**1. Introduction**

Over the last 30 years, the organization of phonetic systems in bilingual speakers has been extensively examined (see [1] for more information). Phonetic category formation refers to the processes by which bilingual or second language leaners come to distinguish phonetic details of share phonemes in each language [2]. Most studies dealt with adult speakers who learn a second language (L2) after they have fully acquired their first language (L1). These studies mainly focused on how the influence of one language on the other depends on the learner's age of exposure to the L2. The current chapter deals with phonetic characteristics of sound produced by bilingual children. Bilingual children are different from adult bilinguals or L2 learners in that their language systems of two languages continue to develop during developmental processes. Thus, it is of interest to examine bilingual children to understand how

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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 formation in bilingual children are also suggested.

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.

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

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

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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

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

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

detailed information on each hypothesis is discussed.

*2.2.1. Unitary Language System hypothesis*
