Babies find their voice when given social push. (Babble Rousers).

Eight-month-old infants utter more complex, speechlike sounds when their mothers encourage them with well-timed touches and smiles rather than with words offered as models to imitate, a new study finds.

This provides the first evidence that nonverbal interactions with caregivers shape babies' vocal learning, says psychologist Michael H. Goldstein of Franklin and Marshall College in Lancaster, Pa. The power of maternal behaviors to ramp up babies' babbling corresponds to the way certain bird species learn to sing, propose Goldstein and his coworkers Andrew P. King and Meredith J. West, both of Indiana University in Bloomington.

"This is a basic type of social learning," Goldstein asserts. "At 8 months of age, kids may already make complex speech sounds occasionally, but they produce them regularly in response to [certain encouraging] interactions with a caregiver."

The new findings, slated to appear in an upcoming Proceedings of the National Academy of Sciences, support the view that learning language doesn't hinge solely on the ability to imitate spoken sounds.

Goldstein's group studied 30 infants averaging 8 months of age and their mothers. Each mother-child pair first played in a room stocked with toys for 10 minutes. Then half the mothers were told to respond immediately during the next 10 minutes to a baby's vocalizations by smiling, moving closer to the child, and gently touching him or her--but not by talking. The rest of the mothers were instructed through headphones to react in the same ways, but usually not right after a baby made sounds.

In those cases in which mothers timed their smiles and touches with their babies' vocalizations, the infants' babbling almost instantly became more mature, the scientists say. Compared with during the initial play period, these babies enunciated substantially more syllables such as "da" and "gu" They also drew deeper breaths while vocalizing, which improved their articulation of speech sounds and removed the nasal tone that many of them had at first.

During a final l0-minute stretch, both groups of mothers played without restrictions with their babies. Those infants that had received nonverbal encouragements continued to produce more syllables and more finely articulated speech sounds than their counterparts, although all infants vocalized less frequently during this final phase.

Several songbird species exhibit a type of social learning that parallels the impact of mothers' behavior on their babies' babbling, the researchers contend. For instance, in brown-headed cowbirds, young males transform immature songs into polished form by heeding the nonvocal responses of adult females, such as subtle wing movements.

The new findings reiterate how the social environment influences the use of language in infancy, remarks psychologist Michael Owren of Cornell University. However, it's not clear that a common mechanism underlies learning in songbirds and babies, he says. Because song learning is restricted to male cowbirds instructed only by females, the process may rely on a specialized procedure, Owren says.

"Goldstein's study is important because it puts more of the responsibility for the infant's language learning on the caretakers, rather than depending entirely on a special mechanism within the infant--imitation," says Indiana University psychologist Susan S. Jones.