"The baby early begins to draw a kind of map of the sounds he hears," Kuhl says. "That map continues to develop and strengthen as the sounds are repeated. The sounds not heard, the synapses not used, are bypassed and pruned from the brain's network. Eventually the sounds and accent of the language become automatic. You don't think about it, like walking. The sounds your wife heard earlier become more and more embedded into the map, until eventually they are almost ineradicable."
Moreover, sounds are sorted by what Kuhl calls "a magnet effect." Those resembling a prototypical sound in the home language are taken in and interpreted as the home sound. Between the magnet effect and the detailed mapmaking, any attempt to introduce a new language creates "interference," in Kuhl's term. With each passing year, redrawing the map becomes more difficult.
But, I asked, what about children who grow up bilingual? All of us have known kids who were fluent in English on the school playground but spoke another language at home. In fact, Sally has been equally facile from childhood in two decidedly different languages—Cebuano, the language of the central Philippines, her mother's home, and Tagalog, her father's tongue, which in its standardized form is the national language spoken around Manila. In those cases, the infant brain simply draws two maps, and the process is particularly easy when a specific language can be identified with the tone, pitch and pronunciation of each parent.
Not that an adult absolutely can't learn another language, cautions Kuhl, who herself is trying valiantly to master Mandarin Chinese. None of us want to believe that our brains are inexorably fixed, and we can't learn new tricks as grown-ups. But becoming fluent and accent-free in a new language becomes increasingly difficult, and the best time to start is as early as possible, like nursery school, while the brain is still developing. By puberty it can be an uphill struggle, as generations of language students have found. "People talk about a 'window of opportunity' for learning language," Kuhl says. "The implication is that if you miss that opportunity, it's too late. I don't agree. It is more difficult with the years. But not impossible."
English is rapidly becoming the language of science and of computers. In Silicon Valley, just down the road from where I live, you hear the accents of Bangalore, Helsinki, Tehran and Taipei randomly mixed in with pure Yankee. The newspapers carry ads for "accent reduction clinics," and as many as half the kids in a classroom may speak another language at home. All of which makes Kuhl's research about early language learning particularly relevant.
When I arrived home from visiting Kuhl, Sally was waiting for me. "Hell-low," I said. "You ought to do something about that accent," she replied.