April 20, 1999, Tuesday
Science Desk Old Brains Can Learn New Language Tricks
By SANDRA BLAKESLEE
Pity the Japanese tourist asking for directions in New York City: ''Which way to Times Square?''
The answer might be: ''Turn left after the next light.''
The next what? Does that mean traffic signal or the next street turning off to the right? To the typical native speaker of Japanese, right and light are hopelessly confused because the English sounds ''L'' and ''R'' are indistinguishable.
But they will not be confused for long. In a fascinating set of experiments, researchers at the Center for the Neural Basis of Cognition in Pittsburgh have found a way to teach native speakers of Japanese to hear the difference between L and R reliably after just one hour of training.
The new findings are ''extremely compelling,'' said Dr. Edward Jones, a neuroscientist at the University of California at Davis and president of the Cognitive Neuroscience Society, who is familiar with the research. They shed light on how the adult brain changes, a phenomenon called plasticity, and on mechanisms that make it resistant to change.
Dr. Helen Neville, a leading expert on brain plasticity at the University of Oregon in Eugene, called the experiments ''cool.'' They show that the adult brain is capable of substantial change, even late in life, she said.
The research is being conducted by Dr. Jay McClelland and his colleagues at the Pittsburgh center, a joint program of Carnegie Mellon University and the University of Pittsburgh. Dr. McClelland, the center's co-director, has long been interested in how the brain learns and, sometimes more importantly, how it fails to learn. He presented his findings last week at the annual meeting of the Cognitive Neuroscience Society, which was held in Washington.
The brain presents a puzzle, Dr. McClelland said, in that many kinds of learning continue or even improve throughout adulthood, but others, like the speech sounds of a language, appear to slow almost to a stop. Few people can learn a second language without an accent after the age of 10. Scientists call this 10-year window a critical period for acquiring the sounds of a language. But what is the neural basis of this critical period?
Clues are found in the way brain cells connect and influence one another. In test tube experiments, scientists take two nerve cells that are connected by a kind of cable called an axon. When the first cell is induced to fire an electric pulse down its axon, the second cell also fires. Soon, physical changes develop in both cells so that the first one almost always makes the second one fire. This is how information is passed throughout the brain in a process known as Hebbian learning.
Early on, brain cells are not well connected, Dr. McClelland said. But when experiences from the outside world begin to flow into the brain, cells begin to fire, and Hebbian patterns get stamped in. As neuroscientists are fond of saying, cells that fire together, wire together, forming circuits.
Later, a cell may come into direct contact with cells from another circuit, but if it is committed to what it has learned, it will not respond even to very strong stimulation from those other cells. In other words, it fails to learn.
This model of Hebbian learning can be applied to the sounds of human language, Dr. McClelland said. Newborn babies can discriminate all the sounds of every language in the world. It is as if there were a space inside their brains that is a blank slate, waiting for sounds to enter. When the sounds of the native language come pouring in, each sound induces some cells to wire up and become dedicated to its peculiar frequency.
Thus, there are clumps of cells that are tuned to the Finnish ''O,'' the Spanish ''D'' or the English ''Th'' -- all of which are difficult for non-native speakers to hear or pronounce. Within a short time, the baby's ability to distinguish all sounds fades away.
Babies in English-speaking families have cells dedicated to hearing both L and R, whereas Japanese babies have only one phonetic category for a similar sound, Dr. McClelland said. To an American, the single Japanese R sound, as in the word riokan, meaning guest room, sounds rather like a D, he said. In any case, as the children in both cultures grow up, their sound categories become more sharply defined.
The brain has a relatively small amount of neural tissue dedicated to speech sounds, Dr. McClelland said, and so carves up that space with strong boundaries.
Sounds are stamped in early because the baby needs them to build the foundation for language comprehension. Thus, Hebbian processes come in early and lock in speech sound circuits. In other parts of the brain, Hebbian processes continue, but those circuits can be more flexible, he said. An adult learns to speak a second language by making new connections in many circuits but cannot supplant the locked-in native sound system with the new sounds of the second language.
Moreover, the unfamiliar sounds of a foreign language actually reinforce the sound system of one's native language, Dr. McClelland said. When a Japanese speaker comes to America, every time he hears an English L or R, his single Japanese R phoneme is activated. Instead of becoming more flexible, his ability to hear L and R diminishes with increasing exposure to English.
The challenge, Dr. McClelland said, is to carve out two spaces in the Japanese speaker's brain when he has only one space for the L and R sounds. If the critical period is over forever, he said, this should be impossible. But if some plasticity or malleability remains, there should be a way to override the embedded circuits using Hebbian learning.
Thirty-four native Japanese speakers came to the Pittsburgh laboratory, where they were given headphones through which they heard pairs of words -- road and load, light and right, and so on -- under one of two conditions.
In one condition, subjects heard regular speech. They had to say or guess when they heard an L-word or R-word by tapping a response into a laptop computer.
In the second condition, subjects heard the same words exaggerated by a computer, so that each sound's peculiar frequency or formant was accentuated. As their ability to distinguish L and R words improved, the words were presented in regular speech. Finally, they heard the words in sloppy or degraded speech so that even native speakers would have to listen hard to hear the difference.
The subjects, all of whom had great trouble with L and R before training, used the computer for three 20-minutes sessions, involving hundreds of word pairs, Dr. McClelland said. No feedback was offered, to make conditions resemble the real world.
Those who heard natural speech barely improved and some actually got worse, Dr. McClelland said. But those who heard the exaggerated speech with gradual training toward more natural speech all improved greatly. After one hour, they could clearly distinguish light and right.
At this point, the successful subjects do not generalize what they have learned to all L and R sounds, he said. But the experiment is just beginning. If they go on to train on numerous pairs of words, they may be able to retrain their entire sound system. It appears that they have begun to carve out new, independent circuits for the L and R sounds.
This approach may be effective for retraining other embedded circuits, like those that underlie racial prejudices or stereotypes, Dr. McClelland said. For example, some people react with fear when they see a stranger, based simply on his dress or skin color. That response may be stamped in. ''We could think about ways of structuring situations to present a stimulus that would originally elicit the fear response and then teach the brain to have a different reaction,'' he said.
And it can definitely be used for second language training.
During the World War II, American soldiers fighting the Japanese infantry adopted the password ''lollapalooza,'' figuring that no Japanese speaker could pronounce it. So much for that idea.
Organizations mentioned in this article:
Center for the Neural Basis of Cognition (Pittsburgh, Pa)
Brain; Research; Language and Languages; Japanese Language; English Language; Education and Schools; Speech
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