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
Fast mapping takes off at the same moment as a dramatic reorganization of the child's brain, in which language-related operations, particularly grammar, shift from both sides of the brain into the left hemisphere. Most adult brains are lopsided when it comes to language, processing grammar almost entirely in the left temporal lobe, just over the left ear. Infants and toddlers, however, treat language in both hemispheres, according to Debra Mills, at the University of California--San Diego, and Helen Neville, at the University of Oregon. Mills and Neville stuck electrodes to toddlers' heads to find that processing of words that serve special grammatical functions, such as prepositions, conjunctions, and articles, begins to shift into the left side around the end of the third year.
From then on, the two hemispheres assume different job descriptions. The right temporal lobe continues to perform spatial tasks, such as following the trajectory of a baseball and predicting where it will land. It also pays attention to the emotional information contained in the cadence and pitch of speech. Both hemispheres know the meanings of many words, but the left temporal lobe holds the key to grammar.
This division is maintained even when the language is signed, not spoken. Ursula Bellugi and Edward Klima, a wife and husband team at the Salk Institute for Biological Studies in La Jolla, Calif., recently demonstrated this fact by studying deaf people who were lifelong signers of American Sign Language and who also had suffered a stroke in specific areas of the brain. The researchers found, predictably, that signers with damage to the right hemisphere had great difficulty with tasks involving spatial perception, such as copying a drawing of a geometric pattern. What was surprising was that right hemisphere damage did not hinder their fluency in ASL, which relies on movements of the hands and body in space. It was signers with damage to the left hemisphere who found they could no longer express themselves in ASL or understand it. Some had trouble producing the specific facial expressions that convey grammatical information in ASL. It is not just speech that's being processed in the left hemisphere, says MIT's Pinker, "or movements of the mouth, but abstract language."
Nobody knows why the left hemisphere got the job of processing language, but linguists are beginning to surmise that languages are constructed the way they are in part because the human brain is not infinitely capable of all kinds of computation. "We are starting to see how the universals among languages could arise out of constraints on how the brain computes and how children learn," says Johns Hopkins linguist Paul Smolensky. For instance, the vast majority of the world's languages favor syllables that end in a vowel, though English is an exception. (Think of a native Italian speaking English and adding vowels where there are none.) That's because it is easier for the auditory centers of the brain to perceive differences between consonants when they come before a vowel than when they come after. Human brains can easily recognize pad, bad, and dad as three different words; it is much harder to distinguish tab, tap, and tad. As languages around the world were evolving, they were pulled along paths that minimize ambiguity among sounds.
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