Language lateralisation in late proficient bilinguals: A lexical decision fMRI study

► Cortical activations during lexical decision tasks were compared between late proficient bilinguals and monolinguals. ► Bilinguals showed weaker language lateralisation during L2 (English) than monolinguals. ► Bilinguals recruited additional brain areas for both native and L2 languages, possibly as a result of greater cortical effort. ► Age of L2 acquisition was the main determinant of language representation in bilinguals. Approximately half the world's population can now speak more than one language. Understanding the neural basis of language organisation in bilinguals, and whether the cortical networks involved during language processing differ from that of monolinguals, is therefore an important area of research. A main issue concerns whether L2 (second language) is processed using the same neural mechanisms that mediate L1 (first language) processing. Moderating factors include the age of L2 acquisition and the level of proficiency. Here we used a lexical decision task with five conditions during functional magnetic resonance imaging (fMRI) to investigate language processing in eight late proficient bilinguals when using Macedonian (L1) and English (L2). Bilinguals had greater bilateral activation during both L1 and L2 processing, and therefore weaker language lateralisation, compared to matched control English monolinguals. A greater amount of overall activation was also seen in bilinguals, especially during L2 conditions. Late proficient bilinguals living in their L2 environment employ a more extensive neural network than monolinguals when processing their second language.