Review of language organisation in bilingual patients: what can we learn from direct brain mapping?

Although the majority of people worldwide are bilingual, the brain representation of language in bilingual persons is still a matter of debate. Since the beginning of the studies conducted on bilinguals, most authors denied that learning a new language requires a new semantic processing or the involvement of new cortical areas. In this paper, we review neurosurgical studies using direct electrocortical or subcortical stimulation techniques for brain mapping in bilingual subjects and compare this data with that obtained from other brain mapping methods. The authors focused on the most controversial issue whether multiple languages are represented in common or distinct cerebral areas. Seven direct brain mapping studies from different teams focused on bilingualism and multilingualism. All these studies showed that even if cerebral representation of language in multilingual patients could be grossly located in the same cortical region, it was possible to individualise distinct language-specific areas by direct cortical stimulation in the dominant frontal and temporo-parietal regions. Task- and language-specific sites were also described, demonstrating an important specialisation of some cortical areas. Using subcortical stimulation, some authors were able to find specific white matter tracts for different languages. Finally, all authors recommend in bilingual patients who need brain mapping for neurosurgical purpose to test all languages in which the subjects are fluent.