**2. Theories of bilingual speech acquisition**

#### **2.1. Speech learning model for phonetic systems in adult bilingual and L2 speakers**

Flege [1] offers the Speech Learning Model (SLM) to account for how individuals learn to produce the vowels and consonants of their L2. The aim of the model is to explain production limitations of experienced L2 learners focusing on more perceptual aspects of learners rather than their motoric constraints. Thus, SLM posits that difficulty to produce a certain L2 phoneme is attributed to a perceptual limitation to discern the sound, and not to production difficulty. A basic assumption in Flege's model is that phonetic elements of the L1 and L2 are related to each other at the level of allophones, and the language-specific aspects of speech sounds are formed in long-term representation called phonetic categories. Since perception plays an important role in the establishment of phonetic categories, if bilingual speakers are able to perceive phonetic differences between L1 and L2, then a new phonetic category can be established for the L2 sound. The likelihood of establishing a new category is further increased by the degree of dissimilarity between an L2 sound and it's closest related L2 sound [1, 3].

Flege further hypothesized that a single phonetic category is used to process similar L1 and L2 sounds due to equivalent classification. If sounds in the L1 and L2 are perceptually linked, then their perceived similarities may block category formation by what Flege refers to as the "mechanism of equivalence classification" [4]. For this process, phonetic category assimilation may occur. Flege and Eefting [5] examined the voice-onset-time (VOT) values of Spanish and English stop consonants as produced by Spanish-English bilinguals. They noted that Spanish-English bilinguals produced stop consonants in English with VOT values resembling those seen in Spanish, suggesting that phonetic category formation was blocked due to similarities in stop consonant production. Flege also predicts two circumstances in which bilingual productions may differ from that of monolinguals: a bilingual's category formation is deflected away from the L1 category in order to maintain phonetic contrast between categories sharing a common L1-L2 phonological space; as a result, representations produced by bilinguals are based on features different from monolinguals. Bohn and Flege [6] investigated the production of German and English vowels by adult German learners of English. They note that these bilingual speakers produced vowels in such a way that they were able to maintain contrasts within the individual's phonological space. During this process, phonetic category dissimilation may take place.

### **2.2. Linguistic system models in bilingual children**

phonetic categories develop and are organized across languages and how L1 and L2 systems interact with each other. Two further questions are raised in investigating phonetic categories in bilingual children. First, currently available studies examining phonetic development in bilingual children mainly focused on stop consonant production; limited evidence regarding whether phonetic characteristics of other categories (e.g., vowels) are similar to stops is available. Second, children who are exposed to two languages are either simultaneous or sequential bilingual. Whether phonetic categories of simultaneous bilingual and sequential bilingual children show similar characteristics is not well-examined either. In order to address these questions, first, I outline the currently dominant theoretical models of phonetic category formation in bilinguals. Then, a comprehensive review of existing literature of phonetic categories in bilingual children is provided. Finally, a proposed model of development of phonetic category formation is formulated. Directions for future research on phonetic category

**2.1. Speech learning model for phonetic systems in adult bilingual and L2 speakers**

Flege [1] offers the Speech Learning Model (SLM) to account for how individuals learn to produce the vowels and consonants of their L2. The aim of the model is to explain production limitations of experienced L2 learners focusing on more perceptual aspects of learners rather than their motoric constraints. Thus, SLM posits that difficulty to produce a certain L2 phoneme is attributed to a perceptual limitation to discern the sound, and not to production difficulty. A basic assumption in Flege's model is that phonetic elements of the L1 and L2 are related to each other at the level of allophones, and the language-specific aspects of speech sounds are formed in long-term representation called phonetic categories. Since perception plays an important role in the establishment of phonetic categories, if bilingual speakers are able to perceive phonetic differences between L1 and L2, then a new phonetic category can be established for the L2 sound. The likelihood of establishing a new category is further increased by the degree of dissimilarity between an L2 sound and it's closest related L2 sound [1, 3].

Flege further hypothesized that a single phonetic category is used to process similar L1 and L2 sounds due to equivalent classification. If sounds in the L1 and L2 are perceptually linked, then their perceived similarities may block category formation by what Flege refers to as the "mechanism of equivalence classification" [4]. For this process, phonetic category assimilation may occur. Flege and Eefting [5] examined the voice-onset-time (VOT) values of Spanish and English stop consonants as produced by Spanish-English bilinguals. They noted that Spanish-English bilinguals produced stop consonants in English with VOT values resembling those seen in Spanish, suggesting that phonetic category formation was blocked due to similarities in stop consonant production. Flege also predicts two circumstances in which bilingual productions may differ from that of monolinguals: a bilingual's category formation is deflected away from the L1 category in order to maintain phonetic contrast between categories sharing a common L1-L2 phonological space; as a result, representations produced by bilinguals are based on

formation in bilingual children are also suggested.

78 Multilingualism and Bilingualism

**2. Theories of bilingual speech acquisition**

While SLM was developed to explain adult bilingual and L2 learners, the linguistic system model is a theory regarding language acquisition in bilingual children. The focus of this theory is whether bilingual children develop one or two linguistic systems in the learning of their respective languages. The one-system model, known as the Unitary Language System (ULS), was originally hypothesized by Volterra and Taeschner [7] and the two-system model, also known as the Dual Language System (DLS) hypothesis, was posited by Genesee [8]. Under the ULS model, during early language development, bilingual children would take received input from both languages and combine the information into a single language system. As the language acquisition process continues, bilingual children develop more advanced linguistic skills, and undergo a differentiation process. It is during this process that these children distinguish between languages and achieve bilingual status. The DLS hypothesis stands as an alternative to the ULS hypothesis. The DLS hypothesis posits that children establish two separate linguistic systems from the beginning of the language acquisition process. Under this model, children receive dual language input and separate this information into two distinct language systems. These children do not undergo a period in which their linguistic systems are merged. They have separate linguistics systems from the onset of the acquisition processes; thus children are always considered to be bilinguals under the DLS hypothesis. Since the ULS and DLS hypotheses are significant for understanding bilingual children, more detailed information on each hypothesis is discussed.

#### *2.2.1. Unitary Language System hypothesis*

Under the ULS hypothesis, Volterra and Taeschner [7] claim that between infancy and the age of 3, children progress through three stages in order to become bilingual. The first stage of language acquisition in bilingual children shares many similarities with the language development of monolingual children. As children receive language input from both languages, they organize the information into one system. Volterra and Taeschner provide evidence for this by noting a lack of translation equivalents during the early stages of language development. Children receiving dual language input appear to avoid learning words in both languages that share the same meaning. Volterra and Taeschner developed three stages based on a study conducted with two Italian-German bilingual sisters and from data taken from Leopold [9]. Speech samples from these three subjects were taken between 1 year and 2 months (1.2) and 3.9. Their parents indicated that they used the one parent, one language policy and thus only spoke to the children in their native languages. Data obtained during their study seemed to suggest that children do in fact learn translation equivalents between languages. Volterra and Taeschner refuted this idea by suggesting that word meanings have contextual ties which influence the child's use of a word; thus they would not be considered a translation equivalent. During the second stage of language acquisition, the child is able to differentiate between the lexicons of each language but still continues to apply the same syntactic rules to both languages. Evidence for this stage of language development in bilingual children is seen in the presence of translation equivalents. The child's language now indicates that he or she has words in both languages with equivalent or corresponding meanings. The presence of translation equivalents indicates that the child is able to distinguish lexical items of one language from the other, and sort them by language. Despite the distinction made between lexical items of each language, the incorporation of grammatical components from one language into the other continues to suggest a unified system. During the third stage, language acquisition in the bilingual child is complete. Both the lexical and syntactic linguistic systems are differentiated. Volterra and Taeschner found that the children from the Italian-German study and the Leopold study distinguished and applied the appropriate syntactic rules of each respective language as early as 3.9. It is at this stage that children become bilingual.

**Languages Ages (years;** 

*Spanish English* Konefal and Fokes [22]

Deuchar and Clark [25]

Fabiano-Smith and Bunta [40]

Muru and Lee

*Korean English* Baker and Trofimovich [35]

Lee and Iverson [19]

Lee and Iverson [2]

Lee and Iverson [38]

*Japanese English* Johnson and Wilson [27]

*French English*

*Chinese English*

Yang and Fox [46]

*Arabic English*

[41]

**months)**

**Types of bilingual**

Yavas [36] 2nd graders Sequential 10 Group Voiceless

simultaneous

3 Sequential/

10, 16 and adults

**Number of bilingual participants** **Types of study**

A Psycholinguistic Perspective on Development of Phonetic Category Formation in Bilingual…

4, 7, 10 Sequential 3 Case study Stop No Unknown

8 Group Voiceless

Sequential 40 Group Vowel Yes Yes

5–6, 10 Sequential 32 Group Stop No Yes

5, 10 Simultaneous 30 Group Stop Yes Yes

5,10 Simultaneous 40 Group Vowel Yes Yes

3 Simultaneous 12 Group Stop/vowel Yes Yes/no

2.10, 4.8 Simultaneous 2 Case study Stop No, parents'

Lee [43] 3 Sequential 12 Group Stop Yes Yes

Watson [23] 5, 6, 8, 10 Simultaneous 20 Case study Stop No Yes Mack [24] 10 Simultaneous 1 Case study Stop Yes Yes

Yang et al. [45] 3.7–5.3 Sequential 1 Case study Vowel No Unknown

Khattab [26] 5, 7, 10 Simultaneous 3 Case study Stop Yes Yes/no

5 Sequential 15 Group Vowel Yes Yes

Harada [37] 6, 8, 10 Sequential 15 Group Voiceless

1.7, 1.11, 2.3 Simultaneous 1 Case study Stop No, parents'

**Sound category**

stop

stop

stops

**Monolingual control group**

http://dx.doi.org/10.5772/intechopen.72112

input speech

input speech

Yes Yes

Yes/no

No Yes

Yes No

**Two systems** 81

Yes

#### *2.2.2. Dual Language System hypothesis*

Paradis and Genesee [10] argue that bilingual children may acquire separate linguistic systems, and pose an additional option to the DLS by further categorizing these systems into autonomous (no interaction between the two language systems) and interdependent (interaction between the linguistic systems). If these linguistic systems are formed autonomously, then we would expect acquisition of each language by a bilingual to mimic that of a monolingual speaker of each respective language. However, if the two linguistics systems interact during language acquisition, we would expect to see three processes such as transfer, acceleration or delay in each language. Transfer occurs when bilinguals incorporate grammatical elements of one language into the grammar of another language. Acceleration happens when grammatical properties occur earlier in bilinguals than in typical developing monolinguals. Delay is the process in which the burden of simultaneous language acquisition causes delays in the grammatical development of bilinguals when compared to monolinguals.
