Prosody and focus in speech to infants and adults.

Abstract

Two studies investigated adults' use of emphasis to mark focused words in speech to infants and adults. In Experiment 1,18 mothers told a story to a 14-month-old infant and to an adult, using a picture book in which 6 target items were the focus of attention. Prosodic emphasis was measured both acoustically and subjectively In speech to infants, mothers consistently positioned focused words on exaggerated pitch peaks in utterance-final position, whereas in speech to adults emphasis was more variable. In Experiment 2,12 women taught another adult an assembly procedure involving familiar and novel terminology. In both studies, stressed words in adultdirected speech rarely coincided with pitch peaks. However, in infant-directed speech, mothers regularly used pitch prominence to convey primary stress. The use of exaggerated pitch peaks at the ends of utterances to mark focused words may facilitate speech processing for the infant. Research on features of early linguistic experiences that influence language acquisition has begun to explore the role of prosody in providing the infant with acoustic cues to linguistic structure in the speech waveform. Faced with the problem of discovering the linguistically relevant units in continuous speech, the preverbal infant may take advantage of features that are regularly correlated with such units as words, phrases, and clauses. According to some versions of this prosodic bootstrapping hypothesis (e.g., Gleitman & Wanner, 1982; Morgan & Newport, 1981), infants can exploit the cues routinely available in spoken English to infer the correct units of analysis for the language without requiring special modifications in maternal speech. Other evidence suggests that the characteristic exaggeration of cues in infant-directed speech may indeed be useful to the infant in partitioning the speech stream. When speaking to infants, mothers use more exaggerated vowel lengthening to mark both phrase boundaries (Morgan, 1986) and clause boundaries (Bernstein Ratner, 1986) than when speaking to adults. The finding that infants are sensitive to clause boundaries in infantdirected but not in adult-directe d speech (Kemler-Nelson, Hirsh-Pasek, Jusczyk, & Cassidy, 1989) suggests that the exaggerated prosody typical of mothers' speech (e.g., Fernald & Simon, 1984) increases the salience of acoustic cues to linguistic structure for the preverbal infant. This recent interest in the usefulness of prosody to infants who are learning language has focused mainly on how cues reveal phrase and clause structure. The focus on syntactic rather than lexical units is not surprising, given that most current theories of language acquisition take for granted the