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Why do bilingual stroke patients sometimes recover only one language?

When a person suffers from a stroke, the blood flow to certain areas is rapidly disturbed either because a blood vessel starts bleeding (haemorrhage) or because a blood vessel is blocked. If the stroke affects brain areas which are important for language, then the language functions can be partially or fully lost. This condition is called aphasia, but it is sometimes reversible at least to some degree with time, treatment and rehabilitation. Bilinguals, that is, people who speak more than one language, are known to recover from aphasia in a number of different ways. The most common case is when the bilingual patient similarly recovers both languages equally well as in parallel aphasia. In some cases, however, the recovery of bilingual is disproportionately favouring one of his/her languages and this type of recovery is called selective aphasia.

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The case of selective aphasia made researchers initially believe that each language of a bilingual person must be located in a different brain area, given that one language recovers better than the other after the stroke. However, with the help of brain imaging scans we now know that this idea is not correct. On the contrary, when a person speaks many languages, they all activate a common network of brain areas. While the whole picture of how the brain generates multiple languages is still partly unclear, we do know a number of factors that seem to influence to what extent the languages of a bilingual patient will recover after a stroke. If a person is less proficient in one of the two languages, this language may not recover as good as the more proficient language. This means that the more automatic a skill is, the easier it is to recover it, whereas something that takes effort such as language that one only speaks rarely is harder to recover. Social factors and emotional involvement are also important if we want to understand which language will recover after a stroke, for instance how often a specific language is used, or what emotions are associated with a specific language. However, it is still unclear exactly how these factors interplay in predicting the recovery success.

One of the current theories on why bilingual aphasics may disproportionately recover one language better than the other suggests that this happens when the stroke damages specific control mechanisms in the brain. When a bilingual person knows two languages, he/she needs to suppress or ‘switch off’ one of the languages while using the other language. If the mechanisms that control this switch are damaged during the stroke, the aphasic patient may no longer be able to similarly recover both languages as the ability to control the language use has been lost. In this case the person may appear to have completely lost one of the languages, but the problem is actually one of control. Recently researchers found that the control mechanisms are more impaired in bilinguals with selective aphasia who recover only one language than in bilinguals with parallel aphasia who recover both languages. Interestingly, when languages recover after stroke, the connections between language and control areas in the brain are re-established. While this interesting finding supports the theory linking selective aphasia to impaired control mechanisms, it is only one of several theories and researchers are currently trying to better understand what other causes may also underlie the surprising recovery patterns that can be seen in bilingual aphasia.

Written by Diana Dimitrova and Annika Hulten

Further Reading

Fabbro, F. (2001). The bilingual brain: Bilingual aphasia. Brain and Language, 79(2), 201-210. pdf

Green, D. W., & Abutalebi, J. (2008). Understanding the link between bilingual aphasia and language control. Journal of Neurolinguistics, 21(6), 558-576.

Verreyt, N. (2013). The underlying mechanism of selective and differential recovery in bilingual aphasia. Department of Experimental psychology, Ghent, Belgium. pdf

About MPI

This is the MPI

The Max Planck Institute for Psycholinguistics is an institute of the German Max Planck Society. Our mission is to undertake basic research into the psychological,social and biological foundations of language. The goal is to understand how our minds and brains process language, how language interacts with other aspects of mind, and how we can learn languages of quite different types.

The institute is situated on the campus of the Radboud University. We participate in the Donders Institute for Brain, Cognition and Behaviour, and have particularly close ties to that institute's Centre for Cognitive Neuroimaging. We also participate in the Centre for Language Studies. A joint graduate school, the IMPRS in Language Sciences, links the Donders Institute, the CLS and the MPI.

 
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This project was coordinated by:

Katrien Segaert 
Katerina Kucera
Judith Holler

Sean Roberts
Agnieszka Konopka
Gwilym Lockwood
Elma Hilbrink
Joost Rommers
Mark Dingemanse
Connie de Vos