Baby Talk
Learning language, researchers are finding, is an astonishing act of brain computation--and it's performed by people too young to tie their shoes
But neural networks have yet to come close to the computation power of a toddler. Ninety percent of the sentences uttered by the average 3-year-old are grammatically correct. The mistakes they do make are rarely random but rather the result of following the rules of grammar with excessive zeal. There is no logical reason for being able to say "I batted the ball" but not "I holded the rabbit," except that about 180 of the most commonly used English verbs are conjugated irregularly.
Yet for all of grammar's seeming illogic, toddlers' brains may be able to spot clues in the sentences they hear that help them learn grammatical rules, just as they use statistical regularities to find word boundaries. One such clue is the little bits of language called grammatical morphemes, which among other things tell a listener whether a word is being used as noun or as a verb. The, for instance, signals that a noun will soon follow, while the suffix ion also identifies a word as a noun, as in vibration. Psycholinguist LouAnn Gerken of the University of Arizona recently reported that toddlers know what grammatical morphemes signify before they actually use them. She tested this by asking 2-year-olds a series of questions in which the grammatical morphemes were replaced with other words. When asked to "Find the dog for me," for example, 85 percent of children in her study could point to the right animal in a picture. But when the question was "Find was dog for me," they pointed to the dog 55 percent of the time. "Find gub dog for me," and it dropped to 40 percent.
Fast mapping. Children may be noticing grammatical morphemes when they are as young as 10 months and have just begun making connections between words and their definitions. Gerken recently found that infants' brain waves change when they are listening to stories in which grammatical morphemes are replaced with other words, suggesting they begin picking up grammar even before they know what sentences mean.
Such linguistic leaps come as a baby's brain is humming with activity. Within the first few months of life, a baby's neurons will forge 1,000 trillion connections, an increase of 20-fold from birth. Neurobiologists once assumed that the wiring in a baby's brain was set at birth. After that, the brain, like legs and noses, just grew bigger. That view has been demolished, says Anne Fernald, a psycholinguist at Stanford University, "now that we can eavesdrop on the brain." Images made using the brain-scanning technique positron emission tomography have revealed, for instance, that when a baby is 8 or 9 months old, the part of the brain that stores and indexes many kinds of memory becomes fully functional. This is precisely when babies appear to be able to attach meaning to words.
Other leaps in a child's linguistic prowess also coincide with remarkable changes in the brain. For instance, an adult listener can recognize eleph as elephant within about 400 milliseconds, an ability called "fast mapping" that demands that the brain process speech sounds with phenomenal speed. "To understand strings of words, you have to identify individual words rapidly," says Fernald. She and her colleagues have found that around 15 months of age, a child needs more than a second to recognize even a familiar word, like baby. At 18 months, the child can get the picture slightly before the word is ending. At 24 months, she knows the word in a mere 600 milliseconds, as soon as the syllable bay has been uttered.
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