Between 6 and 12 months, babies can literally "hear" the specific sounds of all languages spoken. When compared to adults, babies are better at hearing the differences between the sounds of non-native languages. But by 12 months, babies are losing this ability to hear these differences. At the same time, babies are getting better at distinguishing between the sounds of the language or languages they hear regularly. Babies are developing a type of "sound map" in their brains to help them hear the differences between language sounds. This map consists of creating a "perfect example" of each sound they hear, with a target area around this spot. With their sound map for "ee," for example, babies learn to pick out the "ee" distinctly from the other sounds they hear. These perfect examples of speech sounds, called prototypes, have a large effect on how babies hear speech and how they babble. Prototypes help "tune" the child's brain for the language around him, so that he can hear the different sounds of speech clearly. Even when adults don't speak clearly, babies seem to compare the mumbled sounds in grown-ups' speech to the prototypes in their brains and figure out what they're saying. By the time they're 6 months old, babies who hear the sounds of their culture's language have developed a set of speech-sound prototypes they can use as building blocks when they begin to put together their own words, usually sometime around 12 months.
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