The concept of how babies learn to speak is an elusive and mysterious one, even for scientists who yearn to understand how young children are able to master the complexities of language - a difficult enough task for the adult brain and the most powerful computers. In a recent series of experiments, psychologists have found that infants seem to remember relatively complex words, even when they only hear those words in tape-recorded stories without the benefit of any other stimuli. Such findings suggest that babies memorize words that occur frequently in speech - an important prerequisite for learning language.
Although much work has been conducted to investigate how children learn the meanings of words, little research has been aimed at learning how infants focus on the sounds of words, says Peter Jusczyk, professor of psychology at Johns Hopkins University, who co-authored the paper with Elizabeth A. Hohne, a psychologist at AT&T Labs in Holmdel, N.J.
In their work, the two scientists recorded women narrating three different children's stories; these contained words like "peccaries" and "python", and each lasted about 10 minutes. Then the researchers visited the homes of 15 eight-month-old infants, playing the stories to them every day for 10 days. By the end, the babies had heard each story 10 times.
The psychologists identified the 36 words that occurred most often in the stories. Then they arranged those words in lists of 12 words each. Two weeks after the final visit to the infants' homes, the babies were brought to Jusczyk's lab at Johns Hopkins. One at a time, they listened to the lists containing the words that occurred most frequently in the stories. Then they listened to lists of other, similar-sounding words that did not occur in the stories.
A light flashed above the speaker through which the tape recording was played. When the infants looked at the light, the word lists began and continued to play until the infants looked away. Thus, the researchers determined how long the infants had listened to specific lists of words. "What we found was that the babies listened longer - significantly longer - to the lists of words from the stories," Jusczyk says.
Previous research using this technique has shown that infants tend to listen longer to words that are more familiar to them. The researchers, however, wanted to make sure the infants were not listening longer to the story words simply because they found them more interesting, so they brought a new group of infants to the lab who had never heard the stories on tape. When those infants heard the lists of both story and non-story words, they showed no preference and actually listened slightly longer to the non-story words.
"That showed us that the experience the babies had had at home listening to the stories had an impact on what they really remembered," Jusczyk explains, noting that the infants learned the words even though they never had any personal contact with the women who narrated the stories.
At about 18 months, a child's vocabulary and grasp of language suddenly expands. Scientists don't know why. One possible explanation is that children may begin storing the sounds and meanings of words while they are infants, and suddenly, at a year-and-a-half, they are able to connect these words with meanings.
"It's sort of like working on a puzzle. You get a few pieces and then everything falls into place," says Jusczyk, adding that this study shows that infants may store the sounds of words, even when they have not yet learned the meanings.