Acquisition scenarios and status of the languages
The acquisition scenario of bilingual deaf learners does not easily fit into traditional typologies of language acquisition situations. In bilingualism research on hearing learners, age of exposure is commonly used as a criterion to distinguish three different types of acquisition situations: bilingual first language acquisition (exposure to two languages from birth), child second language acquisition (exposure to a second language [L2] after age 3), and adult second language acquisition (exposure to an L2 in adolescence/adulthood) (Paradis et al. 2011). Research on language development in these different scenarios has provided further insights into the impact of age and previously available language knowledge on the evolution of learner systems. Implicit to the differentiation of acquisition scenarios is the attribution of the status of first language (L1) to the language(s) acquired from birth, whereby full access to the language(s) is assumed.
L1 and L2 labels in sign bilingualism. In research on bilingual deaf learners accessibility is commonly considered the defining criterion of the language assigned the L1 label, that is, sign language (Grosjean 2008; Leuninger 2000; Pla- za-Pust 2008b; among others). For the majority of deaf children born to hearing non-signing parents, however, age of exposure to sign language seldom occurs from birth. Whether and when they are exposed to the language, as we learned in previous sections (cf. chapter 1), depends on multiple factors, including parents’ choices about language, medical advice, early intervention, and the availability of sign bilingual education programmes. Thus, sign language is attributed a primary status even though the oral language in its spoken form might be the first language they are exposed to (particularly in the case of children of non-signing parents). Because deaf learners have no or only limited access to the spoken language used in their environment, it is generally assumed that they learn it effectively in its written form only at a later age (in school). Consequently, we are left with a somewhat atypical acquisition situation that eludes clear-cut classification: the acquisition scenario is bilingual to the extent that deaf children are exposed to spoken language and sign language early on (within the critical period). However, the effective acquisition of the oral language in the form of the written language at a later age, bound to a formal context, better fits the traditional concept of child L2 acquisition.
Critical period effects. Variation in age of exposure to a fully accessible L1 in bilingual deaf learners marks a crucial difference to hearing bilingual learners for whom exposure to the L2 majority language might vary, but for whom exposure to a fully accessible L1 from birth can be taken for granted. Consequently, questions concerning the impact of bilingualism on deaf children’s language development are intimately tied to the more fundamental issue of “[h]ow early linguistic experience affect[s] the trajectory of language acquisition over the life span” (Mayberry 2007: 538).
Crucially, a lack of fully accessible language during the sensitive period for language acquisition affects deaf learners’ L1 and L2 competences. The results obtained in several studies undertaken by Mayberry and her colleagues (see Mayberry 2007, for a summary) point to the relevance of the age factor (age of exposure) in sign bilingualism. In these studies, L2 learners of English who differed in their age of exposure to L1 were found to perform equally well on measures of their syntactic L2 knowledge when their age of exposure to the L2 was the same and their exposure to L1 had occurred early on, irrespective of the modality of expression of the L1. However, the performance of those learners who had no exposure to an accessible first language early on was found to be poorer, and at near-chance level for complex syntactic structures, which can be taken as an indication of the relevance of accessible input during the sensitive period for language acquisition (Mayberry & Lock 2003). In addition, the available research indicates that late learners of L1 sign language (at age 5-10 years) may not ever become fully fluent in the language. Based on the evidence obtained, Mayberry (2007: 537) concluded that “the effects of L1 acquisition on both L1 and L2 outcome are apparent across levels of linguistic structure, namely, syntax, phonology, and the lexicon.” Hence, there is a fundamental sense in which “L1 and L2 acquisition are clearly interdependent” (Mayberry 2007: 543).