Two languages in mind: Bilingualism as a tool to investigate language, cognition, and the brain.

A series of discoveries in the last two decades has changed the way we think about bilingualism and its implications for language and cognition. One is that both languages are always active. The parallel activation of the two languages is thought to give rise to competition that imposes demands on the bilingual to control the language not in use to achieve fluency in the target language. The second is that there are consequences of bilingualism that affect the native as well as the second language. The native language changes in response to second language use. The third is that the consequences of bilingualism are not limited to language but appear to reflect a reorganization of brain networks that hold implications for the ways in which bilinguals negotiate cognitive competition more generally. The focus of recent research on bilingualism has been to understand the relation between these discoveries and the implications they hold for language, cognition, and the brain across the lifespan.