Many studies have found that infant-directed (ID) speech has higher pitch, has more exaggerated pitch contours, has a larger pitch range, has a slower tempo, and is more rhythmic than typical adult-directed (AD) speech. We show that the ID speech style reflects free vocal expression of emotion to infants, in comparison with more inhibited expression of emotion in typical AD speech. When AD speech does express emotion, the same acoustic features are used as in ID speech. We recorded ID and AD samples of speech expressing love-comfort, fear, and surprise. The emotions were equally discriminable in the ID and AD samples. Acoustic analyses showed few differences between the ID and AD samples, but robust differences across the emotions. We conclude that ID prosody itself is not special. What is special is the widespread expression of emotion to infants in comparison with the more inhibited expression of emotion in typical adult interactions.

"Baby talk" or speech directedto prelinguisticinfants is high in pitch and has exaggeratedpitch contours (up/down patterns of pitch change) across languages and cultures. Using an acoustic model, we predicted that the large pitch contours of infant-directed speech should improve infants' ability to discriminate vowels. On the other hand, the same model predicted that high pitch would not benefit, and might actually impair, infants' ability to discriminate vowels. We then confirmed these predictions experimentally. We conclude that the exaggerated pitch contours of infant-directed speech aid infants' acquisition of vowel categories but that the high pitch of infant-directed speech must serve another function, such as attracting infants' attention or aiding emotional communication.

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The language children hear presents them with a multitude of co-occurrences between words and things in the world, and they must repeatedly determine which among these manifold co-occurrences is relevant. Social factors--such as cues regarding the speaker's referential intent--might serve as one guide to whether word-object covariation should be registered. In 2 studies, infants (15-20 months and 18-20 months in Studies 1 and 2, respectively) heard novel labels at a time when they were investigating a single novel object; in one case the label was uttered by a speaker seated within the infant's view and displaying concurrent attention to the novel toy (coupled condition), whereas in the other case the label emanated from a speaker seated out of the infant's view (decoupled condition). In both studies, subsequent comprehension questions indicated that infants of 18-20 months registered a stable link between label and object in the coupled conditions, but not in the decoupled condition, despite the fact that covariation between label and object was equivalent in the 2 conditions. Thus, by 18-20 months children are inclined to establish a mapping between word and object only when a speaker displays signs of referring to that object.

For adults and children, speech perception can be significantly influenced by watching a speaker's mouth movements. While recent reports suggest that infants may be able to integrate heard and seen speech, the current research demonstrates that integration is neither as strong or consistent in infants as it is in adults. Three habituation experiments were conducted. In Experiment 1, female (but not male) infants showed evidence of an adult pattern of integration following habituation with an audiovisual /bi/ and testing with audio /bi/-visual /vi/ (perceived as /vi/ by adults). The interpretation of integration was supported, but only in part, by Experiment 2. In Experiment 3, infants were habituated to a mismatched audio /bi/-visual /vi/ combination and tested on concordant /bi/-/bi/ versus /vi/-/vi/ displays. Here, only male infants showed evidence of integration. These results suggest that an initial mechanism supports integration, but that integration is not mandatory for young infants.