Free Welcome to Your Child's Brain: How the Mind Grows From Conception to College by Sam Wang, Sandra Aamodt Page B

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.
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    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 shapesthe 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