During the first year of life, babies can recognize surprisingly small differences in the sounds of similar syllables. But in tomorrow's issue of Nature, researchers report that infants ignore these subtle differences as they begin to learn whole words. The finding could help researchers design materials for children who are slow in learning to speak.
Before infants are even 1 year old, they can distinguish subtle phonetic sounds in all languages. For example, a baby can recognize two "D" sounds in Hindi that sound identical to an English-speaking adult. Babies lose this ability to detect non-native sounds sometime after their first birthday, however, so Christine Stager, a developmental psychologist at the University of British Columbia (UBC), Vancouver, wondered if this might reflect a basic part of language development.
In one of a series of experiments, she and her colleague Janet Werker, also of UBC, tested the perception of 32 infants, 8 months and 14 months old. On a video screen they flashed a shape, and over a loudspeaker said the nonsense word "bih." As the infants became familiar with the word-image, the time they spent observing the picture declined, indicating that they had become bored with the learned material. Next, the researchers changed the sound from "bih" to "dih". Infants who noticed the change in sound, the researchers reasoned, would look at the picture longer. On average, the 8-month-olds stared 2 seconds longer than before, while the 14-month-olds did not change.
Stager surmises that the 8-month-olds focused only on discriminating between the sounds. In contrast, she suspects the 14-month-olds had more on their minds--the word-picture relationship--so their brains didn't notice the subtle change of consonants. "What the 14-month-old child is doing is very efficiently glossing over detail when they are faced with a more difficult task," says Werker. Once the word-picture relationship is mastered and the child's vocabulary is large enough to require making phonetic discriminations again, says Stager, the brain is able to make all the phonetic distinctions it needs.
For children who are slow to grasp word recognition and speech, Werker says it might make sense to create learning materials that focus either on phonetic building or word learning, but not at the same time. And for any kid starting to learn words, it's not unnatural to confuse consonants, adds Peter Jusczyk, a psychologist at Johns Hopkins University. Their brains are simply overloaded--something many adults would appreciate, he says. "If we're put into a situation where we are trying to juggle two or three things at once, they might not get done as well."