4. The neurolinguistics of bilingualism Neurolinguists have long been interested in describing how language is organized in the "bilingual brain" and how this organization differs from that of the monolingual. One approach has been to observe and test bilingual aphasics in order to better understand which languages have been affected by brain injury and which factors best account for the different patterns of recovery of the languages. Another approach has been to study normal bilinguals to ascertain whether language processing occurs mainly in the left hemisphere of the brain (as it appears to do in monolinguals) or in both hemispheres.
As concerns aphasia, researchers have observed various recovery patterns of the bilingual's two or more languages after injury (they have been labelled parallel, differential, successive, antagonistic, selective, mixed) and they have tried to account for the factors that seem to play a role in non-parallel recovery (that is, when the languages are not all recovered together at the same rate). Currently, no single factor has emerged to explain the different types of recovery patterns, and it is not known whether recovery significantly differs following therapy in one language, in the other or in both. It would appear, though, that if a language is not recovered, it is not that it is lost, but simply that it is inhibited, temporarily or permanently.
On the topic of language lateralization, it is now a well-known fact that the left hemisphere of the brain in monolinguals is dominant for language. The question has been whether bilinguals also show strong left-hemisphere dominance for language. Until a few years ago, and based on case studies of bilingual aphasics and on experimental results, some researchers proposed that bilinguals use the right hemisphere in language processing more than monolinguals. However, after further studies that were better controlled, there appears to be clear evidence that monolinguals and bilinguals do not differ at all in hemispheric involvement during language processing. As concerns language organization in the bilingual brain, most researchers agree that the bilingual's languages are not stored in completely different locations. In addition, it would appear that bilinguals have two subsets of neural connections, one for each language (each can be activated or inhibited independently) while at the same time possessing one larger set from which they are able to draw elements of either language at any time. This said, the bilingual brain is still very much terra incognita, and only further experimental and clinical research will tell us how similar it is to the monolingual brain and in what ways it may be different.