Effects of age of learning on voice onset time: categorical perception of Swedish stops by near-native L2 speakers.

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research-article2013

LAS57410.1177/0023830913508760Language and SpeechStölten et al.

Language and Speech

Article

Effects of Age of Learning on Voice Onset Time: Categorical Perception of Swedish Stops by Near-native L2 Speakers

Language and Speech 2014, Vol. 57(4) 425–450 © The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0023830913508760 las.sagepub.com

Katrin Stölten, Niclas Abrahamsson and Kenneth Hyltenstam Stockholm University, Sweden

Abstract This study examined the effects of age of onset (AO) of L2 acquisition on the categorical perception of the voicing contrast in Swedish word-initial stops varying in voice onset time (VOT). Three voicing continua created on the basis of natural Swedish word pairs with /p-b/, /t-d/, /k-/ in initial position were presented to 41 Spanish early (AO < 12) and late (AO > 12) near-native speakers of L2 Swedish. Fifteen native speakers of Swedish served as controls. Categorizations were influenced by AO and listener status as L1/L2 speaker, in that the late learners deviated the most from native-speaker perception. In addition, only a small minority of the late learners perceived the voicing contrast in a way comparable to native-speaker categorization, while most early L2 learners demonstrated nativelike categorization patterns. However, when the results were combined with the L2 learners’ production of Swedish voiceless stops (Stölten, 2005; Stölten, Abrahamsson & Hyltenstam, in press), nativelike production and perception was never found among the late learners, while a majority of the early learners still exhibited nativelike production and perception. It is concluded that, despite their being perceived as mother-tongue speakers of Swedish by native listeners, the late learners do not, after detailed phonetic scrutiny, exhibit a fully nativelike command of Swedish VOT. Consequently, being near-native rather than nativelike speakers of their second language, these individuals do not constitute the evidence necessary to reject the hypothesis of one or several critical (or sensitive) periods for language acquisition. Keywords Age of onset (AO), voice onset time (VOT), categorical perception, near-nativeness, L2 acquisition

Corresponding author: Katrin Stölten, Centre for Research on Bilingualism, Stockholm University, SE10691 Stockholm, Sweden. Email: [email protected]

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1 Introduction 1.1 Age constraints on first and second language acquisition Extensive research on infant perception has shown that newborns are innately equipped with sensitivities to perceive a variety of phonetic contrasts. However, already during their first months of life infants become perceptually attuned to the phonemic contrasts found in their immediate linguistic environment, thereby experiencing an early reorganization of perceptual abilities that makes them increasingly insensitive to non-native contrasts (Kuhl, 2001; Werker & Tees, 1984). A central issue in the field of language acquisition research has been the critical period for language acquisition offered by Lenneberg (1967). According to his original formulation of the critical period hypothesis (CPH), language acquisition is constrained by biological/maturational factors resulting in a decrease and eventual loss of flexibility for cerebral reorganization after the age of 12–13 years. Lenneberg (1967, p. 158) concluded that whereas languages are successfully learned during childhood, “primary, basic language skills not acquired at that time… usually remain deficient for life”. However, while Lenneberg (1967) assumed puberty or, more precisely, the age of approximately 12 years to constitute the upper limit of such a time frame of heightened sensitivity, other proposals exist regarding the time span of a biologically defined critical period for language acquisition. One suggestion is offered by Ruben (1997), who investigated first language (L1) development in children temporarily deprived of continuous L1 verbal input during their first year of life due to fluctuant hearing loss as a result of otitis of the middle ear. When examined at the age of nine years, these children scored significantly lower on tests of verbal memory and phonetic discrimination when compared to a group of children who had had normal hearing during their first year of life. According to Ruben (1997, p. 117), the findings support the idea of several critical periods (i.e., for phonology, morphosyntax and semantics) of which “the earliest specialization is phonological, with a limiting of phoneme discrimination by the end of the first year”. Since the late 1960s, researchers have investigated the question as to whether such theories of one or several critical periods may extend to second language acquisition (SLA). Whereas in some studies the upper limit for nativelike second language (L2) attainment has been associated with puberty or adolescence (for morphology and syntax see e.g., Long (1990), Patkowski (1980); for phonology see e.g., Scovel (1969, 1988), other researchers have suggested an earlier age, possibly around six or seven years, to be crucial for achieving nativelike proficiency in a second language (for morphology and syntax see e.g., Johnson & Newport (1989); for phonology see e.g., Flege, Munro & MacKay (1995)). The general observation that childhood L2 acquirers outperform learners with a late start is strongly supported by the negative correlations found between the learners’ age of onset (AO) of L2 acquisition and any measure of L2 ultimate attainment (Abrahamsson, 2012; Abrahamsson & Hyltenstam, 2009; DeKeyser, 2000; Johnson & Newport, 1989; Stölten, 2005; Stölten, et al., in press). Findings like these have frequently been interpreted with reference to biological constraints in terms of maturation of the brain (Lenneberg, 1967; Pulvermüller & Schumann, 1994; Uylings, 2006). Following the suggestion made by Long (1990, 1993) that all we need to be able to reject the CPH and the idea of maturational changes with age is one single adult language learner with an ultimate attainment of the L2 that is indistinguishable from that of native speakers, a number of studies have claimed to having identified such learners (Birdsong, 2007; Birdsong & Molis, 2001; Moyer, 1999; van Boxtel, Bongaerts & Coppen, 2005; White & Genesee, 1996). In a series of pronunciation studies on highly proficient late L2 learners of English, French or Dutch, Bongaerts and his colleagues (Bongaerts, Mennen & van der Slik, 2000; Bongaerts, Planken & Schils, 1995; Bongaerts, van Summeren, Planken & Schils, 1997) identified individual cases of late L2 learners who passed for native speakers by a panel of native listeners. The researchers concluded that an


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authentic, nativelike pronunciation of the L2 is indeed achievable even by L2 learners with a late start. However, estimations of the incidence of nativelikeness in adult L2 learning are inconsistent in SLA research. Whereas some studies report quite high rates (above 20%) of nativelikeness within the adult learner population (Birdsong, 1992; Birdsong & Molis, 2001; White & Genesee, 1996), other studies suggest more moderate rates (5–20%) (Bongaerts, 1999; Flege, Yeni-Komshian & Liu, 1999; van Boxtel et al., 2005). In a study by Moyer (1999), in which the pronunciation of 24 highly advanced American learners of German was investigated, only one adult L2 learner passed for a native speaker when judged by a panel of native German listeners. Findings like these, along with others (Ioup, Boustagui, El Tigi & Moselle, 1994; Johnson & Newport, 1989; Stölten et al., in press), thus suggest that nativelike post-puberty learners should be very rare or even non-existent. The notion of nativelike L2 ultimate attainment, however, has been a matter of disagreement among researchers, and studies that are frequently referred to as having identified late learners with a nativelike L2 command have been subjected to a variety of criticisms. One problematic issue is that too generous interpretations of the concept of nativelikeness may have contributed considerably to overestimations of the incidence of nativelikeness among late L2 learners (Abrahamsson & Hyltenstam, 2009). In short, whereas some researchers measure nativelikeness on the basis of L2 speakers’ self-identifications (Piller, 2002; Seliger, Krashen & Ladefoged, 1975), others consider as nativelike L2 speakers who are perceived as nativelike by mothertongue speakers of the target language (Bongaerts, 1999; Moyer, 1999). Still others use the notion of nativelikeness only when referring to the proficiency of L2 learners who, after thorough linguistic examination, exhibit actual nativelike competence and behavior (Abrahamsson & Hyltenstam, 2008, 2009; Birdsong, 1999, 2007; Bley-Vroman, 1989; Hyltenstam & Abrahamsson, 2003b; Long, 1990). Although the first and second approach may indeed contribute to the complexity of the concept of nativelikeness, it may be argued that only the third interpretation is relevant if the goal is to challenge the CPH. Another problem with studies reporting on nativelike late L2 learners is the frequent use of much too simple language tests, insufficiently detailed analyses or elicitation techniques not sensitive enough to discriminate between near-native and nativelike proficiency, which most certainly have resulted in ceiling effects and type II errors. It is therefore reasonable to assume that much of the previous research has produced “false positives”, that is, very advanced L2 learners have been misclassified as nativelike rather than near-native (Abrahamsson & Hyltenstam, 2009; Long, 2005). Hyltenstam & Abrahamsson (2003a) introduced the notion of non-perceivable non-nativeness, which refers to L2 learners who are perceptually indistinguishable from native speakers, but who, after linguistic scrutiny, exhibit different competence or behavioral patterns when compared to native control speakers. Furthermore, even if nativelike levels might well be attained by some postpuberty or adult learners, their nativelikeness will most likely be confined to a few limited areas of the target language rather than characterizing their entire L2 repertoire. Until the present time, no published study has managed to identify adult learners who have attained an across-the-board command of their L2 identical to that of native speakers. Given this, no claim can be made for the existence of such learners or, consequently, for their status as the counterevidence necessary to reject the CPH. In other words, assertions about biological/maturational factors constraining (second) language acquisition cannot be dismissed on the basis of the available evidence. Another, perhaps more surprising result from recent research is that among learners with an early start to L2 acquisition (i.e., before puberty), the incidence of non-nativelikeness has been shown to be much more frequent than commonly assumed (Abrahamsson, 2012; Abrahamsson & Hyltenstam, 2008, 2009; Bylund, Abrahamsson & Hyltenstam, 2012; Flege et al., 2006; YeniKomshian, Flege & Liu, 2000). Flege et al. (1999) showed that a majority of native Korean learners of L2 English with an early age of arrival to the United States (less than 10 years old) did not


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reach mean foreign accent ratings comparable to those of native controls. In our own empirical studies on the production of L2 Swedish word-initial voiceless stops by L1 Spanish speakers (Stölten, 2005; Stölten et al., in press), far from all 31 early learners (AO ≤ 11 years) did perform within native-speaker range. In a recent study by Abrahamsson (2012), a grammaticality judgment test (GJT) and a test of categorical perception of the /p-b/ contrast were conducted on a group of 200 randomly selected L1 Spanish early and late learners of Swedish as an L2 (AO 1–30). When results from the two measures were combined, about half (55%) of the early childhood learners (i.e., AO 1–6) exhibited nativelike behavior, while this was the case for only 28% of the late childhood learners (AO 6–10). In addition, in both learner groups, 6% and 25%, respectively, did not perform at the native-speaker level on any of the tests. Obviously, incidences of non-nativelikeness among early L2 learners are not good fits with what is predicted by the CPH. Instead of the assumption that learning outcomes change abruptly after a certain age, alternative suggestions have proposed a continuous age-related decline due to linear deterioration of general cognitive abilities in addition to social/psychological and educational factors (Bialystok & Hakuta, 1999; Marinova-Todd, Marshall & Snow, 2000). However, while Bialystok and Hakuta (1999) seem to interpret a linear decline as evidence against a CPH for language acquisition, such a pattern may just as well be explained by maturational factors (Hyltenstam and Abrahamsson, 2003a). On the other hand, to what extent the decline is, in fact, linear is a question that requires further investigation. What emerges from the results of our recent research is a weak linear decline from birth to age 12, after which a fairly steep decline occurs during adolescence, followed again by a weak decline from late adolescence through adulthood (Abrahamsson, 2012; Abrahamsson & Hyltenstam, 2009). This type of pattern is not fully in line with the CPH but is a very close approximation to what it predicts; in fact, the pattern suggests that nativelike proficiency is still possible if acquisition begins during a limited critical or sensitive (or maturational) period, but becomes successively less probable with increasing AO already during that period.

1.2. Voice onset time and categorical perception in SLA research For stop consonants, voice onset time (VOT) is defined as “…the interval between the release burst of the stop and the onset of glottal vibration” (Lisker & Abramson, 1964, p. 389). It is an established view that languages like English and Swedish treat voiceless word-initial stops as aspirated long-lag stops with relatively long, positive VOTs, whereas their voiced counterparts are realized by unaspirated short-lag stops, that is, with short, positive VOTs1 (for English stops, e.g., Deuchar & Clark (1996), Lisker & Abramson (1964); for VOT measurements on Swedish stops, e.g., Sundberg & Lacerda (1999)). This pattern is contrasted by languages like Spanish and French that treat long lead or prevoiced stops (i.e., long, negative VOTs) as voiced, while short-lag stops are classified as voiceless (Caramazza & Yeni-Komshian, 1974; Deuchar & Clark, 1996; Zampini & Green, 2001). In other words, in English (and Swedish) voiced stops are characterized by an articulatory timing similar to Spanish voiceless stops, leading Zampini & Green (2001, p. 24) to the conclusion that although these languages “show the same phonological contrast for the voiced and voiceless stops, the contrast is realized differently at the phonetic level”. Due to this cross-linguistic variation, L2 learners often experience various degrees of difficulty in accurately producing and perceiving L2 stops that differ from their native language stops in terms of VOT. In short, L2 learners, especially those who had late starts, generally tend to either transfer their native-language voicing patterns to the second language or manifest stop categories that are intermediate to those of the L1 and L2 (Caramazza, Yeni-Komshian, Zurif & Carbone, 1973; Obler, 1982; Williams, 1977b). Individuals who start to learn their L2 early in life usually end up producing and perceiving L2 stops with mean VOTs that resemble native-speaker behavior


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to a considerably greater extent than late bilinguals (Abrahamsson, 2012; Flege, 1991; Flege & Eefting, 1987; MacKay, Flege, Piske & Schirru, 2001; Stölten et al., in press). Although such findings are sometimes related to maturation of the brain (Patkowski, 1980; Scovel, 1969, 1988), the idea of age effects arising from a neurologically-based critical period has been questioned since studies have reported on L2 learners whose level of L2 proficiency does not seem to fit into this overall pattern, as outlined above.

1.3 The study – aims and hypotheses The aim of the present study was to evaluate the claims by some researchers that late L2 learners with a nativelike ultimate attainment exist, and who therefore constitute the evidence necessary to reject the CPH and any theory of maturational constraints on language acquisition. Therefore, the study investigated age effects as well as the incidence of actual nativelike perception of VOT among (late) L2 learners, all of whom seem nativelike in everyday conversation. For more than two decades, researchers have followed Long’s (1990, 1993) suggestion and directed their attention to potential counterexamples to the CPH – that is, late/adult L2 learners who give the appearance of having attained nativelike proficiency in their second language despite having learned it after the alleged critical or sensitive period is over. The question of interest is whether such seemingly nativelike L2 learners can be considered truly nativelike after linguistic scrutiny, because: •• If so, they would constitute evidence that late language acquisition is not constrained or made more difficult by the maturation of the brain (but rather by other, non-biological factors); however, •• If not entirely nativelike, these learners cannot be considered the evidence necessary to reject the CPH or theories of maturational constraints on language acquisition. In line with this research agenda, a large-scale project was designed with the aim of exploring the role of AO for L1 Spanish speakers’ ultimate attainment of L2 Swedish (Abrahamsson & Hyltenstam, 2009). Contrary to group studies with randomly selected L2 acquirers, the project focused exclusively on potentially nativelike early and late L2 learners, that is, individuals who pass for mothertongue speakers of their L2 in everyday oral communication with actual mother-tongue speakers. The rationale behind this focus was that there is no meaning in investigating noticeably non-nativelike L2 speakers intensively in order to reaffirm their non-nativelikeness (Long 1993; cf. Abrahamsson & Hyltenstam, 2008, 2009; Hyltenstam & Abrahamsson, 2003a). By the use of an initial screening test for potential nativelikeness (see Section 2.1 Participants), L2 proficiencies were kept fairly high and as uniform as possible across the AO range, thereby diminishing the risk for obvious differences in L2 proficiency (i.e., high-level vs. low-level) having a major impact on the outcomes. Furthermore, the project made use of different techniques, including the initial screening followed by a multi-task methodology comprising challenging tests and in-depth analyses of the participants’ L2 attainment of a wide range of L2 linguistic components and skills (Abrahamsson & Hyltenstam, 2009). As part of the project, the present paper is concerned with L2 learners’ categorical perception of the voicing contrast in Swedish word-initial stops, varying along a continuum of VOT. Perception experiments investigating language effects in L2/bilingual listeners are usually based on test stimuli comprising synthetically generated stop-vowel syllables (Caramazza et al., 1973; Flege & Eefting, 1987; Flege, Schmidt & Wharton, 1996). However, as stated by Elman, Diehl and Buchwald (1977), synthetic nonsense syllables may not constitute the most ideal test stimuli for establishing and maintaining a specific language set throughout the entire identification test, especially when bilingual listeners are concerned. The researchers therefore preferred a sample of five naturally-produced test syllables with varying VOT in their study on English-Spanish


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bilinguals (Elman et al., 1977). Instead of test syllables, Hazan and Boulakia (1993) used minimal word pairs based on edited natural speech tokens that are meaningful in the languages in question (i.e., French and English). Depending on whether the bilinguals were based in Great Britain or in France, the participants were tested in their country of residence. According to the researchers, this methodology makes certain “that speakers were better placed in the respective language modes” (Hazan & Boulakia, 1993, p. 19). Therefore, the perception study in hand made use of edited natural speech tokens that are meaningful in Swedish. In addition, great care was given to the presentation of stimuli, that is, they were presented in an entirely Swedish environment. This combination of procedures, we believe, is especially important in a context in which highly proficient L2 speakers with varying AOs are tested since it ensures that the individuals are positioned in the right language mode. The present study also differs from many perception studies in the number of stop pairs and VOT stimuli being investigated. Whereas some studies have examined two (Volaitis & Miller, 1992) or three stop continua (Abramson & Lisker, 1973; Caramazza et al., 1973), listening tests are generally restricted to one stop pair (Elman et al., 1977; Flege et al., 1996; Hazan & Boulakia, 1993; Zampini & Green, 2001). Moreover, among those studies examining categorical perception in L2/bilingual listeners, stimuli often vary in intervals of either 5 ms (Caramazza et al, 1973; Flege et al., 1996) or 10 ms (Flege & Eefting, 1987; Hazan & Boulakia, 1993; Mack, 1989) along the VOT continuum. This study, however, included all three minimal stop pairs in Swedish (i.e., labial, dental and velar) and intervals of 5 ms VOT were used for each continuum. Furthermore, stop identifications were first examined and compared with a group of native speakers of Swedish and the results were then analyzed in relation to previous data obtained from the same L2 speakers’ production of the three Swedish voiceless stops /p t k/ (reported in detail in Stölten (2005), Stölten et al., in press). By combining these procedures, the present study allows for a more comprehensive, in-depth phonetic analysis of the VOT system that goes beyond evaluations based on small samples of everyday oral language use, thereby reducing the risk for type II errors and “false positives” in the search for truly nativelike proficiency. VOT is generally described as an important, salient cue for voicing in both production and perception of initial stops across a wide range of languages (Abramson & Lisker, 1973; Caramazza & Yeni-Komshian, 1974; Lisker & Abramson, 1964). It may therefore be assumed that manipulations of VOT should be sufficient to discriminate voiced from voiceless stops in stimuli created on the basis of natural productions. However, for reasons of reliability and validity, it is of first priority to ensure that the test design used in the present study, including the preparation of VOT stimuli and the procedure of the listening tests (for details, see Section 2.3 on stimuli and procedure below), meets this expectation. Therefore, the first part of the study is concerned with the question of whether the listeners, irrespective of their status as native speakers or L2 learners of Swedish and regardless of their AO of L2 acquisition, perceive the edited natural stimuli in a categorical manner on the basis of VOT. Only when the reliability and validity of these methods have been established can VOT be considered an adequate measure for the investigation of Spanish-Swedish bilinguals’ sensitivity to the L2 voicing contrast and can conclusions be made concerning the central hypotheses of the study regarding AO effects. As SLA research has consistently demonstrated, learners who begin their L2 acquisition during childhood generally attain higher proficiency levels than adult learners. Although exceptions to this overall pattern have been reported, AO is the strongest variable affecting ultimate attainment in L2 acquisition. Hypothesis 1 therefore postulates: 1. Even among L2 learners with a seemingly nativelike pronunciation, as measured by native-listener judgments, category boundary placement is affected by the L2 learners’ AO and the status as L1 or L2


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speakers of Swedish, in that the native speakers will show the highest and the late L2 learners the lowest (i.e., the most non-Swedish-like) average category crossover values, with the early L2 learners exhibiting values somewhere in between.

With reference to the CPH and its application to L2 acquisition, neither late learners with an actual nativelike command of their L2 nor early learners with a proficiency level below that of native speakers are predicted by the hypothesis. However, as shown in previous research, late learners are sometimes identified as having a nativelike L2 command of a few or even a variety of linguistic phenomena, and early learners are not always found to perform at a native-speaker level. Such findings motivate the formulation of hypothesis 2: 2. Even among L2 learners with a seemingly nativelike pronunciation, as measured by native-listener judgments, (a) very few (if any) late L2 learners, and (b) most (but not all) early L2 learners will exhibit average category boundary values within the range of native-speaker categorization.

In a recent study by Abrahamsson and Hyltenstam (2009) investigating the same group of highly advanced L1 Spanish learners of L2 Swedish, the incidence of actual linguistic nativelikeness was found to decrease considerably when test results from ten different measures of L2 proficiency (e.g., grammaticality judgment, formulaic language, stop production and perception) were combined. Only three of the 31 early learners (AO 1–11) and none of the 10 late learners (AO 13–19) exhibited complete nativelike command of their L2 when scrutinized in detail and across the board of language proficiency. Based on empirical findings from L1 and L2 research, aspects of phonetics/phonology have been suggested to be most sensitive to critical period effects (Scovel, 1988). It is therefore of interest to analyze in greater detail the incidence of nativelikeness when only phonetic/phonological L2 competence is under investigation. In other words, presuming stop production and perception to be especially vulnerable to the AO factor, hypothesis 3 predicts that: 3. Even among L2 learners with a seemingly nativelike pronunciation, as measured by native-listener judgments, when results from their stop perception are combined with data from their productions of word-initial voiceless stops, and when all three places of articulation are considered, (a) the incidence of nativelikeness will become virtually non-existent among late L2 learners, but (b) a majority of the early L2 learners will still perform within native-speaker range.

2 Method 2.1 Participants From a pool of 195 potential participants (132 females and 63 males) identified through advertisements distributed in two daily newspapers,2 41 learners of L2 Swedish (32 females and nine males) were selected for the present study. The advertisements targeted only speakers with Spanish as L1 who self-identified as nativelike in Swedish as their L2. Selections were based on a screening test in which the candidates (107 participants with an AO3 ≤ 11 years, 88 with an AO ≥ 12 years) had to pass as mother-tongue speakers of Swedish by at least six of 10 linguistically/ phonetically untrained native listeners. The listeners’ task was to judge 20–30 s monologues on a specified topic extracted from each of the L2 speakers. From the 104 individuals who passed as native speakers (87 participants with AO 1–11, 17 participants with AO 12–47), 41 were selected who could be matched with regard to background factors.4 At the time of testing, the participants were adults with a mean age of 32 years (range 20–50), and their mean length of


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residence (LOR) in Sweden was 25 years (range 12–42). All had acquired the variety of Swedish spoken in the Stockholm area and had an educational level of no less than senior high school (i.e., 12 years of education). Since their day of immigration, the L2 speakers had never more than just occasionally lived outside Sweden or the Stockholm area. Except for chronological age, none of the differences between the group of the 31 early (AO 1–11) and 10 late (AO 13–19) L2 learners were statistically significant, suggesting that the groups were well-matched with regard to background variables.5 For further details on the selection and methodology of the main project, see Abrahamsson and Hyltenstam (2009). A control group was included, consisting of 15 native speakers of Stockholm Swedish.6 These were matched with the experimental group regarding age (mean = 30 years, range = 23–46), sex (11 females and four males) and educational level (senior high-school diploma as a minimum). The native speakers had no linguistic or phonetic training. There was a strong bias toward Chilean Spanish (27 participants) in the experimental group. Other Latin American countries of origin that were represented included Peru (six participants), Colombia (two), Argentina (one), Bolivia (one), Mexico (one) and Uruguay (one). Two participants reported Spain as their home country. Comparisons made by Rosner, López-Bascuas, García-Albea and Fahey (2000) between Castilian Spanish and three Latin American dialects7 showed that a certain variation in terms of VOT exists among Spanish varieties. The overall observation is that whereas Latin American perceptual VOT boundaries are negative, Castilian listeners experience positive category crossovers. These findings support the conclusion made by Rosner et al. (2000) that Spanish dialects should be treated separately where VOT is concerned. In the present study, however, this was disregarded since differences in VOT between languages like Swedish or English and any Spanish variety generally are more salient than the differences found among Spanish varieties. Further motivation for treating the Spanish varieties as sharing a single approximately similar perceptual VOT system comes from Williams (1977a, 1977b) comparing the voice timing patterns in the production of labial, dental and velar wordinitial stops by speakers of four Latin American dialects: Guatemalan, Venezuelan, Peruvian and Puerto Rican Spanish. No significant cross-dialectal variation was found regarding the realization of VOT for any of the stops. Furthermore, comparisons of bilabial stop categorizations between Peruvian Spanish and Puerto Rican monolinguals showed that most of the listeners required voicing lead proportions of 14 ms or more in order to make a voiced decision 75% or more of the time, suggesting that “prevoicing is a sufficient positive cue for the voiced stop category in Spanish” (Williams, 1977a, p. 175).

2.2 General procedure The entire testing procedure took place in a sound-attenuated room at Stockholm University and lasted for approximately four h per person. The participants were tested individually for a wide range of linguistic levels and components of Swedish. Prior to linguistic scrutiny, each individual went through a hearing test with an Oscilla SM 910 Inmedico A/S, Bromma, Sweden screening audiometer, and a decrease of no more than 10 dB for one frequency in one ear was considered acceptable. Data collection and testing was conducted by a male native speaker of Stockholm Swedish. Before undertaking these 4-h sessions, a test battery was designed containing some 20 different instruments for speech elicitation and language testing. In addition to the categorical perception task reported on here, other test components comprised the production of word-initial voiceless stops, speech perception in noise, grammaticality judgments, idiomatic expressions and proverbs (for details, see Abrahamsson & Hyltenstam, 2009).


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2.3 Stimuli and procedure for the test on categorical perception The speech stimuli were prepared on the basis of recordings of isolated, naturally-spoken minimal word pairs read aloud by a female native speaker of Stockholm Swedish. The recordings took place in an anechoic chamber at the phonetic laboratory at Stockholm University where the phonetically trained speaker was instructed to produce repetitions of six Swedish words – three minimal pairs with a voiced and voiceless stop, respectively, located in word-initial position: par /pɑr/ (pair; couple) and bar /bɑr/ (bar; bare; carried); tal /tɑl/ (number; speech) and dal /dɑl/ (valley); kal /kɑl/ (naked; bald) and gal /ɡɑl/ (crow-s; call-s, verb in present tense). In order to achieve pronunciations adequate for manipulations of VOT, the speaker was instructed to pronounce the stops with a distinct voicing contrast. All readings were recorded through a cardioid condenser microphone (Brüel & Kjær) and intensified with a Brüel & Kjær 2209 amplifier. The signals were then digitized at 22,050 Hz with a 16 bits per sample representation. One production of each of the words par (word length ≈ 520 ms), tal (≈ 550 ms) and kal (≈ 560 ms) was chosen for waveform editing using Adobe Audition version 1.5 software (Adobe Audition version 1.5 software, Adobe ® Audition TH © 1992–2004 Adobe Systems Incorporated). Burst durations are known to vary with place of articulation, where velar stops appear to have longer burst durations than labials and dentals/alveolars (Jiang, Chen & Alwan, 2006; Klatt, 1975). Moreover, the average number of multiple voiceless stop bursts in elicited speech has been shown to increase with place of articulation, with velar stops generating the highest rate of burst multiplicity (Sönmez, Plauché, Shriberg & Franco, 2000). In order to ensure that no multiple release bursts were present in the edited speech stimuli, parts of the dental and velar overall burst durations were deleted from the waveform and thus adjusted to the 5-ms burst of the bilabial stop. The previously measured VOT intervals for the three original voiceless stop productions ([ph] ≈ +68 ms, [th] ≈ +56 ms, [kh] ≈ +83 ms) were then increased to +100 ms VOT by generating multiple copies of center proportions of the voicing lag interval. Thereafter, the aspiration was progressively shortened in steps of 5 ms. Voicing lead, on the other hand, was simulated by copying the prevoiced portion from the corresponding voiced stop and attaching it to the waveform in front of the voiceless burst transient. The original voicebar durations (/b/ ≈ –110 ms, /d/ ≈ –82 ms, /ɡ/ ≈ –132 ms) were first equalized to a maximum of −100 ms VOT and thereafter decreased in increments of 5 ms. For each voicing continuum, a set of 30 speech stimuli ranging from +90 to −60 ms VOT was chosen that covered the VOT systems of both Spanish and Swedish.8 All waveform modifications were checked perceptually by a group of native speakers of Swedish who confirmed the naturalness of the speech stimuli. In a forced-choice identification task, the stimuli were presented binaurally through headphones (Koss TX/PRO) at a comfortable listening level and were automatically randomized for each listener. The VOT continua were tested one by one, starting with the labial stop series followed by the dental and finally the velar stops. The perception trials were conducted within the first hour of testing and were separated by 10-min breaks during which the participants worked on other linguistic tasks. All stimuli were preceded by the carrier phrase Nu hör du…(“Now you will hear…”), which had been recorded by the same female speaker. Due to time restrictions, each item was presented only once. The task of the listeners was to decide for each stimulus whether they heard the voiced or voiceless member of the word pair and indicate their judgments by pressing a corresponding button on a keyboard. The test took about 15 min (3 × 5 min) to complete.

3 Results 3.1 Reliability and validity of the VOT stimuli Identification functions were calculated individually and then averaged for each of the three participant groups, that is, the early (AO 1–11) and late (AO 13–19) L2 learners as well as the Swedish Downloaded from las.sagepub.com at LAKEHEAD UNIV on March 18, 2015

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control group. As presented in Figure 1(a) to (c), group labeling functions were expressed in terms of the percentage of “voiced” (/b/, /d/, /ɡ/) responses in relation to VOT.9 The figures show that all three listener groups perceived the Swedish word-initial stops categorically on the basis of VOT. Detailed analyses of individual labeling functions further revealed that a majority of the participants actually exhibited clear categorical perception patterns. When allowing for one stimulus response to disrupt the otherwise clear-cut categorization functions (i.e., response curves that were characterized by an abrupt and sharp change from one phoneme category to the other at a specific point on the VOT scale), a total of 158 (i.e., 96%) out of the 165 response curves (55 listeners × 3 voicing continua) were identified as labeling functions that separated the VOT continuum clearly into two distinct stop categories. As can be seen in Table 1, all 15 participants in the native-speaker group perceived well-defined phoneme categories for the three stop continua. Regarding the group of early and late L2 learners, this observation was made for the dental and labial stop pairs. The early L2 learners exhibited the lowest ratings for the bilabial voicing continuum in that 26 participants (i.e., 87%) showed distinct categorization patterns. However, when all three voicing continua were combined, bimodal distributed labeling functions occurred with high rates for all three participant groups: 100% for the native speakers, 94% for the early L2 learners and 93% for the late L2 learners. Furthermore, the rate of perceived distinct phoneme categories was fairly high for all three voicing continua: 54 (i.e., 98%) of the 55 listeners exhibited discernible categorization patterns for the dental, 53 (i.e., 96%) for the velar and 51 (i.e., 93%) for the labial stop pair. A one-way ANOVA test showed that differences were not significant for participant group (F2,162 = 1.403, p = .25) or for stop continuum (F2,162 = 1.038, p = .36). Finally, examination of the individual response patterns revealed that 48 (i.e., 87%) of the 55 participants displayed discernible bimodal distributed labeling functions for all three VOT continua, whereas the remaining seven individuals did so for two of the three stop pairs. Thus, no listener was found with inconsistent categorization patterns for all three continua. However, as can be seen in Figure 1(a) to (c), the group identification functions did not appear as straightforward as might be predicted from the individual results reported in the previous section. The reason for this was that although most of the listeners showed distinct categorizations, they differed as to where on the VOT continuum an abrupt shift in phoneme category was perceived. Thus, intra-group variation affected both the shape of the group response curves and the rate of change between stop categories. Comparisons between the three voicing continua revealed considerably different mean labeling functions; regardless of listener group, category boundaries represented by the estimated 50% crossover points between phoneme categories10 indicated a shift toward longer, positive VOTs as the place of stop articulation moved further back (see Table 2). Moreover, the categorization curves became steeper and more monotonic as they moved from labial (Figure 1(a)) over dental (Figure 1(b)) to velar (Figure 1(c)) stop perception. As illustrated in Table 2, this alteration was mirrored in an overall decrease of standard deviations, indicating an increasing intra-group consistency within all three participant groups from labial to velar stop perception. It is worth noting that the native speakers exhibited lower standard deviations regardless of stop categorization when compared to the whole group of L2 speakers (see Table 2), but this difference was not statistically significant (Levene’s p > .05). Interestingly, whereas all listener groups showed high perceptual consistency with regard to the “voiceless” parts of the VOT continua (i.e., stops varying between +35 and +90 ms VOT were unanimously identified as voiceless; see Figure 1(a) to (c)), higher variability appeared on the “voiced” side of the response curves. In sum, the results showed that the three edited natural stop pairs were perceived categorically by all listener groups and reliability and validity of the test design were confirmed. With this methodological issue being clarified, we now turn to the central part of this study, that is, the investigation of AO effects on VOT.


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(a) Percentage /b/ responses

100 80 Nave speakers

60

Early L2 learners

40

Late L2 learners

20 0 -60

-20

0

20

40

60

80

Smulus VOT (ms) 100

Percentage /d/ responses

(b)

-40

80 Nave speakers 60

Early L2 learners

40

Late L2 learners

20 0 -60

-40

-20

0

20

40

60

80

Smulus VOT (ms)

Percentage /g/ responses

(c)

100 80 Nave speakers

60

Early L2 learners

40

Late L2 learners

20 0 -60

-40

-20

0

20

40

60

80

Smulus VOT (ms)

Figure 1. Mean identification functions of the (a) labial, (b) dental, and (c) velar stop contrasts.

3.2 AO effects on VOT With reference to Table 2, hypothesis 1 was supported by the finding that the native speakers of Swedish changed phoneme category at the highest mean VOTs, while the late L2 learners showed the lowest perceptual crossovers and thus deviated the most from native-speaker categorization. Also in accordance with the hypothesis, the early L2 learners perceived phoneme category boundaries at mean crossover values between those for the late learners and the native speakers. A one-way ANOVA revealed that the overall group differences were statistically significant for the bilabial (F2, 52 =


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Table 1. Number and percentage of distinct categorization functions according to stop continuum and groups of native speakers, early and late L2 learners, and all groups combined. Native speakers (n = 15) /p-b/ /t-d/ /k-ɡ/ ∑

Early L2 learners AO ≤ 11 years (n = 30)

(%) 15 15 15 45

Late L2 learners AO ≥ 13 years (n = 10)

All three groups combined (n = 55)

(%)

(%)

(%)

100.0 100.0 100.0 100.0

26 30 29 85

86.7 100.0 96.7 94.4

10 9 9 28

100.0 90.0 90.0 93.3

51 54 53 158

92.7 98.2 96.4 95.8

Table 2. Means (M) and standard deviations (SD) of the bilabial, dental, and velar category boundaries (in ms VOT) for native speakers, early and late learner groups and all L2 learners combined. Native speakers (n = 15)



All L2 learners combined (n = 40)

M (ms VOT)

SD

M (ms VOT)

SD

M (ms VOT)

SD

M (ms VOT)

SD

/p-b/ /t-d/ /k-ɡ/

+7.23 +15.34 +24.62

11.0 6.0 5.3

–2.93 +10.74 +20.17

12.2 9.6 6.1

–17.57 +1.28 +14.86

15.5 9.1 5.0

–5.43 +9.05 +19.97

14.3 10.1 7.2

Table 3. Multiple post hoc comparisons between stop continuum and native speakers, early learners and late learners. Fisher’s protected LSD

Tukey’s HSD

/p-b/

/t-d/

/k-ɡ/

/p-b/

/t-d/

/k-ɡ/

Native-early Native-late Early-late

.013 .000 .002

.102, ns .000 .005

.017 .000 .014

.034 .000 .006

.228, ns .001 .013

.045 .000 .036

11.807; p < .001), dental (F2, 52 = 7.847; p = .001) and velar voicing continuum (F2, 52 = 8.815; p = .001). Except for the non-significant difference between the native speakers and the early L2 learners in the case of the dental stop continuum, the Fisher’s protected LSD and Tukey’s HSD post hoc test confirmed that all other group differences reached statistical significance (see Table 3). In order to analyze the perception data in greater detail and thus to focus on hypothesis 2, the individually estimated category boundaries were plotted against the L2 learners’ AO as illustrated in Figure 2(a) to (c). As shown, negative correlations between category boundary placement and AO were present in all three voicing continua. The correlations were statistically significant for the bilabial (r = –.47, p < .01) and the dental stop continuum (r = –.34, p < .05), and very close to significant for velar stops (r = –.31, p = .054).11 Defining the lowest native-speaker category crossover value as the minimum criterion for nativelikeness (–13.0 ms for the bilabial, +5.4 ms for the dental, and +17.8


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Table 4. Number and percentages of nativelike category boundaries according to stop continuum and early vs. late learning. Nativelike category boundaries



Number

(%)

Number

(%)

All three stop continua Two of the stop continua /p-b/ & /t-d/ /t-d/ & /k-ɡ/ /p-b/ & /k-ɡ/ One of the stop continua /p-b/ /t-d/ /k-ɡ/ None of the stop continua

22

71.0

2

(20.0)

4 – –

12.9

2 – –

– 2 3 –

6.5 9.7

1 1 3 1

(20.0) 10.0 10.0 30.0 10.0

ms for the velar stop continuum; see dashed lines in Figure 2(a) to (c)), a majority of the L2 learners perceived phoneme boundaries within the range of native-speaker categorization regardless place of stop articulation.12 However, differences were found between the early and late L2 learners. As predicted by hypothesis 2a, only a small minority, that is, two individuals (AO 13 and 16) of the 10 late L2 learners changed phoneme category at VOT values within the range of native-speaker categorization for all three voicing continua (Table 4). Within the group of 31 early L2 learners this was the case for a majority (22, 71.0%) of the participants, thereby confirming hypothesis 2b. This group difference turned out to be statistically significant: χ2(1) = 8.09, p < .01. In the group of the early learners, four participants exhibited nativelike category crossovers for two of the three stop continua, whereas this pattern occurred with two of the late L2 learners (AO 13 and 15). Within both learner groups, five early (16.2%) and five late (50%) learners perceived a change in stop category within native-speaker range for one of the three voicing continua (Table 4). Finally, one participant (AO 14) did not exhibit nativelike perceptual boundaries for any of the three stop pairs. In order to investigate hypotheses 3a and 3b, the perception results were combined with the data derived from two earlier studies on the same individuals’ production of Swedish voiceless stops (Stölten, 2005; Stölten et al., in press). Table 5 displays to what extent the early and late L2 learners exhibited non-/nativelike production and/or perception.13 When compared to Table 4, the incidence of entire nativelikeness, that is, nativelike production and perception, had decreased by approximately 20% for both the early and the late L2 learners. However, the most striking finding was that whereas still as many as 16 (51.6%) of the early learners exhibited entirely nativelike behavior on both measures, this was the case for none of the late learners (see Table 5), and this difference was statistically significant (χ2(1) = 8.47, p < .001). In other words, hypotheses 3a and 3b were confirmed. Furthermore, the incidence of non-nativelikeness was more common within the group of the late L2 learners (40.0%, four individuals with AO 13, 14, 15 and 19) than it was within the early learner group (6.5%, two individuals with AO 1 and 8). This group difference was significant (χ2(1) = 6.83, p < .01). Table 5


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Table 5. Number and percentages of non-/nativelike stop production and/or perception for the group of early and late L2 learners. Non-nativelike production AND perception

Nativelike production OR perception

Nativelike production AND perception

Number

(%)

Number

(%)

Number

(%)

Early L2 learners AO ≤ 11 years (n = 31) Late L2 learners AO ≥ 13 years (n = 10)

2

6.5

13

42.0

16

51.6

4

40.0

6

60.0

–

Production data from Stölten, 2005 and Stölten et al., in press.

further shows that nativelike production or perception was higher for the late L2 learners (60.0%) than for the early learners (42.0%), but this differences was not significant (χ2(1) = 1.0, p = .32). Within this group, four late (i.e., 40.0%) and seven early learners (22.6%) reached native-speaker level for production, but not perception (see Appendix 1). As also displayed in Appendix 1, percentages of nativelikeness for stop perception only were more or less the same for the late and the early learners. Taken together, a total of 25 (i.e., 15 early and all 10 late learners) out of 41 L2 learners did not reach native-speaker level for production and perception. In this group, all AOs (except for AO 6) were represented (see Appendix 1). Comparisons between stop production and perception further revealed that nativelike results generally appeared to be slightly more frequent in production (see Appendix 1), but this finding was not statistically significant (χ2(1) = .47, p = .49).

4 Discussion 4.1 Reliability and validity of the VOT stimuli Before turning to our main hypotheses regarding AO effects on VOT, one methodological issue is shortly commented on as it has important implications for the analysis of categorical perception. For reasons of reliability, categorical perception is usually tested through the use of forced-choice identification and/or discrimination tasks in which the randomized stimuli are presented several times (Flege & Eefting, 1986; Flege et al., 1996; Williams, 1977a). Unfortunately, the present categorical perception task had to be restricted in order to fit into the time limits of the whole test session (a total of 4 h). Therefore, only identifications were performed, and each stimulus was presented only once. However, despite these methodological limitations, the study still managed to show that, when allowing for one stimulus response disrupting the otherwise clear-cut categorization patterns, nearly all (i.e., 96%) of the participants perceived a sharp crossover between stop categories. The study also showed that 48 out of the 55 participants (87%) exhibited clear categorization patterns for all three VOT continua, whereas the remaining seven (i.e., 13%) did so for one or two of the minimal word pairs. It is interesting to note that the identification functions without distinct categorizations were all found among L2 learners. However, as indicated by non-significant inter-group differences, the participants, irrespective of their status as native speaker or L2 learner of Swedish or of their AO of L2 acquisition, perceptually categorized edited natural


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(b) 80

Category boundary (ms VOT)


(a) 60 40 20 0 -20 -40 -60

0

2

4

6

8

80 60 40 20 0 -20 -40 -60

10 12 14 16 18 20

0

2

4

Age of onset (AO)

6

8 10 12 14 16 18 20

Age of onset (AO)


(c) 80 60 40 20 0 -20 -40 -60

0

2

4

6

8 10 12 14 16 18 20

Age of onset (AO)

Figure 2. The relationship between age of onset (AO) and estimated phoneme category boundaries (in ms VOT) for the (a) labial, (b) dental, and (c) velar stop continua. The unfilled diamonds at AO 0 represent average VOTs of the 15 native speakers; dashed line, shortest native-speaker category boundary placement.

Swedish word-initial stops on the basis of VOT. These findings not only support the view of VOT being a sufficient and salient acoustic cue for the voicing distinction in stop consonants, but the empirical evidence also suggests high validity and reliability of the test design, including both the preparation of edited natural VOT stimuli and the performance on the listening tasks. However, the present study described the group-labeling functions as becoming steeper and more monotonic when moving from labial over dental to velar stop series. This observation, accompanied by a decrease in standard deviations, thus suggest that the listener groups improved in perceptually classifying stop pairs into distinct phoneme categories with an increasing posterior place of articulation. Since the three test blocks have been presented in the same order to all participants, that is, first the labial followed by the dental and finally the velar stop continuum, it cannot be ruled out that the observed improvement was an effect of the participants experiencing an increase in familiarity with the experimental task over time. However, regardless of the stop


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continuum, the native speakers showed a slightly higher intra-group homogeneity (indicated by lower standard deviations) than the L2 speakers. Although not significant, the overall pattern of higher variability among the advanced L2 speakers is congruent with observations generally made in studies on different measures of L2 proficiencies (Abrahamsson, 2012; Hyltenstam & Abrahamsson, 2003b; Yeni-Komshian et al., 2000). One explanation is offered in Obler’s (1982) study of stop production and perception in Hebrew–English bilinguals. Among other findings, results from a stop identification task of synthetic VOT stimuli showed that the bilinguals experienced a wider range of perceptual uncertainty than did the monolingual Hebrew and the monolingual English group. In other words, there was a broader range along the VOT continuum within which the bilinguals might consider a stop as either voiced or voiceless, depending on language context, leading Obler (1982, p. 345) to the conclusion that “[t]he bilinguals, then, are maximizing the extremes of the two languages”.

4.2 AO effects on VOT It is a well-established fact that children are more successful language learners than adults in that they eventually end up with a higher and more complete mastery of their L2. Negative correlations between age of L2 acquisition and ultimate attainment have not only been seen as an indication of AO being a decisive variable affecting L2 learning; rather, findings like these have also been interpreted as caused by maturational constraints (Abrahamsson & Hyltenstam, 2009; DeKeyser, 2000; Hyltenstam & Abrahamsson, 2003a; Johnson & Newport, 1989; Long, 1990, 1993; Patkowski, 1980; Stölten et al., in press). However, in contrast to studies demonstrating AO effects among randomly selected L2 learners with varying degrees of L2 proficiency, the present study revealed that even among learners with a seemingly nativelike L2 proficiency, overall negative correlations between AO and ultimate attainment emerge when their L2 competence is scrutinized in detail, in this case in terms of L2 stop perception. Although the correlations were statistically significant only for the labial and the dental voicing continua, a negative effect of increasing AO on the categorical perception of all three Swedish stop continua was still confirmed by the data. That is, regardless of the stop continuum, significant group differences were found, in that the late learners exhibited the most deviant or non-Swedish-like perceptual boundaries. As predicted by hypothesis 1, the native speakers exhibited the highest and the late L2 learners the lowest average crossovers, with the early L2 learners in between. Also in previous investigations of the same group of advanced L2 learners’ production of Swedish voiceless stops, negative correlations between AO and VOT measures as well as differences between learner groups were observed (Stölten, 2005; Stölten et al., in press). Findings like these suggest that even among L2 learners who give the impression of having attained a nativelike L2 proficiency (including pronunciation), L2 stop production and perception tend to deviate from target-like values with increasing AOs. These findings are in agreement with data reported by Flege et al. (1996), investigating the categorical perception of early and late Spanish-English bilinguals in comparison to a group of English and Spanish monolingual speakers. The bilinguals who had learned English during childhood performed like the English monolinguals to a higher degree than did the Spanish L1 speakers who had learned English in adulthood. It was suggested that the early bilinguals were more effective in establishing new phonetic stop categories in a second language. Applying this explanation to the present study, it may be assumed that even among highly proficient speakers of L2 Swedish, learners with higher AOs experience greater difficulties in accurately establishing L2 phonetic categories. Comparisons between participant groups further revealed that eight out of the 10 late (i.e., AO 13–19) L2 speakers did not exhibit average category crossovers within the range of native-speaker categorization for all three stop continua. In fact, within this group one learner perceived all three


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voicing continua in a non-nativelike manner. It is interesting to note that this high rate of nonnativelikeness could be observed despite the fact that this study made use of a more inclusive definition of nativelikeness than usually preferred in group studies (cf. footnote 12). However, two participants were identified who, despite a late start to L2 acquisition, perceived all stop categories in a way comparable to native-speaker behavior. Thus, in accordance with hypothesis 2a, only a small minority of the late, seemingly nativelike L2 learners had attained an actual nativelike proficiency level concerning stop classification. Regarding the early L2 learners (AO ≤ 11), the results obtained were also in line with hypothesis 2b, in that most (i.e., 71%) of the early learners exhibited average boundary values within the range of native-speaker categorization for the three voicing continua. However, as many as nine early learners did not reach nativelike categorizations for all three stop pairs. In our previous studies (Stölten, 2005; Stölten et al., in press), a minority of late L2 learners also demonstrated entirely nativelike stop productions. However, in the same study, far from all early learners performed within the native-speaker range. Studies like these are thus in line with several works (Abrahamsson & Hyltenstam, 2008, 2009; Humes-Bartlo, 1989; Hyltenstam & Abrahamsson, 2003b; Ioup, 1989), demonstrating that the incidence of non-nativelikeness among L2 speakers with an early start to their L2 acquisition is higher than assumed by other researchers who describe the ultimate attainment in early L2 learners as mainly uniform and comparable to that of native controls (Johnson & Newport, 1989; Oyama, 1976). Regarding the late learners, the present perception data and the earlier production data each independently indicate that although nativelike command is rarely found, it still seems possible in some exceptional late learners. Thus, looking at these two measures separately, it might be tempting to interpret such exceptional learners as possible counterevidence to the CPH and to the existence of maturational constraints. However, as we have suggested earlier, in addition to detailed analyses, the combination of different measures of L2 linguistic proficiency is crucial, especially in the investigation of highly advanced L2 learners, in order to avoid overestimations of nativelikeness and “false positives” that may lead to such hasty conclusions about the existence/nonexistence of maturational and/or critical period effects (Abrahamsson & Hyltenstam, 2009). Based on the assumption that phonetics and phonology are most sensitive to critical period effects (Scovel, 1988), we would expect nativelike command to decline considerably, particularly in late L2 learners, when the perception data are combined with data from stop production (Stölten, 2005; Stölten et al., in press). In fact, complete nativelikeness dropped remarkably (approximately 20%) for both the early and the late learner group. Consider again that these observations were made by choosing a way of analysis in both the production and perception test that makes use of a more inclusive criterion for nativelikeness (cf. footnote 12). Whereas it was still a majority of the early learners who exhibited nativelike production and perception (hypothesis 3b), the most dramatic change was found for the late learners of whom none demonstrated entirely nativelike command (hypothesis 3a). Thus, if nativelike L2 stop production and perception is achieved, it is exclusively found among L2 learners with an early start to L2 acquisition (before the age of 12 years). However, if seen from the opposite point of view, only half of the early-learner group actually performed within the native-speaker range on production and perception. Among those participants who did not exhibit nativelike behavior on both measures, all AOs (except for AO 6) were represented. Moreover, seven early learners reached nativelike command only for production and six did so only for perception. Despite the finding that the incidence of non-nativelikeness regarding both production and perception was significantly lower in the early-learner group, there were still two individuals (AO 1 and 8) who did not reach native-speaker level on any of the tasks. Although these findings indicate that the domain of L2 phonetics/phonology is especially vulnerable to age-of-onset effects, the data obtained do not immediately seem to fit into the pattern predicted by the CPH. However, what the results do imply is that entirely nativelike command with


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respect to L2 stop production and perception is extremely rare in late learners and much more uncommon among early L2 learners than generally documented in SLA research. While the findings from the present study strengthen the position taken by those researchers who claim that absolute nativelikeness is, in principle, never attained by late learners (Bley-Vroman, 1989; Gregg, 1996; Hyltenstam & Abrahamsson, 2003a, 2003b; Long, 1990), they undermine claims by others that nativelikeness among late L2 learners is a quite common phenomenon and that such learners constitute reliable evidence against biological/maturational constraints on L2 acquisition (Bialystok, 1997; Birdsong, 1992, 2007; Birdsong & Molis, 2001; Bongaerts, 1999). As regards the different patterns obtained for VOT production and VOT perception, the present data could be evaluated with reference to Flege’s (1995, p. 238) proposition of perception leading production in the process of L2 learning in that “without accurate perceptual ‘targets’ to guide the sensorimotor learning of L2 sounds, production of the L2 sounds will be inaccurate”. According to Flege (1991), early bilinguals eventually discover perceptual dissimilarities, allowing them to produce L2 segments more authentically, whereas late learners continue to perceptually identify and thus produce L2 sounds as realizations of an already established L1 category. The present study revealed instances that might be interpreted in this way: entirely nativelike stop perception and production were apparent only among early L2 learners, allowing for the suggestion that new phonetic categories become well-established in some childhood learners, while this was not found in late L2 learners. However, among those who exhibited non-nativelike command on both measures, two individuals were found with AOs of one and eight years. In other words, despite an early start, L2 category formation identical to that of native speakers has obviously not taken place in these learners. Within the group of L2 learners who mastered production or perception, eight (six early and two late) performed at the native level on stop perception only. It may be concluded that in these individuals perception precedes production, that is, they seem to have established L2 phonetic categories perceptually but still have not mastered the production of the L2 stops in a nativelike manner. These findings are in accordance with previous research supporting the primacy status of perception over production (Borden, Gerber & Milsark, 1983; Flege, 1991, 1995; Flege & Eefting, 1987; Werker & Tees, 1984). However, even more L2 learners (seven early and four late) showed the reverse pattern, in that they produced L2 stops accurately but did not exhibit entirely nativelike perception. The pattern observed is in line with other studies (Caramazza et al., 1973; Mack, 1989; Sheldon & Strange, 1982; Zampini & Green, 2001) and allows for the suggestion that in some, even very successful, L2 learners, perceptual difficulties may persist after the mastery of authentic L2 stop production. According to Sheldon & Strange (1982, p. 257), such discrepancy between production and perception accuracy also implies that “perceptual mastery is not necessarily a causative factor in the acquisition of productive skills”. Findings like these may be interpreted with reference to Obler’s (1982) study on Hebrew–English bilinguals and their production and perception of word-initial stops in both languages. Although the bilinguals did not produce the stops identical to the monolingual Hebrew or the monolingual English group, the results still showed that their stop productions shifted towards the monolingual controls. Concerning the categorical perception task, the bilinguals perceived a change in stop category from voiced to voiceless at different VOTs when tested in English than when tested in Hebrew, but the difference between the languages was not as great as the difference observed between the two monolingual groups. From these findings, Obler (1982) suggested that production and perception are processed through different systems. Whereas L2 learners are able to separate L1 and L2 sounds due to a dual system for production, they seem to have a more unified system for perception resulting in perceived phoneme boundaries intermediate to those of the monolingual norms for the L1 and L2 (Obler, 1982). Other explanations are offered in Sheldon’s (1985) reply to Borden et al. (1983), who investigated the production and perception of /r/ and /l/ in L1 Korean adult learners of L2 English. As highlighted by Sheldon (1985), Borden et al.’s (1983) conclusion of perception preceding


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production was based on data gathered after training the /r-l/ contrast. However, prior to training, some individuals were found whose production scores were equal to or even better than the perception scores. From these findings Sheldon (1985) concludes that the fine-tuning of L2 perception seems to take more time than the fine-tuning of L2 production in language learners who have not received any training on the production and perception of the L2 contrast. After intense perceptual training of the /r-l/ contrast, however, high-level perception accuracy may well be attained. Sheldon (1985, p. 112) further addresses motivational factors as being responsible for more accurate production than perception. As the researcher says, “since mispronunciation is more noticeable, and more stigmatizing, the learner may become more interested in and able to improve it than to improve perception”. In other words, difficulties in L2 production and perception obviously influence social functioning differently. According to Strange (1995), differences in production and perception may be related to language experience, such that substitution patterns in production can be predicted by perceptual category assimilation in inexperienced L2 learners, whereas no such correlation between mastery of perception and production might exist in more experienced learners. The fact that the present study focused on highly advanced and probably very experienced L2 learners may thus explain why no consistent relational pattern between stop perception and production was found in the data.

5 Conclusions This study aimed at investigating the role of AO on L2 phonetic/phonological acquisition in L1 Spanish early and late highly proficient speakers of L2 Swedish who had been preselected on the criterion that they were perceived as native speakers of Swedish. The analysis revealed that category perception is affected by the L2 learners’ AO and the status as L1/L2 speakers of Swedish (Hypothesis 1). In other words, there was a general age effect on the L2 learners’ categoricalperception behavior mirrored by negative correlations (significant for the bilabial and the dental VOT continuum and close to significant for the velar stop pair) and also the overall differences between listener groups were significant for all three voicing continua. The conclusion is made that even among L2 learners, who give the appearance of having attained a verbal command of the target language indistinguishable from that of native speakers, the ability to establish nativelike L2 phonetic categories on the basis of VOT is limited and becomes increasingly constrained with higher AOs. A closer analysis of the group of early and late L2 acquirers revealed that only a small minority of the seemingly nativelike late learners achieved actual nativelike L2 proficiency in terms of category boundary placement (hypothesis 2a), while most, but far from all, early L2 speakers demonstrated an actual nativelike behavior when their perception of the L2 was analyzed in detail (hypothesis 2b). However, when the perception data were combined with production data derived from previous studies on the same group of advanced L2 learners (Stölten, 2005; Stölten et al., in press), the incidence of complete nativelikeness on both measures decreased considerably with no individuals with nativelike command found among the late learners. In view of these findings, the present study questions earlier, quite high estimates of the existence of nativelikeness in adult learners (Birdsong, 2007; Birdsong & Molis, 2001; Bongaerts, 1999; White & Genesee, 1996) and even more moderate estimates, like 5% of all adult L2 learners (Selinker, 1972). Instead, the present data point in the same direction as in our previous research, namely, that complete nativelikeness in late learners is, in principle, not found when L2 linguistic proficiency is scrutinized in detail (Abrahamsson & Hyltenstam, 2009; Hyltenstam & Abrahamsson, 2003a, 2003b). The results further indicate that an analysis of stop perception together with an analysis of production – of all three places of articulation – may be sufficient to discriminate late, otherwise native-sounding L2 learners from native speakers.14 Moreover, incidences of non-nativelikeness observed in very low


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AOs suggest that not even an early start to L2 acquisition warrants an entirely nativelike outcome. Findings like these also support the view of phonetic/phonological components being especially vulnerable to age-of-onset effects which, in turn, may be related to the early maturation of motor and auditory cortexes of the brain. If we agree with Long (1990, 1993) that the best (or at least one) way to reject the CPH or any theory of maturational constraints on (second) language acquisition would be to identify learners who attained a fully nativelike command of the L2 despite having begun learning after the alleged critical period, then the present data cannot be used to support a non-maturational position. Indeed, all our participants passed a verbal screening test, in which a majority of native listeners had perceived them as native speakers of Swedish, and in this respect they resemble the few late L2 speakers reported in the studies by Bongaerts and his colleagues (see Bongaerts, 1999) who passed as native speakers when reading a few sentences in the L2. However, and as we have argued before (see e.g., Abrahamsson & Hyltenstam, 2009), our position remains that the ultimate attainment of the small population of adult, seemingly nativelike learners who potentially constitute the evidence necessary to reject a long-lived and extremely important hypothesis within SLA and linguistics generally, needs to be scrutinized in detail and with great care before any conclusions can be drawn. This is what we, as linguists, are trained to do – to go beyond what is immediately obvious or easily perceivable, and to investigate language at a level where the skills of linguistically naïve listeners are no longer helpful, but where instead our more specialized and sophisticated linguistic/ phonetic tools, measurements and analyses must be employed. If such late learners turn out to have an ultimate attainment that clearly (albeit minimally) lies outside that of native control participants, the only reasonable conclusion – given that we still accept the research agenda suggested by Long (1990, 1993) – must be that the CPH or any theory of maturational constraints on language acquisition cannot be rejected, at least not on the basis of the present results. One may of course speculate whether the small differences we find between our near-native learners and native speakers proper are at all due to maturation, but maybe instead to other, nonmaturational factors not investigated in the present study such as, for example, quantity and quality of L2 input (MacKay et al., 2001; see also Long, 1990), language dominance (Antoniou, Tyler & Best, 2012; Hazan & Boulakia, 1993), bilingualism (Flege, 1995, 1999; Ortega, 2010; Pallier et al., 2003; Ventureyra, Pallier & Yoo, 2004), and amount of L1 and L2 use (Flege, Frieda & Nozawa, 1997; Flege, Munro & MacKay, 1995; Guion, Flege, & Loftin 2000; YeniKomshian et al., 2000). Clearly, more investigation is needed in order to clarify to what extent these factors may have influenced the results. Whether governed by biological/maturational factors or not, the study suggests that nativelike L2 ultimate attainment is constrained from a very early stage in language development, at least where the establishment of L2 phonetic categories is concerned. Acknowledgements We are grateful to Anna Ericsson for lending us her voice, Hassan Djamshidpey for help with the recordings, and Johan Roos for data collection. We would also like to thank the staff at the phonetics laboratory at Stockholm University, especially Olle Engstrand, for suggestions concerning stimuli preparation and Ulla Sundberg, Hartmut Traunmüller and Francisco Lacerda for discussions on data analysis and statistical treatment. Finally, thanks also to our colleagues at the Centre for Research on Bilingualism, Stockholm University, for their comments on the present paper and to Lamont Antieau for checking and correcting our English writing.

Funding This work was supported in part by a grant from the Bank of Sweden Tercentenary Foundation (grant number 1999-0383:01).


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Stölten et al. Notes

1. However, Helgason & Ringen (2008) reported that all their native Swedish speakers showed a strong tendency for realizing phonologically voiced stops in Swedish as prevoiced rather than as unaspirated stops when occurring in an initial position. Findings like these (cf. Fant, 1973) imply that the voicedvoiceless distinction in word-initial Swedish stops may be manifested by prevoicing vs. long-lag aspiration (rather than short-lag vs. long-lag), thereby increasing the contrast between the categories. 2. The newspapers were Metro, a free morning paper with 625,000–720,000 daily readers (the Stockholm edition), distributed throughout the Stockholm public transportation system (local trains, subway, buses, etc.) as well as in other public places (e.g., shopping malls), and Aftonbladet, the leading tabloid newspaper in Sweden, with 325,000 daily readers in the Stockholm area. 3. Age of onset (AO) refers to the L2 learners’ age at first exposure to Swedish, which in most cases coincides with their immigration to Sweden. 4. Since nativelike command was clearly biased toward lower AOs, there were several gaps in the upper half of the AO continuum (12–20+ years), with no nativelike L2 speakers with AO 12, 18 or 20+ years. Of the 17 late learners who passed as native speakers in the screening experiments, only 10 individuals who met all the background criteria could be selected. 5. In accordance with previous studies conducted within the project, it was decided to make use of the same AO groups in the present study. By following this procedure, appropriate generalizations and comparisons between studies can be accomplished. Thus, group comparisons were not conducted with the intension of making any statements on whether or not age 12 should be considered to be the critical age for phonetic detail. 6. The concept of ‘native speaker’ is used in the sense that the control subjects had (a) spoken only Swedish at home during childhood, (b) been educated through Swedish as the medium of instruction at school, (c) lived their whole life in a context in which Swedish had been the majority language, and (d) learned foreign languages only in school contexts. Thus, pure monolingualism was not required (Abrahamsson & Hyltenstam, 2009). 7. The Latin American dialects upon which Rosner et al. (2000) focused were Guatemalan, Venezuelan and Peruvian Spanish (data from Williams, 1977a, 1977b). 8. In view of the fact that all VOT manipulations were performed on the basis of aspirated voiceless stop productions, speech stimuli representing a zero VOT, i.e., when voicing starts simultaneously with the beginning of stop release, were not included in the continua. Thus, the shortest negative and positive VOTs were −5 ms and +5 ms (corresponding to the voiceless burst duration). However, pilot testing confirmed that this discontinuity along the VOT dimension did not result in a disturbance for listeners. 9. The inspection of individual identification functions revealed one listener with exceptionally deviant response patterns. Regarding the labial stop continuum, this L2 learner almost exclusively pressed the “voiceless” button (except for one stimulus) throughout the entire test, and in the case of the dental stop pairs, the listener shifted sporadically between the buttons, suggesting that the participant somehow struggled with the experimental task. Therefore, this early L2 learner (AO 5) was excluded from ANOVAs and correlations. 10. Category crossover values were calculated using the maximum likelihood estimation technique termed probit analysis (Finney, 1947). Probit analysis is a curve-fitting procedure that provides quantitative estimates of the 50% crossover points of binary response curves and has previously been used in categorical perception studies (Caramazza et al., 1973; Hazan & Boulakia, 1993). 11. It should be added that LOR was also analyzed since it is sometimes mentioned as a predictor of L2 proficiency (for an overview, see Piske, MacKay & Flege, 2001). However, statistical analyses did not show any impact of LOR on categorical perception (r = –.01, p = .95 for /p-b/; r = –.05, p = .77 for /t-d/; r = .15, p = .36 for /k-ɡ/). 12. As commented on by one reviewer, it is common practice in group studies to make use of a statistical measure that takes into account the variance within the control group (i.e., two or three standard deviations). Compared to this measure, the definition of nativelikeness based on the range of native-speaker perception used in the present study is obviously a more inclusive criterion. However, in accordance with


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Abrahamsson and Hyltenstam (2009, p. 277), the reason for choosing this more liberal definition was that “[a] nativelikeness criterion based on the NS [native-speaker] absolute range should be viewed as a stronger guarantee against Type I errors and “false negatives” (i.e., claims of nonnativelikeness for L2 users who in fact behave like at least some native speakers)”. In fact, results showed that despite this less stringent definition nativelike L2 command was rarely found among highly advanced L2 speakers (see Discussion). 13. The notion of nativelikeness used in Table 5 refers to those individuals who demonstrate entirely nativelike command with regard to all three places of articulation in stop production and perception (for individual results, see Appendix 1). 14. This is an example of what Abrahamsson and Hyltenstam (2003a, 2009) mean by referring to nonperceivable non-nativeness.

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Appendix 1 Table 6. Results for the 41 participants’ stop production and perception. Stop production

Stop perception

Total

ID

AO

Labial

Dental

Velar

Labial

Dental

Velar

Production

Perception

002 122 126 012 041 052 101 013 031 118 042 089 076 194 157 016 030 100 043 096 086 045 033 145 180 102 172 090 051 007 127 015 188 114 103 049 081 107 001 173 070

1 1 2 3 3 4 5 6 6 6 7 7 8 9 10 11 2 2 3 8 9 10 11 14 14 15 17 4 4 5 5 7 9 13 16 1 8 13 14 15 19

+ + + + (+) + + + + + + + + (+) + + + + + + + + + + + + + + + + + – – + – + – – + – –

+ + + + (+) + + + + + + + + + + + + + (+) + + + + + + + + + + + + + – + + + + + + – –

+ + + + (+) + + + + + + + + + + + + + + + + + + + + (+) + – – – – – – – – – – – – – –

+ + + + + + + + + + + + + + + + + – + – + – + – – + – + + + (+) + + + + – – + + – –

+ + + + + + + + + + + + + + + + + – + + + – + – – + – + + + (+) + + + + + – + – + –

+ + + + + + + + + + + + + + + + – + – – – + – + – – + + + + (+) + + + + – + – – – +

+ + + + (+) + + + + + + + + (+) + + + + (+) + + + + + + (+) + – – – – – – – – – – – – – –

+ + + + + + + + + + + + + + + + – – – – – – – – – – – + + + (+) + + + + – – – – – –

+ /– = within/below native-speaker range; (+) = missing data. In accordance with earlier studies on the same L2 learners, missing data (+) were treated as if they were within native-speaker range (cf. Abrahamsson & Hyltenstam, 2009; Stölten et al., in press); ID = participant identification number; AO = age of onset; Total = all three places of articulation within native-speaker range. (Production data from Stölten, 2005 and Stölten et al., in press).


Voice onset time and speakers' age: data from Hungarian.

Effects of Age, Sex and Syllable Number on Voice Onset Time: Evidence from Children's Voiceless Aspirated Stops.

Voice onset time in Swedish children with phonological impairment.

Perception and production of English stops by tonal and non-tonal Korean dialect speakers.

Speech perception and production by sequential bilingual children: a longitudinal study of voice onset time acquisition.

Age of learning affects the authenticity of voice-onset time (VOT) in stop consonants produced in a second language.

Effect of Voice Onset Type on Vocal Attack Time.

On pushing the voice-onset-time (vot) boundary about.

Age and time effects on implicit and explicit learning.

The irrationality of categorical perception.

Foreign language learning in French speakers is associated with rhythm perception, but not with melody perception.

Measurement of voice onset time in maxillectomy patients.

Resistance of voice onset time variability to intoxication.

Modeling phoneme perception. I: Categorical perception.

Getting the gist of multiple hues: metric and categorical effects on ensemble perception of hue.

Effects of intensity on the categorical perception of stop consonants and isolated second formant transitions.

Categorical Perception for Emotional Faces.

The Sound of Voice: Voice-Based Categorization of Speakers' Sexual Orientation within and across Languages.

Dissociating Attention Effects from Categorical Perception with ERP Functional Microstates.

Coupled attribute similarity learning on categorical data.

Incidental Learning of Gender Agreement in L2.

Relation between voice-onset time and vowel duration.

Effects of target typicality on categorical search.

Vocalic transitions in the perception of voiceless initial stops.