Researchers have analyzed the mouth motions of babies babbling, smiling or making other noises and have concluded that the baby sounds are springing directly from infant efforts to master speech and language. Their findings appear Friday in the journal Science.
When a baby is babbling, it "is hard at work figuring out the sound system of their language and how those sounds are put together," said Laura-Ann Petitto, a researcher at Dartmouth College and senior author of the study. "The baby is actually building an understanding of the sounds of their language."
Petitto said her new study, using 10 babies between the ages of 5 months and 12 months, shows that the distinctive sounds of an infant's babbling are prompted by signals from one part of the brain while other sounds or mouth motions are directed by other parts of the brain.
"This suggests that language functions specialize in the brain at a very early age," she said.
In the study, Petitto, a professor in psychology and brain sciences, and co-author Siobhan Holowka analyzed the mouth motion of infants as they babbled, made other vocal sounds and as they smiled.
The infants, five from an English-speaking family and five from a French-speaking family, were filmed as they made sounds. The film was then slowed down to enable the researchers to closely analyze each mouth motion.
They found that when the babies were babbling, the motion was more emphatic on the right side of the mouth. For smiling, the left side of the mouth was opened more. For random vocalizations, the mouth was symmetrical, with both sides activated equally.
This difference in motion, said Petitto, directly reflects the parts of the brain being used in each activity of the mouth.
If the baby is producing vocal babbling, then the right side of the mouth has a greater opening and it is pulled down a little more, Petitto said. "However, if the baby is smiling, the left side of the mouth is going to have a slightly greater opening and you also get a contraction around the left eye."
When the baby is producing sounds with no babbling content, "then the mouth is full, wide open," she said.
Petitto said that since the left side of the brain controls the right side of the mouth, then the findings suggest that the left side of the brain is sending the babbling signals.
"The left hemisphere of the brain is dedicated to language learning," she said, noting that the study shows "it comes on much earlier than we thought."
Smiling is prompted by the right side of the brain, the seat of emotions, she said.
"Our study shows that as early as five months old, there is participation of the emotional centers in the brain," the researcher said.
Elena Plante, a speech language pathologist at the University of Arizona, said the study demonstrates that "the control of movement of the face and mouth for babbling, which is a precursor of spoken language, seems very different" from the control of movement for activities like smiling or crying.
She said that is consistent with other studies that have suggested that mouth movements for speech are different than for other types of vocalizations.
"This really pushes that model down to a much lower age than we have been able to look at before," said Plante.
Petitto said that experts long ago identified between an infant's babble and other sounds a baby makes. A true babble has a vowel and a consonant and a single syllable-like sound that is repeated over and over, she said.
"When a baby is going da-da-da-da, or ba-ba-ba-ba or ga-ga-ga-ga, you can easily hear it," Petitto said. "That's a babble and it contrasts with a sound made by the mouth that ... is a single sound such as 'ahhhhhhhhhhh."'
Petitto said the study used babies from both English and French speaking families to make sure that the mouth movements during babbling were not specific to one language. She said she believes the mouth motion is universal.
She said it is hoped that by accurately interpreting infant babbling, researchers will be able to develop a system to diagnose speech problems at a very young age and, perhaps, start treatment early.
"When children have language problems, one of the greatest difficulties is that you have to wait until they produce language to discover they have language problems," she said.