Defining bilingualism in early childhood

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Phonological development

It is by now well established that babies are born with a language-general sensitivity to speech sounds. Evidence from a vast amount of studies on monolingual infants shows that, throughout their rst year of life, infants’ perception becomes tuned to the sounds of their native language. That is, they gradually perfect their ability to recognise the phonemes (i.e., the sound categories) that are used to distinguish words, while they simultaneously lose sensitivity to contrasts that are unused in their language (for a review, see Maurer and Werker, 2014). Vowel contrasts begin to stabilize around 6 months of age, while consonants take slightly longer, with most contrasts being in place by the end of the rst year of life. Phonological development not only involves learning the sounds of one’s native language, but also the rules that dictate how these sounds can be combined to form syllables and words { i.e., its phonotactics { and how the pronunciation of certain sounds or words may change depending on the phonological context. In monolinguals, sensitivity to the phonotactic rules of their native language emerges around the age of 9 months (Jusczyk, Luce, & Charles-Luce, 1994). Other phonological rules may take longer to acquire, such as the French liaison, which may take years to master (Chevrot, Dugua, & Fayol, 2009). Research on phonological development in children growing up with two languages is much more scarce, with only a handful of studies exploring phonological perception, principally in infants and toddlers, and another handful focusing on phonological production, mainly in preschoolers. In the following subsections we will review both lines of research.

Phonological perception in bilinguals

Evidence on phonological perception suggests that, like monolinguals, bilingual infants tune to the phonemic categories of their native languages during the rst year of life (for a review, see Werker, 2012 and Hammer et al., 2014). However, their developmental trajectories are not yet well understood, with studies showing di erent patterns depending on the experimental method, the phonological contrast, and the language pair. Such is the case of the previously discussed vowel contrast /e/-/E/. Bosch and Sebastian-Galles (2003b) rst investigated the perception of this contrast in di erent age groups of Spanish-Catalan bilingual infants, as well as in Spanish and Catalan monolinguals, using a familiarization-preference procedure. Their results con rmed, rst of all, that at 4 months all three groups of infants could discriminate this contrast, regardless of the language they were exposed to. Furthermore, by the time they were 8 months old, both monolingual groups had learnt their native categories, that is, Catalan infants maintained their discrimination of /e/-/E/, while Spanish monolinguals (for whom this contrast is not phonemic) had already lost sensitivity to it. Interestingly, bilingual infants who had been exposed to both languages since birth (and hence to both phonological categories) failed at discriminating these vowels at 8 months, but succeeded again at 12-months-old, suggesting that their phonological development follows a \U-shaped » pattern: they go from language-general perception, to a temporary collapse of the two categories, and nally a recovery of the phonemic distinction. A similar U-shaped acquisition has been attested in Spanish-Catalan bilinguals for the consonant contrast /s/-/z/, which also exists in Catalan but not in Spanish (Bosch & Sebastian-Galles, 2003a), and the /o/-/u/ vowel contrast, which exists in both languages (Bosch & Sebastian-Galles, 2005), but not in a more distant vowel contrast, /e/-/u/ (Bosch & Sebastian-Galles, 2005).
A subsequent study revisited the /e/-/E/ contrast with a di erent experimental paradigm (an an-ticipatory looking task), and found evidence that Spanish-Catalan bilingual infants can discriminate this contrast at 8-months-old (Albareda-Castellot, Pons, & Sebastian-Galles, 2011), suggesting that speci c task demands may in uence their ability to attend to this contrast. Moreover, Sundara and Scutellaro (2011) studied Spanish-English bilinguals on this same contrast, which also exists in English. They showed, even using a similar experimental paradigm to the one used by Bosch and Sebastian-Galles (2003b), that Spanish-English bilingual 8-month-olds had retained this distinction. The authors suggested that a possible explanation for Spanish-English bilinguals’ success (while Spanish-Catalan infants had failed at this age) is that Spanish and English are easier to discriminate due to the fact that they belong to two di erent rhythmic classes, as previously discussed in Section 1.2. Thus, Spanish-English infants’ ability to sort out their input by language may have facilitated the separation of these phonemic categories.
Other contrasts seem to be less problematic. For instance, French-English bilingual infants retain their discrimination of a dental vs alveolar contrast distinguishing the French and English realisations of /d/, while monolingual French and English infants lose it by 10 months of age (Sundara, Polka, & Molnar, 2008). In another study with French-English bilinguals, Burns, Yoshida, Hill, and Werker (2007) explored infants’ ability to discriminate /b/ from /p/. An interesting property of this contrast is that the voice onset time (VOT) boundary separating these two categories is di erent in French and English, thus leaving an intermediate range of VOT values in which French adults usually perceive a /p/, while English adults hear a /b/. Three age groups (6-8 months, 10-12 months and 14-20 months) of French-English bilingual and English monolingual infants were tested on their discrimination of this intermediate category against samples from the two extremes of the /b/-/p/ spectrum. Their results showed that, while both monolinguals and bilinguals behaved similarly in the youngest age group, by 10 to 12-months-old each group had tuned to their native language(s). That is, monolingual English infants treated the intermediate category as a /b/ (i.e., they discriminated it only against /p/), while bilinguals discriminated both contrasts, thus showing phonological knowledge of both of their languages. Bilinguals’ success in these contrasts may have been due to the fact that their target languages (French and English) belong to di erent rhythmic classes, but more evidence of similar contrasts across di erent language pairs would be needed before we could understand the role of language distance on phonological development.
Some recent studies have begun to explore the e ects of the speci c language pair in a more systematic way, but are still scarce (Havy, Bouchon, & Nazzi, 2016; Liu & Kager, 2015). For instance, Liu and Kager (2015) explored the perception of this same /b/-/p/ intermediate contrast in bilingual infants learning Dutch and an additional language which either shares the same VOT values as Dutch for these two consonants (French, Spanish), or has a di erent VOT boundary (Chinese, English, German). Their results showed that the perception of these contrasts depended on the language pair (with those sharing similar realisations of these consonants showing a stable contrast discrimination at 11-months-old, although with noisy results at 8-9 months), as well as on their language dominance, a ecting principally those infants learning a language pair with di ering realisations of these sounds. More studies of this kind are needed in order to form a full picture of the developmental trajectory of phonemic categories in young bilinguals.
Beyond the development of sound categories, and past the rst two years of life, not much is known about young bilinguals’ phonological perception. Indeed, a language’s phonological system is not limited to its composing sounds. Languages may di er in the possible combinations of phonemes within a syllable – that is, their phonotactics – as well as on phonological rules that may alter the surface form of a word when pronounced within a sentence. Sebastian-Galles and Bosch (2002) explored Spanish-Catalan bilinguals’ perception of Catalan phonotactic rules. Ten-month-old bilinguals, as well as Spanish and Catalan monolinguals, were tested on their discrimination of legal versus illegal consonant clusters. Their results showed that Catalan-dominant bilinguals, as Catalan monolinguals, succeeded in discriminating these two types of clusters, while Spanish-dominant bilinguals and Spanish monolinguals failed. To the best of our knowledge, no other study has explored the perception of phonological rules in young bilinguals, leaving a big gap in our understanding of their phonological development.

Phonological production in bilinguals

Many studies on phonological production have focused on preschoolers (Fabiano-Smith & Barlow, 2010; Fabiano-Smith & Goldstein, 2010; Fabiano-Smith, Oglivie, Maiefski, & Schertz, 2015; Goldstein, Fabiano, & Washington, 2005; Goldstein & Washington, 2001; MacLeod & Fabiano-Smith, 2015; Munro, Ball, Muller, Duckworth, & Lyddy, 2005; Nicoladis & Paradis, 2011), while some others have looked at infants and toddlers, although most often through case studies (Deuchar & Clark, 1996; C. E. Johnson & Lancaster, 1998; Kehoe, 2002; Kehoe, Lleo, & Rakow, 2004; Maneva & Genesee, 2002; Paradis, 2001; Schnitzer & Krasinski, 1994, 1996). In a study of a French-English bilingual infant, Maneva and Genesee (2002) found evidence of some language-speci c phonological features in the infant’s babbling, depending on the language of the parent that the child was interacting with. For instance, the child produced more stop + vowel syllables when interacting with his English-speaking parent, and more approximant + vowel syllables when interacting with his French-speaking parent. This suggests that language di erentiation may take place from the onset of speech production.
Once children start producing words, most speech production studies have focused on the development of phonemic categories. Some have conducted longitudinal studies on a small number of children, analysing their natural productions (e.g., C. E. Johnson and Lancaster, 1998; Kehoe, 2002; Kehoe et al., 2004; Schnitzer and Krasinski, 1994, 1996). For instance, Schnitzer and Krasinski (1994, 1996) analysed the productions of two children acquiring Spanish and English, and found evidence of a merged consonant system in one of them, but not in the other. Others have used elicited speech or picture naming tasks to collect production data in speci c age groups (e.g., Fabiano-Smith and Goldstein, 2010; Fabiano-Smith et al., 2015; Goldstein et al., 2005; Goldstein and Washington, 2001). For example, Goldstein and Washington (2001) used a picture naming task to assess the phonological production of Spanish-English bilingual preschoolers. They observed that half of the children had a complete consonant repertoire in at least one of their two languages, and all of them produced the full vowel repertoire of both languages. Furthermore, they found some evidence (although rare) of cross-linguistic e ects re ected in consonant substitutions. In a similar study with 3 to 4-year-old Spanish-English bilinguals, Fabiano-Smith and Goldstein (2010) found that their phonological accuracy in word production was higher in sounds that were shared by both languages. Overall, the available evidence from production studies suggests that bilingual children begin to develop phonological categories of both of their languages from early on, yet their dual phonological systems may sometimes interact.
Furthermore, a couple of studies have analysed other phonological properties of young bilinguals’ productions. Paradis (2001) studied French-English 2-year-olds patterns of truncation during a nonce-word repetition task. Truncations (e.g., nana for \banana ») are typical of toddlers’ productions and are known to be in uenced by language-speci c word-prosodic properties (Allen & Hawkins, 1980). While the truncation patterns of the bilingual toddlers in each of their languages were found to generally match those of the corresponding monolingual French and English toddlers, there was also some evidence of cross-linguistic transfer. Thus, their phonological systems appeared to be di erentiated, yet not fully independent. In another study using a picture naming task, Nicoladis and Paradis (2011) explored French-English bilingual children’s production of liaison, a complex phonological rule in French that causes word- nal silent consonants to be pronounced before a vowel-initial word (e.g., petit [p@ti] \small » is pronounced [p@tit] in petit ours \small bear »). They found that, in general, young children’s French vocabulary | but not their age -{ correlated positively with their production of liaison. Interestingly, when matched by vocabulary, bilinguals applied liaison less often than their monolingual peers’, but only in low-frequency collocation frames. However, the sample size of this matched comparison was very small (6 monolinguals and 6 bilinguals between the ages of 3 and 5), making it di cult to draw conclusions.
In summary, while current evidence from perception and production studies suggests that young bilinguals start acquiring phonological properties of both of their languages from early on, their de-velopment does not seem to be equivalent to that of two monolinguals in one. Complex patterns of phonological acquisition emerge depending on the speci c language pairs and phonological properties that BFL learners set out to discover, sometimes showing a short-lived delay in the development of a phonological feature of one of the two languages, and sometimes showing cross-linguistic in uences. However, the number of studies conducted so far remains very limited, leaving many open questions regarding their phonological development.

Lexical development

The acquisition of a lexicon involves multiple cognitive abilities that develop in early childhood. In order to learn words, infants must learn to segment sound sequences from the continuous acoustic signal, store a mental representation of their phonological form, assign a meaning to them, and even-tually be able to produce them orally. Monolingual infants achieve these feats during the rst two years of life, although lexical development is a process that continues throughout the entire childhood. Evidence from behavioral experiments shows that monolingual infants begin to use transitional prob-abilities and prosodic cues to segment new words from spoken utterances between the ages of 6 and 9 months (Curtin, Mintz, & Christiansen, 2005; E. K. Johnson & Jusczyk, 2001; Sa ran, Aslin, & Newport, 1996; Thiessen & Sa ran, 2003, 2007), and that they are able to recognise the sound of familiar words without a visual referent at 11 months (Halle & de Boysson-Bardies, 1994; Swingley, 2005; Vihman, Nakai, DePaolis, & Halle, 2004; Vihman, Thierry, Lum, Keren-Portnoy, & Martin, 2007). Furthermore, the development of word-referent mappings for very frequent words may start to develop as early as 6 months (Bergelson & Swingley, 2012; Tinco & Jusczyk, 1999, 2012). Towards the second year of life, once infants have acquired their rst few lexical items, they can use a variety of learning strategies to discover the meaning of new words. For example, young monolinguals have a mutual exclusivity bias: upon hearing a new word, children are likely to assume that the new la-bel cannot refer to an object or concept for which they already know the word, and therefore must denote a new referent, be it a new object, or a part or property of a known object (Halberda, 2003; Markman, Wasow, & Hansen, 2003; Mather & Plunkett, 2011; Merriman, Bowman, & MacWhinney, 1989). Speech production also emerges in the rst year of life. Infants typically produce canonical babbling (i.e., the repetition of syllables composed of a consonant and a vowel, such as \dadada ») by the age of 6 to 7 months (Eilers et al., 1993), say their rst words around their rst birthday, and by the time they are 18-months-old they can produce, on average, some 50 words (Fenson et al., 1994).

Bilinguals’ milestones and learning strategies

As in many other areas of research on childhood bilingualism, data on lexical acquisition in young bilinguals is limited. Nevertheless, current evidence suggests that BFL learners achieve several of the previously mentioned milestones at the same age as their monolingual peers. Examples of this are the onset of canonical babbling (Oller, Eilers, Urbano, & Cobo-Lewis, 1997), the recognition of familiar word forms at 11 months (Vihman et al., 2007), and the age of production of their rst words (Petitto et al., 2001).
Bilingual children may, however, di er from young monolinguals in many aspects of their lexical development. One of them is their use of word learning strategies. For instance, using a visual disambiguation task, Byers-Heinlein and Werker (2009) studied monolingual, bilingual and trilingual 17-month-olds’ use of mutual exclusivity. Infants were presented with two pictures on a screen, one corresponding to a familiar object and the other to a novel object, while hearing a phrase prompting them to look at one of them. Their results showed that monolinguals and multilinguals di ered in their use of mutual exclusivity: while monolinguals looked signi cantly longer at the novel object upon hearing a novel word, bilinguals did so only marginally, and trilinguals did not give evidence of using mutual exclusivity at all. Similar results, showing a reduction or absence of a mutual exclusivity bias in bilinguals, have been found in other experiments with infants and pre-schoolers (Davidson & Tell, 2005; Houston-Price, Caloghiris, & Raviglione, 2010; Kandhadai, Hall, & Werker, 2017). The fact that bilinguals rely less on mutual exclusivity to guess new word meanings has been argued to stem from their language experience: unlike monolinguals, bilinguals are likely to hear more than one label per object – i.e., one label per language – and may thus not develop a strong constraint on the number of words that can map to the same concept (Byers-Heinlein & Werker, 2009; Houston-Price et al., 2010).
Other studies have explored the interactions between bilinguals’ phonological and lexical development. For instance, using a cross-modal word learning paradigm, Fennell, Byers-Heinlein, and Werker (2007) observed that 17-month-old bilingual infants exposed to English and a second language failed at learning a new minimal pair, bih – dih, di ering only in the /b/-/d/ contrast (which is discriminated by bilingual infants at 14-months-old, Fennell, 2005), while monolingual English infants succeeded at this age. Bilinguals eventually achieved this feat at the age of 20 months, indicating a delay in their ability to use phonetic detail for word learning. The authors attributed this delay to the increased demands of language acquisition in bilingual settings. However, subsequent studies testing other consonant contrasts and language pairs show varied results. In some experiments with French-English bilinguals, no evidence was found of a bilingual delay in minimal pair learning using the /b/-/g/ contrast (Mattock, Polka, Rvachew, & Krehm, 2010) or the /k/-/g/ contrast (Fennell & Byers-Heinlein, 2014) when the speech samples were produced by a bilingual speaker. Havy et al. (2016) explored the e ect of cross-linguistic similarities on bilinguals’ ability to learn minimal pairs. They tested 16-month-old bilinguals acquiring French plus a Romance language with similar realisations of their stop consonants /p,t,k,b,d,g/ (Spanish, Italian and Portuguese), or French plus a Germanic language (English, German) which di er in the realisation of these stops. The task consisted in learning a minimal pair of words with a 1-feature contrast either in voicing (e.g., /p/-/b/) or in place of articulation (e.g., /p/-/t/). Their results showed that infants learning a close language pair (e.g., French-Spanish) succeeded in this word learning task, while infants learning French plus a Germanic language failed to learn the word-object pairings. This suggests that similarities between properties of the two languages might play a role in lexical acquisition, but given the contradictory results between this and previous studies, more evidence would be needed before a conclusion can be drawn.
A related line of research has begun to explore the role of bilingual phonological acquisition on word recognition. Ramon-Casas, Swingley, Sebastian-Galles, and Bosch (2009) studied Spanish, Catalan and Spanish-Catalan 18- to 25-month-old infants’ ability to detect a vowel mispronunciation in familiar nouns. Infants were tested on a preferential looking task, in which the name of one of two objects presented on screen was either pronounced correctly or with an /e/-/E/ vowel change. Consistent with their native phonological systems, monolingual Catalan infants detected the mispronunciations in Catalan nouns (producing shorter looking times towards the target image), while Spanish monolinguals did not notice the vowel change in Spanish nouns. Bilingual toddlers were tested only on Catalan words. Despite their familiarity with this vowel contrast (which, as we mentioned in Section 1.3.1, they can detect by the age of 12 months), they did not detect the mispronunciations. When tested on other more salient vowel distinctions present also in Spanish (e.g., /e/-/i/ or /e/-/a/), Spanish monolinguals and bilinguals both succeeded in this task. An additional experiment testing 3-year-old bilinguals indicated that Catalan-dominant preschoolers did detect the /e/-/E/ change. However, in these series of experiments, the Catalan words on which bilinguals were tested all had cognates in Spanish, e.g., the word \bee » is pronounced [@’BE«@] in Catalan and [a’Bexa] in Spanish. In a follow-up experiment, Ramon-Casas and Bosch (2010) tested 2-year-old bilinguals on a similar task but using only Catalan words that have no cognates in Spanish. In this case, bilingual toddlers succeeded in detecting the /e/-/E/ mispronunciation. These ndings suggest that cognates may have underspeci ed phonological representations, yet this does not impede the acquisition of phonological categories.

Vocabulary size and composition in bilinguals

Another aspect of lexical development in bilingual children that has attracted much attention is the size and growth rate of their dual vocabulary. Although bilinguals may reach some of the rst lexical milestones at the same time as their monolingual peers, their two vocabularies may not necessarily develop at the same speed. In order to assess young children’s vocabulary, several methods have been used, depending on the age of the child. A widely used method in infants and toddlers is the use of vocabulary lists lled out by parents, most typically the MacArthur-Bates Communicative Development Inventories (aka CDI, Fenson et al., 1994), which are available in many languages. In older toddlers and preschoolers, some studies have used alternative methods that do not depend on parental report, such as the Computerized Comprehension Task designed by Friend and Keplinger (Friend & Keplinger, 2003; Poulin-Dubois, Bialystok, Blaye, Polonia, & Yott, 2013) or the Peabody Picture Vocabulary Test (aka PPVT, Dunn, Dunn, Lenhard, Lenhard, and Suggate, 2015).
In one of the rst studies of its kind, Pearson, Fernandez, and Oller (1993) conducted a semi-longitudinal study with 25 Spanish-English bilingual infants between the ages of 8 and 30 months, and compared them with a group of monolinguals. Using the Spanish and English CDIs, they com-puted four vocabulary measures: English vocabulary, Spanish vocabulary, Total vocabulary (TV, i.e., English plus Spanish vocabularies combined), and Total conceptual vocabulary (TCV, i.e., Total vocabulary minus translation equivalents, resulting in the number of concepts for which they know at least one label). Although statistical analyses were limited due to the small sample size in each age group, their results showed that, when considering their TV or TCV, bilinguals’ lexicons were comparable to that of monolinguals. However, the vocabulary scores in each of their two languages, when analysed individually, were sometimes smaller than that of their monolingual peers. Further-more, they compared the evolution in time of bilinguals and monolinguals’ production scores, from the age of 16- to 26-months-old, showing similar growth rates for Total vocabulary across both groups of infants. Since then, a number of studies have investigated bilinguals’ vocabulary scores in early childhood, nding, in general, that bilinguals’ vocabulary in one or both of their languages may be smaller than that of their monolingual peers, while their Total vocabulary scores are often comparable or even larger (Bialystok, Luk, Peets, & Yang, 2010; De Houwer, Bornstein, & Putnick, 2014; Ho et al., 2012; Junker & Stockman, 2002; Marchman, Fernald, & Hurtado, 2010; Poulin-Dubois et al., 2013). Furthermore, infants’ vocabulary in each of their two languages may develop at di erent speeds (David & Wei, 2008; Pearson & Fernandez, 1994). Research has aimed at understanding which factors in uence the development of each language’s vocabulary, suggesting strong e ects of relative amount of exposure, among many other input and environmental factors (e.g., Ho and Core, 2013; Pearson, Fernandez, Lewedeg, and Oller, 1997; Place and Ho , 2011), which we will discuss in Chapter 3.
A related line of research has focused on the content of the bilinguals’ dual lexicon. While individual di erences exist between children, and particularly in the growth rate of each of their two languages, bilinguals have been reported to acquire semantic categories in both languages in the same order as they typically appear in monolinguals, e.g., social words emerge before nouns, and these before verbs (Conboy & Thal, 2006; David & Wei, 2008). Furthermore, several studies explored the existence of translation equivalents (TEs) in young bilinguals’ lexicons (Bosch & Ramon-Casas, 2014; De Houwer, Bornstein, & De Coster, 2006; Legacy et al., 2017; Pearson, Fernandez, & Oller, 1995; Poulin-Dubois et al., 2013). Translation equivalents are words that can be considered to represent the same meaning across two languages, such as the English word tree and the French word arbre. Research shows that soon after they produce their rst words, bilinguals start acquiring translation equivalents (De Houwer et al., 2006; Pearson et al., 1995), thus indicating that they can accept and learn more than one label for the same object. The existence of TEs in the bilinguals’ lexicon has been interpreted as evidence that young bilinguals are able to di erentiate, at least to some extent, their two languages (Genesee & Nicoladis, 2006; Patterson & Pearson, 2004).
In summary, lexical acquisition in young bilinguals seems to follow, as a whole, a similar timeline as in monolinguals. However, due to the fact that bilinguals encounter each language in di erent proportions and contexts, each respective vocabulary may develop at its own rhythm. Furthermore, bilinguals’ lexical development shows signs of early language di erentiation, but subtleties exist in the way their dual phonological and lexical systems interact. When forming their rst phonological representations of words, similarities and partial overlaps between linguistic properties of the two languages may pose a particular challenge for the bilingual learner. Once more, the scarcity of data available so far limits the possibility to draw conclusions, but current evidence outlines the importance of investigating the interaction between bilinguals’ phonological and lexical development.

Table of contents :

1 Introduction 
1.1 Defining bilingualism in early childhood
1.1.1 The challenge of bilingual first language acquisition
1.2 Early language discrimination
1.3 Phonological development
1.3.1 Phonological perception in bilinguals
1.3.2 Phonological production in bilinguals
1.4 Lexical development
1.4.1 Bilinguals’ milestones and learning strategies
1.4.2 Vocabulary size and composition in bilinguals
1.5 Thesis overview
2 Modelling early language discrimination 
2.1 Introduction
2.2 I-vector pipeline
2.3 Computational experiments
2.3.1 Experiment 1: A proof of concept (Article 1)
2.3.2 Experiment 2: Generalizing to other language pairs
2.3.3 Experiment 3: Language discrimination with filtered speech
2.3.4 Experiment 4: Language discrimination across speakers
2.3.5 Experiment 5: The role of the background model
2.3.6 Experiment 6: The impact of a bilingual background (Article 2)
2.4 General discussion and future directions
2.A Appendix A: Pipeline details
2.A.1 Feature extraction
2.A.2 Background Model
2.A.3 Total Variability space
2.A.4 I-vector extraction
2.B Appendix B: Expectation-Maximization algorithm for GMMs
2.C Appendix C: P-values from simulations in Exp. 4
3 Dual language input and its impact on lexical development 
3.1 Introduction
3.2 Methods
3.2.1 Subjects
3.2.2 Materials and procedures
3.2.3 Coding and pre-processing
3.3 Results and discussion
3.3.1 Language Environment Questionnaire
3.3.2 Language Diaries
3.3.3 Vocabulary scores
3.3.4 Eects of Language Exposure on Vocabulary Scores
3.4 Discussion
3.A Appendix A: Language Diary sample page
3.B Appendix B: Language Environment Questionnaire
3.C Appendix C: Comparison of weighting options
3.D Appendix D: Correlation results for indirect input measures
4 Perception of language-specific phonological rules 
4.1 Article
4.2 Additional studies
4.2.1 Pilot experiment 1
4.2.2 Pilot experiment 2
4.2.3 Pilot experiment 3
4.2.4 Pilot experiment 4
4.2.5 Pilot experiment 5
4.2.6 General discussion
5 General discussion 
5.1 Language separation in the first year of life
5.2 Separation of phonological rules
5.3 Conclusion
Bibliography 

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