When timing is everything: Age of first-language acquisition effects on second-language learning

The present paper summarizes three experiments that investigate the effects of age of acquisition on first-language (L1) acquisition in relation to second-language (L2) outcome. The experiments use the unique acquisition situations of childhood deafness and sign language. The key factors controlled across the studies are age of L1 acquisition, the sensory–motor modality of the language, and level of linguistic structure. Findings consistent across the studies show age of L1 acquisition to be a determining factor in the success of both L1 and L2 acquisition. Sensory–motor modality shows no general or specific effects. It is of importance that the effects of age of L1 acquisition on both L1 and L2 outcome are apparent across levels of linguistic structure, namely, syntax, phonology, and the lexicon. The results demonstrate that L1 acquisition bestows not only facility with the linguistic structure of the L1, but also the ability to learn linguistic structure in the L2. The investigation of sign languages has altered our understanding of the nature of language in a profound way. Sign languages have hierarchical structure identical to that of spoken languages despite the fact they are gestured and watched. Specifically, sign languages have multilayered and interrelated, linguistic organization: a lexicon with sublexical structure (i.e., phonology), grammatical and derivational morphology, syntax, and semantics (Klima & Bellugi, 1979; Liddell, 1980; Padden, 1988; Stokoe, Casterline, & Cronneberg, 1965; Supalla & Newport, 1978). Using the architecture of sign language, psycholingusitic research has discovered that the human mind packs and unpacks meaning in sign language via its linguistic structure, in a fashion identical to how speakers produce and perceive meaning from spoken language (for a review, see Emmorey, 2002). Consistent with these linguistic and psychological discoveries, developmental research has found that the milestones of sign language acquisition by children exposed to it from birth are no different from those for spoken language (Mayberry & Squires, 2006; Meier, 1987; Petitto, 1991; Reilley, McIntyre, & Bellugi, 1990). Given the linguistic, psycholinguistic, and developmental parallels between sign and spoken language, it comes as no surprise that the neurocortical underpinnings of sign language are © 2007 Cambridge University Press 0142-7164/07 $15.00 Applied Psycholinguistics 28:3 538 Mayberry: When timing is everything highly similar to those of spoken language (MacSweeney et al., 2002; Petitto et al., 2000). Sign language research has thus unearthed a key cognitive principle. Linguistic structure, its acquisition, processing, and neurocortical representation transcend sensory–motor modality. The structure and processing of language are properties of the human mind as it communicates with other minds. The properties of the sensory and motor systems clearly provide the raw materials from which language builds its architecture, but they do not determine its form, function and acquisition, or social and neurocortical use. Sign language research can potentially yield another critical insight into the human mind, which is the role of early linguistic experience in language acquisition. How does early linguistic experience affect the trajectory of language acquisition over the life span? The present paper focuses on the role of early linguistic experience in second-language (L2) learning. We ask here whether and how age of acquisition (AoA) of the first language (L1) affects the outcome of L2 learning. We also ask whether sensory–motor modality is a relevant factor in the transfer of linguistic skills from the L1 to the L2. Specifically, we ask whether a visual L1 can support subsequent learning of a spoken L2. Before summarizing a series of three experiments conducted in two languages, it is necessary for us to consider the unique circumstances of sign language acquisition vis-à-vis spoken language acquisition. THE BIOLOGICAL AND CULTURAL CONTEXTS OF SIGN LANGUAGE