens.

PRACTICAL TIP: TEACH FOREIGN LANGUAGES EARLY IN LIFE

From the perspective of neuroscience, it’s absurd to wait until high school to begin studying a foreign language. By adolescence, students must work much harder to learn a new language, and most of them will never master it completely. If you want your child to speak another language fluently, by far the best approach is to start early in life.