Babies take a critical step toward learning to speak before they can say a word or even babble. By 3 months of age, infants flexibly use three types of sounds — squeals, growls and vowel-like utterances — to express a range of emotions, from positive to neutral to negative, researchers say.
Attaching sounds freely to different emotions represents a basic building block of spoken language, say psycholinguist D. Kimbrough Oller of the University of Memphis in Tennessee and his colleagues. Any word or phrase can signal any mental state, depending on context and pronunciation. Infants’ flexible manipulation of sounds to signal how they feel lays the groundwork for word learning, the scientists conclude April 1 in the Proceedings of the National Academy of Sciences.
Language evolution took off once this ability emerged in human babies, Oller proposes. Ape and monkey researchers have mainly studied vocalizations that have one meaning, such as distress calls.
“At this point, the conservative conclusion is that the human infant at 3 months is already vocally freer than has been demonstrated for any other primate at any age,” Oller says.
Oller’s group videotaped infants playing and interacting with their parents in a lab room equipped with toys and furniture. Acoustic analyses identified nearly 7,000 utterances made by infants up to 1 year of age that qualified as laughs, cries, squeals, growls or vowel-like sounds.
Trained experimenters separately judged whether each sound an infant made, and the facial expression accompanying that sound, was positive, negative or neutral.
Overall, infants produced the flexible trio of emotion sounds much more often than laughs or cries. Babies most frequently uttered vowel-like sounds, which were less distinctive than babbling that starts at around 7 months of age.
Neuroscientists previously reported that monkeys, apes and humans share an ancient brain pathway linked to emotional sounds such as laughing and crying. In the new study, babies’ laughs overwhelmingly expressed positive feelings and cries usually conveyed negative feelings.
Ancient humans must have evolved new neural connections that supported early voluntary control of sounds other than laughing or crying to communicate emotions, remarks psychologist Michael Owren of Emory University in Atlanta.
“This groundbreaking work shows that, from the beginning, human infants have flexible vocal chops that put them on a very different developmental course than found in monkeys and apes,” Owren says.
Psychologist David Lewkowicz of Florida Atlantic University in Boca Raton remains unconvinced that infants, especially at 3 to 4 months old, flexibly communicate with sounds. The infants’ distinct approaches to matching sounds with facial expressions might reflect confusion more than intention, Lewkowicz says, because babies don’t perceive and understand adultlike emotions until at least 6 months of age and don’t see a relationship between others’ facial and vocal expressions until around 8 months. Parents could also have subtly influenced how their babies vocalized in the lab, he suggests.
In diaries kept during the study period, parents reported knowing that their babies flexibly employed the sounds studied by Oller’s group. In lab exchanges, babies made these sounds only upon getting close to parents’ faces.
Further research needs to examine how babbling develops from parents’ responses to the early vocal flexibility reported in the new study, says psychologist Michael Goldstein of Cornell University.