The bilingual brain

Bilingualism can be defined simply as the regular use of two languages and is a phenomenon that affects almost every nation in the world, but finds a natural geographical context in Switzerland. At the descriptive level, bilingualism can have various forms that are often differentiated in terms of when and how the two (or more) languages were learnt and how proficient a bilingual is in the second (L2) language compared with the first one acquired (L1) or the mother tongue. The emergence of neuroimaging techniques today offers a great opportunity to investigate the brain mechanisms associated with the extraordinary human ability to acquire and manage the use of several languages in a very precise way in everyday life. In particular, they permit determination of the brain representations of the different languages as a function of the level of bilinguals' proficiency. In this article, we discuss two main questions regarding the bilingual brain: how the two languages are organized in the brain in terms of neural network, and how the brain functions in order to smoothly select correctly the desired language in a given context without mixing the two or three languages in an uncontrolled manner. We review data which suggest that (1.) the different languages' representations are fairly similar for L1 and L2, and concern the classical language areas, although a greater convergence is found when L2 proficiency increases, (2.) that the majority of bilinguals' aphasic symptoms can be explained by deficits in languages control mechanisms and that (3.) language control is subserved by a left lateralised fronto-subcortical circuit which is at least partially within the general cognitive control system.