Tongue-tied: When bilinguals switch languages involuntarily

People who can communicate in more than one language – bilinguals - are intriguing to psychologists. A new study focuses on how bilinguals switch between tongues – how come they don't mix up words from different languages? A prevailing view is that bilinguals have a kind of switch at the front of their brain that inhibits the language(s) not currently in use. Now Kuan Kho and colleagues report the case of two bilingual patients who, during the course of brain surgery for epilepsy, appear to have had their 'switches' involuntarily flipped.

Prior to surgery, patient A – a Dutch-English bilingual - underwent a Wada test that involves anaesthetising one half of the brain at a time. When his left-hemisphere was anaesthetised he first went mute for a few minutes, then he fully recovered English (his second language), but struggled with Dutch. Asked to recall a story told to him earlier, he was only able to do so in English. Any Dutch he did come up with, he spoke in an English accent!

Patient B, a French-Chinese bilingual, was having his brain prodded with an electrode to identify which neural tissue was involved in language before the surgeons got to work. The patient was asked to count. He began in French, then when he reached seven (...quatre, cinq, six, sept), the stimulation was applied to his left inferior frontal gyrus, at which point he involuntarily switched to Chinese (, jiu, shi). When the stimulation ended, he reverted to French.

These case studies support the notion that, in bilinguals, specific regions at the front of the left hemisphere act as a language switch. The observations are also consistent with another recent study, which documented involuntary language switching in two bilingual patients who received transcranial magnetic stimulation to their left dorsolateral prefrontal cortex, as a treatment for depression.

Kho, K.H., Duffau, H., Gatignol, P., Leijten, F.S.S., Ramsey, N.F., van Rijen, P.C. & Rutten, G-J.M. (2007). Involuntary language switching in two bilingual patients during the Wada test and intraoperative electrocortical stimulation. Brain and Language, 101, 31-37.
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