The Language Switch
Bilinguals switch from one language to the other without any problems, a sudden leap into foreign vocabulary is usually corrected quickly. What limits the linguistic confusion? You are to be envied, our fellow human beings who grew up with two mother tongues: while we run out of vocabulary in the foreign ice cream parlour, the bilinguals remain sovereign. Her bilingual brain rarely leaves her speechless. Two dictionaries in your head can also invite you to mix vocabulary and grammar. How do bilinguals manage to reconcile two languages in one head?
Bilinguals could of course have two brain regions - one for each idiom. However, this assumption turned out to be wrong. Brain researchers have known for a long time that the same language centers are always active in the bilingual brain. But how does the brain distinguish German from English? Why doesn't it get mixed up? The research team led by Cathy Price from University College London began their experiments with these questions.
The neurologists presented word pairs to three groups of bilinguals on a monitor. These either related to each other in terms of content, such as "trout" and "salmon", or were completely unrelated, such as "spoon" and "shower". Both terms appeared in the same or in different languages. The researchers then observed more closely which neurons in the brain passed on the optical stimuli: they analyzed a group of German-English bilinguals using positron emission tomography, and they used magnetic resonance imaging to look into the brains of German-English and Japanese-English bilinguals.
As expected, the language centers in the left temporal lobe were similarly busy - no matter which language the bilinguals were confronted with. But then the scientists noticed an area of the brain that they had never seen before in connection with bilingualism: the left caudate nucleus. If the test subjects perceived terms that differed from one another because they meant different things or belonged to a different language, the neurons in this brain area fired up significantly more. Apparently, the scientists found what they were looking for in the caudate nucleus: the control center of bilingualism.
Like the signalman recognizes his trains, the caudate nucleus apparently recognizes incoming speech stimuli and sets the course so that the message can be specifically forwarded. At the same time, the brain blocks words from other languages to avoid vocabulary mixing. This is the only way that two dictionaries can be dormant in the same region of the brain.
The researchers were able to confirm their theory in a trilingual woman whose left caudate nucleus was damaged. Although this woman had not lost her multilingualism, she jumped from one language to another, often uncontrollably.
If we didn't have the caudate nucleus, we might be able to save on foreign language lessons at school - and would have to buy our holiday ice cream with our hands and feet. So even if it's only enough for a few foreign-language chunks - as long as they belong to a single language, our brain has already done a lot.