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How bilingual brains switch between tongues

    * 16:16 09 June 2006
    * NewScientist.com news service
    * Roxanne Khamsi

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The next time you listen to the Beatles sing “Michelle” you can thank an area of your brain called the left caudate. It could be what enables you to follow the lyrics as they switch from English to French, claim researchers at University College London in the UK.

Previous brain-scan research into how the brain flips from one language to another has failed to identify any one region responsible, suggesting that the neural circuits for different languages are highly overlapping in the brain.

Now Cathy Price and her colleagues have combined brain scans with behavioural tests and discovered that the left caudate becomes more active as people shift from thinking in one language to another. This area is thought to influence how we articulate words in association with another brain structure known as the thalamus.

The research team recruited 35 bilingual people – 25 spoke German and English, 10 spoke Japanese and English. The participants viewed pairs of words while undergoing brain scans using either positron emission tomography (PET) or functional magnetic resonance (fMRI) imaging.
Double meaning

When volunteers read two words with the same meaning but in different languages, or two words in the same language with unrelated meanings, the left caudate region in their brains became more active than when they read two words from the same language with a similar theme. This held true across both language groups.

“Our results suggest that the left caudate monitors the language in use and increases its activation when there is a switch between languages. This shows that the area is signalling a change in language,” Price claims.

Researchers did not detect increased activity in the right hemisphere's caudate. They suggest this is because the brain's language centres – that connect to and from the caudate – are located in the left hemisphere of the brain.
Prime behaviour

Evidence of the left caudate’s role in language was backed up by behavioural tests on the volunteers. They were faster to answer questions about the second word in each pair if it was related to the first word. While viewing the word "spoon", for example, they were quicker to say that it had a closed handle if the preceding word was “ladle” instead of either “bathtub” or the word for “ladle” in a different language.

The left caudate’s role in language processing is further backed up by the case of a trilingual woman with a damaged caudate region, who involuntarily switched between three different languages while speaking, says Price.

However, Robert Kluender at the University of California at San Diego, US, says the study only looked at a very limited aspect of language processing and therefore more research is needed. “It remains to be seen whether these results will scale up to other levels of linguistic analysis, in particular sentence-level processing,” he says.

Journal reference: Science (DOI: 10.1126/science.1127761)
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