Brain Switch: How Bilingual Brains Juggle Sound Systems

Imagine hearing a sound like “ka” while watching a movie. If you are bilingual in English and Spanish, that sound might mean something different in each language. Now imagine switching between English and Spanish movies, each with the same sound playing in the background. How does your brain decide what language that sound belongs to? That is the question at the heart of our project—how bilingual brains manage competing sound systems.

Here is how it works.
In our study, English monolinguals and Spanish-English bilinguals passively listened to speech sounds while watching short films in either English or Spanish. The same two sounds were played in both settings, but those sounds represent different speech categories in each language. For example, English separates the sounds “ga” and “ka” using a timing cue called Voice Onset Time (VOT), but in Spanish, the same cue does not carry the same weight. We used brain recordings to see how participants processed these sounds depending on the language they were immersed in.

And what we found was striking.
Bilinguals’ brains were highly sensitive to the language context. When watching an English movie, their brains treated the “ga” and “ka” sounds as very different. But during the Spanish movie, the same brains showed reduced sensitivity—almost as if the sounds were not different anymore. This suggests that bilinguals can shift their sound processing depending on which language they think is active. We saw this flexibility in brain regions linked to executive control—the part of the brain that helps with things like decision-making and task switching. Monolinguals, on the other hand, did not show this same flexibility. Their brains processed the sounds the same way regardless of language context.

Why does this matter?
Because it shows that bilinguals are not just juggling two languages at the word level—they are managing competition between sounds even before words are formed. Their brains use control systems to sort out which language is in play and adjust how they listen accordingly. This tells us that bilingualism taps into general brain systems for managing conflict and making choices, not just language-specific systems.  Listen to the Notebook LM Podcast

Now we are taking the idea one step further.

In our new project, we introduced a third language—Mandarin—but not as part of the speech sounds themselves. The sounds stayed exactly the same: the same “ga” and “ka” syllables from before. What changed was the context. This time, participants watched a movie in Mandarin, a language none of them understood, while hearing the same speech sounds in the background.

This allowed us to ask a new question: Can simply being in an unfamiliar language context change how the brain processes sounds? And more specifically, do bilinguals show different brain responses than monolinguals when the context is completely unknown?

We think the answer may be yes.
Because bilinguals are experienced at toggling between languages, their brains might be more sensitive to contextual cues—even when the language is unfamiliar. They may be better at detecting when a new “mode” is needed, activating more flexible processing strategies. If bilinguals adjust their brain responses to the same sounds simply because the language of the movie changed, that would suggest their experience helps them stay mentally agile, even with unfamiliar input.

That is the idea behind Brain Switch—a study exploring how experience with two languages may help the brain stay open and adaptable when encountering a third.

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