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Liver or River?

There's an interesting article in this month's issue of Scientific American on recent research into the organization of neural networks for phoneme recognition:

Some neuroscientists think they are close to explaining, at a physical level, why many native Japanese speakers hear "liver" as "river," and why it is so much easier to learn a new language as a child than as an adult...

Paul Iverson of University College London presented maps of what people hear when they listen to sounds that span the continuum between the American English phonemes /ra/ and /la/...

What emerged was a map of how our experience with language warps what we think we hear. Americans labeled half the sounds /la/ and half /ra/, with little confusion... But the map for Japanese speakers showed an entirely different perceptual landscape. "The results show that it's not that Japanese speakers can't hear the difference between /r/ and /l/," Iverson says. "They are just sensitive to differences that are irrelevant to distinguishing the two" -- differences too subtle for Americans to perceive. Japanese speakers, for example, tend to pay more attention to the speed of the consonant.

Frank H. Guenther of Boston University reported building a neural network model that may explain how phonetic categories arise naturally from the organization of the auditory cortex. In his simulation, neurons are rewarded for correctly recognizing the phonemes of a certain language...

When trained using Japanese speech sounds, the model neurons organized very differently from those trained on English. A pronounced dip in sensitivity appeared right at the border of /ra/ and /la/. This may reflect how "our auditory systems get tuned up to be especially sensitive to the details critical in our own native language," Iverson says. "When you try to learn a second language, those tunings may be inappropriate and interfere with your ability to learn the new categories."

If you haven't tried to learn Japanese, then you might not know that the Japanese pronounciation of /ra/ lies somewhere in between the English pronounciations of /ra/ and /la/. (It's often described as saying /ra/ while tapping the tongue on the upper palate just behind the teeth.) For many English speakers -- myself included -- it's a difficult thing to get right, just as it's hard for many Japanese speakers to correctly say the English /la/.

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