The singsong speech adults use when talking to infants seems to get a baby's attention and even bring a smile. But a report published in today's issue of Science* suggests that this "parentese" may be more than just a tool of endearment. It can, the researchers propose, help infants learn the key features of vowel sounds.
Adult speech is far from uniform, with countless subtle variations on each sound, such as the "a" in "cat" or the "o" in "cot." As children learn language, they must master which phonetic differences to pay attention to, and which to ignore. In earlier work, Patricia Kuhl of the University of Washington, Seattle, and her co-workers found that by 6 months of age, Swedish and American babies learn to categorize vowel sounds, paying attention to meaningful distinctions, while ignoring other variations. Now Kuhl has probed further by studying the parentese of three different languages--English, Swedish, and Russian--to see if the distorted tones provide cues that may be useful for vowel pronunciation.
The team focused on formants, the resonant frequencies that, like notes in a musical chord, make up each vowel sound. If vowel sounds are plotted on a graph, with the frequencies of the two dominant formants represented on the x and y axes, the result is a "vowel triangle," with the sounds "ah," "ee," and "oo" at the corners. Kuhl's group found that, in all three languages, mothers talking to their babies produced exaggerated versions of vowels, emphasizing the features that distinguish them from each other. This nearly doubled the area of the vowel triangle. "It looks like the mothers are increasing the value of the signal," says Kuhl.
The mothers' speech also provided many examples of each vowel sound. This, Kuhl proposes, may help babies learn the features that make each sound special and to ignore the phonetic variations that fall within a given vowel sound. Indeed, by 20 weeks of age, babies' babbling contains distinct vowel sounds that form their own--albeit higher pitched--vowel triangle.
The work "illustrates a close tie between the input and what the child is doing," says language researcher Peter Jusczyk, of Johns Hopkins University. He cautions, however, that it hasn't quite proven that parentese serves an instructive role. "The fact that parents do it doesn't necessarily mean that it is essential for language learning," adds Stanford developmental psychologist Anne Fernald. That hypothesis might be tested, she says, with studies across cultures that use different amounts or types of parentese.