Within the first babbling streams of chaotic baby talk, infants normally manage to disgorge two real words: Mama ... Dada.
A scientist at Hopkins now reports that the sounds that give parents such a thrill actually mark the very beginning of human word comprehension. It is now believed that the origins of language--linking sound patterns with specific meanings--stem from discrete associations infants make, beginning with parents, at 6 months of age.
The findings, which are reported this month by Ruth Tincoff and Peter W. Jusczyk (pictured at right) in the journal Psychological Science, provide a new benchmark in the study of human language.
"Six months is the youngest age anyone has been able to show that children seem to pair sounds with a specific meaning," said Jusczyk, a professor of psychology and a pioneer in the study of infant speech perception and language development. "Most of the previous work on comprehension indicated it was 8 or 10 months of age when kids started to attach labels to particular objects. The difference here is that the words name important social figures. This suggests that infants begin forming a lexicon with sound patterns linked directly to socially significant people, such as their parents."
In an earlier study, Jusczyk found that at 4 months, babies respond to their own names. But the response is largely undifferentiated from other kinds of speech, "just like a child might respond to 'Hi!' without knowing what it means," Jusczyk said. "You can offer certain nonsense words or sounds with an infant, and they'll get excited because it's part of a routine. Maybe the child has the wrong hypothesis about what's going on. We thought it might be the same for 'Mommy' and 'Daddy,' but until we conducted this most recent study to see whether they associate the words with a specific parent, we didn't really know."
To do the test, Jusczyk and his team created an experiment with 24 6-month-old infants.
During the tests at his lab, each infant sat on the lap of one parent while watching a videotape on two TV monitors showing separate images of the child's mother and father. When a synthesized voice spoke the words "Mommy" or "Daddy," researchers recorded the amount of "looking time" the infant gave to the named or unnamed parent.
The results showed that infants' "first looks" were directed toward the named parent significantly more often than would be expected by chance.
However, experimenters didn't stop there. They also had to rule out another possibility.
"When infants begin producing speech," Jusczyk said, "they often overextend labels to a variety of objects. That means, for example, that the word 'dog' might be used to name sheep or cows or any four-legged animal. We wondered if 'Mommy' might be used the same way to include all women and 'Daddy' to name all men."
To test the hypothesis, a new group of 24 6-month-old babies was exposed to the videos of the previous children's parental pairs. Again, a synthesized voice uttered the words "Mommy" and "Daddy," and researchers scored the infants' "looking time"--this time, glances toward other infants' parents.
In this experiment, the infants did not appear to associate "Mommy" and "Daddy" with either parent.
"Does 'Mommy' mean women, and 'Daddy' mean men?" Jusczyk asked. "Or do babies respond to the words with precision, like my name is Peter and my wife's name is Ann Marie? In some ways, the second study would have been interesting no matter what we found. But in fact, we discovered that infants pinpoint 'Mommy' and 'Daddy' explicitly. They understand the words to mean, 'my Mom' and 'my Dad.' By age 6 months, those two words aren't just for anybody."
In his book The Discovery of Spoken Language (MIT Press, 1997), Jusczyk explores these kinds of key questions about infant language acquisition, a field of psychological science that is only 30 years old.
In their search for the origins of language development, he and his colleagues around the world are discovering that early speech recognition is the culmination of furious intellectual efforts that actually begin in the womb, where the rhythms of the mother's voice and native language are communicated to the child through the reverberations of her very bones.
It is believed now that the ability to decipher language comes to children as a kind of natural endowment, which rapidly turns more sophisticated between the ages of 6 weeks and 2 years.
"Learning is different from so many of our other activities," Jusczyk said. "When you learn mathematics, for instance, you have to master a bunch of abstract principles. With language, a person has to master a lot of specific rules, but they come through perceptual capacities and a kind of innate understanding that we seem to be born with."
In that sense, naming "Mommy" and "Daddy" begins the revelation of human beings' most extraordinary trove of riches.
The research, funded by the National Institute of Mental Health and the National Institute of Child Health and Human Development, was a focus of discussion in a recent segment of Good Morning America.