The development of phonetic representation in bilingual and monolingual infants

The development of native language phonetic representations in bilingual infants was compared to that of monolingual infants. Infants (ages 6–8, 10–12, and 14–20 months) from English–French or English-only environments were tested on their ability to discriminate a French and an English voice onset time distinction. Although 6to 8-month-olds responded similarly irrespective of language environment, by 10–12 months both groups of infants displayed language-specific perceptual abilities: the monolinguals demonstrated realignment to the native English boundary whereas the bilinguals began discriminating both native boundaries. This suggests that infants exposed to two languages from birth are equipped to phonetically process each as a native language and the development of phonetic representation is neither delayed nor compromised by additional languages. How infants learn language and perceive speech is a fundamental question in human language research. At a primary level, infants must learn to identify the units of sound that comprise their language. In particular, they are required to learn which speech sounds signal changes in meaning (e.g., they must be able to distinguish b from t in order to differentiate ball from tall). The languagespecific set of sound categories are referred to as the phonetic representation. Phonetic representations underlie learning of lexical items and the acquisition of word meanings necessary to establish a vocabulary. Despite the fundamental role phonetic representations play in language acquisition, there has been very little research on their development in a bilingual context. © 2007 Cambridge University Press 0142-7164/07 $15.00 Applied Psycholinguistics 28:3 456 Burns et al.: Phonetic representation The present work is concerned with the development of phonetic representations in infants being raised in a bilingual environment from birth.1 Previous work has demonstrated that infants are able to distinguish different languages from a very young age, potentially allowing for an early awareness of the duality of the language experience. Newborn infants can discriminate languages from different rhythmical classes (Mehler et al., 1988), and infants of 4 months can discriminate two languages from the same rhythmical class if they have familiarity with one (Bosch & Sebastián-Gallés, 1997a) or both (Bosch & Sebastián-Gallés, 2001). This suggests that bilinguals may be able to distinguish between the two sets of phonetic representations present in the language environment. Although there is increasing work on language development and language processing in bilinguals (for recent reviews, see Genesee & Nicoladis, 2007; Sebastián-Gallés & Bosch, 2005) there is little work on the phonetic development of infants developing in a bilingual environment (although see Bosch & Sebastián-Gallés, 2003, and below). We thus turn first to the substantial literature documenting the corresponding processes in monolingual infants. DEVELOPMENT OF SPEECH PERCEPTION IN MONOLINGUALS Even very young infants display some initial organization of the acoustic–phonetic space. Oneand 4-month-old infants discriminate along a voice onset time (VOT) continuum in a categorical manner, discriminating a change that crosses the English boundary, from 20 to 40 ms, but not changes of equal magnitude from −20 to 0 ms or from 60 to 80 ms (Eimas, Siqueland, Jusczyk, & Vigorito, 1971). This initial state appears to be language general; young infants discriminate categorical changes in VOT that do not denote meaning in the native language in such diverse language environments as English (Aslin, Pisoni, Hennessey, & Perey, 1981), Spanish (Lasky, Syrdal-Lasky, & Klein, 1975; Rivera-Gaxiola, Silva-Pereyra, & Kuhl, 2005) and Kikuyu (Streeter, 1976). From this language-general starting point, monolingual perception has been shown to develop in accordance with the properties of the ambient language (Werker & Tees, 1992). Effects of experience are observed in vowel perception by 6 months of age (Kuhl, Williams, Lacerda, Stevens, & Lindblom, 1992; Polka & Werker, 1994) and in consonants by 10–12 months of age (Anderson, Morgan, & White, 2003; Best, McRoberts, LaFleur, & Silver-Isenstadt, 1995; Kuhl et al., 2006; Rivera-Gaxiola et al., 2005; Werker & Lalonde, 1988; Werker & Tees, 1984). One well-documented aspect of the functional reorganization underway (Werker, 1995) is the maintenance of native contrasts and the decline of nonnative contrasts. Recent evidence has also emerged for experiential facilitation of native contrast discrimination (Kuhl et al., 2006; Polka, Colantonio, & Sundara, 2001; Rivera-Gaxiola et al., 2005; Sundara, Polka, & Genesee, 2006). Polka et al. (2001) first presented evidence for facilitation with work on English and French infants’ and adults’ discrimination of the English /d/–/ / contrast, where, for the English participants, the native contrast discriminated in infancy appeared to strengthen at some point between 10 and 12 months and adulthood. Subsequently, Kuhl et al. (2006) tested infants learning either English or Japanese and found that both groups displayed similar discrimination of the English /ra/–/la/ contrast at 6–8 months. Applied Psycholinguistics 28:3 457 Burns et al.: Phonetic representation However, although the Japanese infants’ discrimination had declined predictably by 10–12 months, the English-learning infants’ discrimination increased correspondingly between the same ages (see Sebastián-Gallés, 2006, for a discussion). This observed facilitation is consistent with the concept of enhancement first described by Aslin and Pisoni (1980), where boundaries that are initially present are reinforced by experience and become more prominent. (See also Narayan, 2006; Narayan & Werker, 2007, for evidence that some native contrasts may not be initially discriminated.) Aslin and Pisoni (1980) also describe a process of realignment, where linguistic experience may shift a boundary from its initial language-general location to one consistent with the ambient language. This process appears to be particularly relevant in the case of VOT contrasts where category boundaries are placed differently along the continuum in different languages (Lisker & Abramson, 1970). Indeed, 4to 6.5-month-old Spanish-learning infants discriminated bilabial stops from −60 to −20 and 20 to 60, but not an equivalent magnitude in change from −20 to 20, which crossed the boundary discriminated by Spanish adults (Lasky et al., 1975). This infant pattern of discrimination is not consistent with the native Spanish phonetic boundaries, suggesting that initial perceptual boundaries may shift to align with native boundaries at some point between 4 and 6.5 months and adulthood. Thus, it appears that native phonetic perception refines from its initial language-general state in a number of different ways (maintenance, decline, facilitation, realignment) to result in languagespecific perception. These findings, of course, primarily describe processes documented in monolinguals. In a bilingual environment the task of the infant could be more complex, and it is an open question whether the development of bilingual infants’ phonetic representations progresses differently from monolingual development. DEVELOPMENT OF SPEECH PERCEPTION IN BILINGUALS The literature on the phonetic development of infants being raised in bilingual environments is generally sparse. However, a unique trajectory has been documented in the vocalic development of bilingual infants compared to monolinguals. Bosch and Sebastián-Gallés (2003) tested infants learning both Spanish and Catalan, and monolingual Catalan infants on their discrimination of a Catalan-specific vowel contrast, /e/–/E/. At 4 months of age, infants in both monolingual and bilingual environments were able to discriminate the contrast. Similarly, both groups discriminated the contrast at 12 months of age. However, at 8 months the groups diverged: the monolingual-learning infants were able pull apart the two vowel categories, whereas the infants being raised in bilingual environments did not. It appears that at 4 months both groups were discriminating in a languagegeneral fashion, and that language-specific responding is observed by 8 months in monolinguals. However, for the bilinguals, language-specific responding was delayed until 12 months of age. Bosch and Sebastián-Gallés argued that this temporary failure to discriminate is because of a more crowded distribution in the phonetic space of the bilingual infants (Catalan /e/, Catalan /E/, and Spanish /e/), which may initially cloud the distinction, and data on English monolingual Applied Psycholinguistics 28:3 458 Burns et al.: Phonetic representation 8-month-olds’ discrimination of vowels in a tight space corroborates this account (Sabourin, Bosch, Sebastián-Gallés, & Werker, 2006). Delays in facilitation of a native-language speech sound contrast have also been demonstrated in 4-year-old bilingual children as compared to monolinguals (Sundara et al., 2006). Similarly, Fennell, Byers-Heinlein, and Werker (2007) demonstrated that although monolingual infants succeed in a minimal pair associative word learning task with the /b/–/d/ contrast by 17 months of age, bilingual infants do not succeed until 20 months of age. Again, this pattern of responses likely results from the greater complexity of the phonetic space in bilingual representation, resulting in a longer process of reorganization. These findings lead to the question of whether the more crowded distribution in the bilingual phonetic space has an effect on the language-specific phonetic category representations that are formed by bilingual infants. Is the process the same, or possibly simply longer, for infants from bilingual environments than infants being raised in monolingual environments, with the end result being two phonetic representations, one from each language? Alternatively, does th