Vocabulary size matters: The assimilation of second-language Australian English vowels to first-language Japanese vowel categories

Adult second-language (L2) learners’ perception of L2 phonetic segments is influenced by firstlanguage phonological and phonetic properties. It was recently proposed that L2 vocabulary size in adult learners is related to changes in L2 perception (perceptual assimilation model), analogous to the emergence of first-language phonological function (i.e., attunement to the phonological identity of words) associated with the “vocabulary explosion” at 18 months. In a preliminary investigation of the relationship between L2 perception and vocabulary size, Japanese learners of Australian English identified Australian English vowels, provided goodness of fit ratings, and completed a vocabulary size questionnaire. We adopted a “whole-system” approach, allowing learners to apply all native vowel system possibilities to the full L2 vowel system. Learners with a larger L2 vocabulary were more consistent in their vowel assimilation patterns, compatible with the L2 perceptual assimilation model. Second-language (L2) learners have foreign accents that are influenced by their particular native (first) language (L1; Flege, 2002; Flege, Munro, & MacKay, 1995; Munro, Flege, & MacKay, 1996). It is less obvious, but equally important, that learners of an L2 also have an “accent” in their perception of the new language (Jenkins, Strange, & Polka, 1995), which is systematically related to the perceived © Cambridge University Press 2010 0142-7164/10 $15.00 Applied Psycholinguistics 32:1 52 Bundgaard-Nielsen et al.: Vocabulary size matters similarities between the phonological segments of their L1 and L2 (Best & Strange, 1992; Flege, 1987). Although learners may find some nonnative contrasts easy to discriminate, it is common for two or more L2 phones to be perceived as identical or similar to just one native phoneme (Bohn, 1995; Bohn & Flege, 1992; Flege, Bohn, & Jang, 1997; Flege & MacKay, 2004; Goto, 1971; Guion, Flege, AkahaneYamada, & Pruitt, 2000; Werker & Tees, 1984). However, difficulties in perceiving nonnative phones do not always persist as L2 proficiency increases (Flege et al., 1995; Ingram & Park, 1997; Tsukada et al., 2005), although it is unclear what drives this change and results in an increased nativelike perception or production. The purpose of this paper is to investigate the role of vocabulary size in how L2 learners learn to perceive these nonnative phones. In particular, the paper investigates the role of vocabulary size on L2 vowel perception, taking into consideration the entire L1 and L2 vowel systems, rather than a subset of nonnative and native consonants or vowels. We focus on a whole vowel system for a number of reasons. As discussed in detail below, there is ample evidence that a learner’s L1 vowel inventory (size and organization) influences how L2 vowels are perceived, and vowels are less discretely perceived (and articulated) than consonants (see, e.g., Strange, 1998a, 1998b), likely resulting in vowels being inherently more interconnected as a system. If this is the case, then failure to include the entire vowel system in a perceptual experiment could result in an ecologically invalid estimation of the perceptual flexibility L2 learners. There is abundant evidence that the size and organization of the L1 vowel inventory influences how L2 learners perceive the vowel contrasts in their new language. For example, native speakers of Spanish, a language with no temporal or tense–lax spectral contrasts, struggle to discriminate between British English /i/ and /i/, because both are perceived as instances of Spanish /i/ and their native phonology is not attuned to durational differences as being significant for vowel identification (Escudero & Boersma, 2004). In contrast, native speakers of Serbian, a language that also lacks this vowel contrast but does have a long and short version of /i/, discriminate between these vowels very well on the basis of duration because they are attuned to durational differences as a marker of phonological identity (Krebs-Lazendic & Best, 2008). Moreover, native speakers of German, a language with an /i/–/i/ contrast, discriminate /i/ and /i/ quite well on the basis of spectral differences, although the German realization of these two vowels is slightly different than the English pronunciations (Bohn & Flege, 1992; Iverson & Evans, 2007). In addition, the number of vowels in learners’ L1 influences their L2 vowel perception. The perceptual difficulty experienced by an L2 learner is partly determined by the size of the L1 vowel inventory relative to the L2 vowel inventory. Thus, it is harder for speakers of L1s with smaller vowel inventories (such as Spanish) to acquire a rich L2 vowel inventory relative to speakers of L1s with larger vowel inventories (such as German and Norwegian). This is because several L2 vowels may be perceived as similar to just one L1 vowel category and consequently will be hard to discriminate (Iverson & Evans, 2007). Perceptual sensitivity to L2 vowel duration has been shown to be a more highly salient cue to vowel identity than spectral information. For example, Japanese learners of English systematically distinguish American English tense (/i..., ei, æ, Applied Psycholinguistics 32:1 53 Bundgaard-Nielsen et al.: Vocabulary size matters A..., O..., oυ, u... /) and lax (/i, ε, ∧, υ/) vowels (Strange et al., 1998), Australian English (AusE) vowels /æ/ and /ɐ... / (Ingram & Park, 1997), and Canadian English /i/ and /i/ (Morrison, 2002) on the basis of duration, even though the durational differences per se are not phonemic in any of these three dialects of English, for which the relevant dimensions are tense and lax, leading English listeners to rely on spectral rather than purely temporal differences. The high saliency of durational differences is thus particularly strong for speakers of languages where duration is phonemic (including vowel length contrasts), such as Estonian (McAllister, Flege, & Piske, 2002), although it has also been documented to a lesser extent for speakers of L1s where vowel duration is not phonemic but who have a tense–lax distinction, such as English and German (Bohn, 1995; Bohn & Flege, 1992; Cebrian, 2006; Flege et al., 1997; Gottfried & Beddor, 1988; Goudbeek, Cutler, & Smits, 2008), and even less so for speakers of languages without length (gemination) and tense–lax distinctions, such as Spanish. Nevertheless, differences and interactions between the L1 and L2 phonological systems of a learner does not mean that an L2 learner’s perception will never improve or come to more closely resemble that of a native speaker of the L2. A large body of literature documents that a learner’s perceptual difficulties are partly determined by their level of familiarity with and use of the L2, and that increased L2 exposure and use typically leads to improved perception and production in the L2 (Flege et al., 1995: Ingram & Park, 1997; Tsukada et al., 2005). Unfortunately, it is not clear how this change in perception (and production) occurs, and the underlying mechanisms have not been adequately addressed by the two primary theoretical models of L2 production and perception: the speech learning model (SLM; Flege, 1995), which focuses on experienced L2 speakers, and the perceptual assimilation model (PAM; Best, 1995), which focuses on naive listeners. Recently, an extension of PAM, PAM-L2 (Best & Tyler, 2007), which focuses on L2 development from within the general PAM framework, has attempted to fill this theoretical gap as well as provide testable hypotheses for how perceptual changes come about. PAM (Best, 1994, 1995) assumes that L1 acquisition is essentially the finetuning of the perceptual systems to those articulatory gestures in the L1 that are meaningful and that naive perception of L2 phones will reflect this L1 tuning in terms of the phonetic realizations and the phonological organization of the speaker’s native language. PAM-L2 (Best & Tyler, 2007) also assumes that perceptual learning is possible at all ages but will be influenced by the entire language learning history of the individual. PAM-L2 further posits that an increased L2 vocabulary may “exert forceful linguistic pressure” (Best & Tyler, 2007, p. 32) on the learner to attune to articulatory, phonetic, and phonological differences in the L2 that have previously been ignored in the L1. That is, for successful L2 comprehension the learner must differentiate between an increasing number of contrasting L2 words that initially sound identical through the L1 phonological system. We propose that this consequently causes the learner to rephonologize, that is, establish an L2 phonology, by modification of or addition to the learner’s existing L1 phonological system. This vocabulary-driven rephonologization is proposed to work in a similar fashion to the emergence of L1 phonological function that appears to be driven by the Applied Psycholinguistics 32:1 54 Bundgaard-Nielsen et al.: Vocabulary size matters “vocabulary explosion” in children at around 18 months (e.g., Metsala & Walley, 1998). We use the term phonological function to mean the ability of adults and even word-learning 19-month-old toddlers to recognize words on the basis of their phonological (abstract) identity, rather than on the basis of familiarity with the surface, phonetic patterns of the specific utterances they have experienced (see Best, Tyler, Gooding, Orlando, & Quann, 2009). The vocabulary-driven linguistic pressure may be particularly strong in densely populated lexical neighborhoods with many minimal pairs, especially where several contrastive L2 phones are assimilated into the same L1 category. It is important to note that we do not imply that reattunement and rephonologization in L2 learners necessarily leads to the formation of a complete new set of L2 phonological categories, but rather that the learner settles on a phonological system for the L2, perhaps by stretching the L1 inventory as far as possible, and perhaps by forming new L2 categories. Given that most theories of L2 acquisition (such as PAM/PAM-L2 and SLM) assume that L2 acquisition is based on the same processes of acquisition as is L1 acquisition (although the language history of an individual most often results in L2 acquisition being affected by the L1 of that learner), the suggestion that L2 reattunement and rephonologization is closely associated with L2 vocabulary development does have some support. Research into L1 acquisition in young infants and children suggests a circular or cyclic relationship between segmental speech perception, vocabulary size, and speech production abilities (see, e.g., Smith, McGregor, & Demille, 2006). Early vocabulary size has been found to predict the speech perception abilities of children between the ages of 2 and 6 (Metsala, 1999), highlighting the interdependency of vocabulary development and speech perception. Segmental perception (of phonetic contrasts) in the first year of life has also been found to predict vocabulary size in the second year of life (Tsao, Liu, & Kuhl, 2004), suggesting that successful phonetic attunement facilitates word learning, which in turn supports the onset of phonologization at the time of the “vocabulary expansion” (see, e.g., Stager & Werker, 1997; Swingley, 2003). The present study tests the PAM-L2 based hypothesis that a larger L2 vocabulary drives a process of rephonologization for adult L2 learners. We do so by examining the similarities and differences in L2 speech perception in two groups of speakers (differing in the size of their L2 vocabulary, but not on any other central measure such as age of acquisition, L2 immersion duration, and number of years of foreign language acquisition) who speak a language with a limited vowel inventory and duration-based phonological vowel distinctions (Japanese), and who are acquiring an L2 with a larger vowel inventory but no pure duration-based phonological distinctions (AusE). This L1–L2 combination has been traditionally seen as posing a significant challenge to Japanese learners, who will have to adjust their limited vowel inventory of just five unique, and relatively pure, vowels as these occur in five spectrally similar short–long pairs (/i, i..., e, e..., a, a..., u, u..., o, o... /; International Phonetic Association, 1999). However, when the possible Japanese bimoraic combinations (/ie, ia, iu, io, ei, ea, eu, eo, ai, ae, au, ao, ui, ue, ua, uo, oi, oe, oa, ou/) are added to the L1 Japanese learners’ category inventory, the task of mapping the 18 AusE vowels Applied Psycholinguistics 32:1 55 Bundgaard-Nielsen et al.: Vocabulary size matters (monophthongs /i..., i, e, e..., ..., @, æ, ɐ, ɐ..., o..., υ, − u..., O/ and diphthongs /i@, æi, Ae, oi, @− u, æO/; Cox, 2006) may be less taxing. However, it complicates the picture that all AusE monophthongs are characterized by some degree of spectral change during production and are generally differentiated on the basis of Frequency 1 (F1) and F2, with the exception of /i... /–/i/ and /ɐ/–/ɐ... /, which are differentiated mainly on the basis of duration (Cox, 2006). In a cross-language mapping experiment, we adopted a “whole-system” approach and presented the learners with the entire AusE vowel inventory and all Japanese monomoraic or bimoraic categories possible. This approach allows a more complete assessment of the perceptual flexibility of the learners than would the presentation of a preselected subset of the L2 vowels because it provides the Japanese learners with the full range of L2 input as well as the opportunity to exploit all native sensitivities as well as L2 vowel differences, spectral and durational. In line with the PAM-L2 hypothesis that L2 vocabulary size contributes centrally to L2 phonological reattunement (Best & Tyler, 2007), we expected L2 learners with a larger L2 vocabulary to more consistently identify L2 vowels in terms of their L1 vowel categories than learners with a smaller L2 vocabulary. This is based on the reasoning that the more advanced acquisition of L2 vocabulary by the former subgroup would have already driven them to perceptually attune to nonnative phonetic and phonological differences and to begin to reorganize their native phoneme inventory to accommodate the L2 vowel system, at least to some degree (and/or to begin to establish new L2 vowel categories for certain L1–L2 differences, as discussed below). This is compatible with both PAM and SLM, which propose (a) that learners are likely to perceive (and produce) nonnative phones on the basis of their similarity to or dissimilarity from existing L1 phones (on a scale from new to similar to identical), and (b) that more advanced learners may be more successful in integrating the L2 phones into their existing phonological system (or establishing new L2 phonemes), because their increased experience with the (vocabulary of their) L2 may have prompted them to better attune to the meaningful (i.e., phonological) differences in the L2 and more successfully use their L1 categories and sensitivities to differentiate between (and possibly create novel categories for) these L2 phones. In the case of L2 vowel acquisition, we further suggest that it is unlikely that an L2 vowel will be perceived as an entirely new category (i.e., as unrelated to any L1 vowel category) in early acquisition, but rather that “difficult” L2 vowels will be initially be perceived as somewhat similar to a number of L1 vowels, in line with research indicating that vowels are produced in a manner less discrete than consonants and rather continuously perceived. We also suggest that such vowels will be more consistently identified as belonging to just one L1 category through further perceptual reattunement and rephonologization, which may allow the learner to identify those aspects of the uncategorized L2 phone that most systematically relate to just one L1 category (facilitating L2 comprehension), or realize that the phone does not systematically relate to any L1 category, leading perhaps to the formation of a new L2 category. Further, in line with PAM and PAM-L2, we expected that the Japanese learners would be highly sensitive to durational differences as well as to some spectral differences in the AusE vowel system. Specifically, we expected that short AusE Applied Psycholinguistics 32:1 56 Bundgaard-Nielsen et al.: Vocabulary size matters monophthongs would be identified as Japanese monomoraic vowels, long AusE monophthongs would be identified as bimoraic identical (“long”) Japanese vowels, and diphthongs would be identified as instances of bimoraic Japanese vowel combinations. In agreement with PAM, our expectation was that the perceptual assimilations of AusE vowels by the learners would reflect not only gradient phonetic similarities (acoustic or articulatory) of their native vowel inventory with the nonnative phones but also the perceived L1/L2 similarities on an abstract, lexically distinctive phonological level.