An association between mothers’ speech clarity and infants’ speech discrimination skills

The quality of speech directed towards infants may play an important role in infants’ language development. However, few studies have examined the link between the two. We examined the correlation between maternal speech clarity and infant speech perception performance in two groups of Mandarin-speaking mother–infant pairs. Maternal speech clarity was assessed using the degree of expansion of the vowel space, a measure previously shown to reflect the intelligibility of words and sentences. Speech discrimination in the infants (6–8 and 10–12-month-olds) was measured using a head-turn task. The results show that mothers’ vowel space area is significantly correlated with infants’ speech discrimination performance. Socioeconomic data from both parents show that the result cannot be attributed to parental socioeconomic factors. This study is correlational and therefore a causal relationship cannot be firmly established. However, the results are consistent with the view that maternal speech clarity directly affects infants’ early language learning. Language acquisition is driven both by biological mechanisms and environmental adaptations and the interplay between these factors is of strong interest to theorists. Current theories of language acquisition – capitalizing on data indicating that infants acquire phonetic, word and phrase structure information simply by listening to ambient language – suggest that the language addressed to infants may play a critical role in the language acquisition process (Jusczyk; 1997; Kuhl, 2000; Kuhl, Tsao, Liu, Zhang & de Boer, 2001; Snow, 1994). Systematic investigation of the nature of the speaking style adults use when addressing infants, often termed ‘motherese’ or infant-directed (ID) speech, shows that it is syntactically and semantically simplified when compared to adult-directed (AD) speech (Cross, 1977; Ferguson, 1964; Snow, 1994). In addition, ID speech has a unique acoustic signature characterized by a higher fundamental frequency (pitch), exaggerated intonation contours, and a slower tempo (Fernald & Simon, 1984; Grieser & Kuhl, 1988; Stern, Spieker, Barnett & Mackain, 1983). The pitch patterns of ID speech attract and hold infant attention, as shown in tests in which infants are given a choice between listening to ID versus AD speech (Cooper & Aslin, 1994; Fernald & Kuhl, 1987). Recent studies demonstrate that the acoustic modifications in ID speech go beyond the prosodic patterns that capture infant attention. Studies show that the phonetic units in infant-directed speech are exaggerated, making the individual sounds of language more distinct from one another (Burnham, Kitamura & Vollmer-Conna, 2002; Kuhl, Andruski, Chistovich, Chistovich, Kozhevnikova, Ryskin, Stolyarova, Sundberg & Lacerda, 1997). The exaggerated articulations of vowels, for example, ‘stretch’ the acoustic space that encompasses vowels, enhancing the distinctive features that distinguish them (Kuhl et al. , 1997). Speech modifications in language addressed to children – modifications seen at the syntactic, semantic and phonetic level – appear to simplify and enhance linguistically relevant cues. These studies address Chomsky’s (1965) classic ‘poverty of the stimulus’ argument. They suggest that while adult-directed speech is often ungrammatical and phonetically reduced, infant-directed speech is much less so. However, the fact that adults across many languages and cultures make these modifications when addressing infants and children does not establish whether they have an impact on language learning. Few studies have investigated the potential link between language input to children and children’s language development. Previous studies have reported that the quantity of language input from parents (i.e. the number of words that a child hears) is associated with children’s vocabulary size at 2 years of age (e.g. Hart & Risley, 1995; Hoff Address for correspondence: Huei-Mei Liu, Department of Special Education, National Taiwan Normal University, Taipei, Taiwan; e-mail: [email protected] DESC_290.fm Page 1 Wednesday, April 30, 2003 9:53 AM