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Purpose: Although a growing body of literature has indentified the positive effects of visual speech on speech and language learning, oral movements of infant directed speech have rarely been studied. This investigation used 3-dimensional motion capture technology to describe how mothers modify their lip movements when talking to their infants.
Method: Lip movements were recorded from twenty-five mothers as they spoke to their infants and other adults. Lip shapes were analyzed for differences across speaking conditions. The maximum fundamental frequency, duration, acoustic intensity, and first and second formant frequency of each vowel were also measured.
Results: Lip movements were significantly larger during infant directed speech than during adult directed speech, although the exaggerations were vowel specific. All of the vowels produced during infant directed speech were characterized by an elevated vocal pitch and a slowed speaking rate when compared to vowels produced during adult directed speech.
Conclusion: The pattern of lip shape exaggerations did not provide support for the hypothesis that mothers produce exemplar visual models of vowels during infant directed speech. Future work is required to determine if the observed increases in vertical lip aperture engender visual and acoustic enhancements that facilitate the early learning of speech.