neuroscience research on second language acquisition and bilingualism that appear to hold promise for designing training programs for adult learners to acquire a second language. On the surface, one might approach the problem of second language learning in adults with some pessimism. A theme in research on second language learning is that it is much easier for individuals to acquire a second language in early childhood than as adults. Even successful adult learners often fail to grasp subtle grammatical distinctions in the second language and speak it with a noticeable accent (e.g., Johnson and Newport, 1989; Piske, MacKay, and Flege, 2001). A full discussion of the reasons that the age of acquisition appears to constrain second language learning is beyond the scope of this paper: for two different theoretical perspectives on the effects of the age of acquisition, see Birdsong (2005) and DeKeyser and Larson-Hall (2005).
Rather than dismiss second language learning as an unattainable task for most adults, I focus on those aspects of the learner and the learning context that appear to enable at least some adult learners to acquire functional skills in a second language. I also examine the consequences of becoming proficient in a second language, not only for the second language, but also for processing in the first language and for cognition more generally. I suggest that the recent research on bilingualism and second language learning provides evidence for a degree of plasticity in the organization of the language system that makes it feasible for adult learners to achieve some measure of success.
The observation that has perhaps most critically changed understanding of bilingual language processing is that bilinguals do not appear to be able to switch off one of their languages when using the other language. The activity of the language not in use has been documented in even highly skilled language tasks, such as reading (Dijkstra, 2005), listening (Marian and Spivey, 2003), and speaking (Kroll, Bobb, and Wodniecka, 2006). The findings from a range of studies, including those that examine performance in the native language, suggest that a bilingual person is a mental juggler. A major goal of the research on this topic has been to determine the factors that eventually control the selection of the language that the bilingual intends to use. Bilinguals do not generally use the unintended language randomly, but they are also able to code switch with others who are similarly bilingual (see, e.g., Muysken, 2000; Myers-Scotton, 2002), suggesting that both of the languages are highly accessible.