Bilinguals May Use One Shared Grammar System in the Brain Instead of Switching Between Two

The same neural system handled English, Spanish and even invented words.

A Spanish speaker learning English may say, “I have 20 years,” instead of “I am 20 years old.” This is a common mistake that can even sound charming. But a new study suggests such slips may not be solely owed to a lack of second language practice and may instead point to something more profound: when people speak two languages, the rules of one can sometimes leak into the other.

Neuroscientists studying the bilingual brain have often asked whether we developed separate grammar systems — one for English, one for Spanish — and switch between them, or there’s a deeper set of tools, applying the same basic machinery to different languages

The second answer may be closer to the truth. Using a brain-scanning technique that tracks activity millisecond by millisecond, researchers found that highly proficient Spanish-English bilinguals used the same left-sided brain network when they made words grammatically correct in either language.

“Our research suggests that brains have a single grammatical engine that fuels all of the languages we speak — rather than separate engines for each one,” Esti Blanco-Elorrieta, an assistant professor of psychology and neural science at NYU and the study’s senior author.

“We show that the same brain patterns support grammar in English and Spanish, indicating that human language may be built from neural computations that transcend any one language.”

Grammar Brain

The study focused on a simple but revealing grammatical operation: making a noun match the number in a phrase.

A participant might see the word “boat” on a screen. Then they would hear the word “two.” The correct response was “boats.” If they saw “boats” and heard “one,” they had to strip the word back to “boat.”

Spanish trials worked the same way. A participant might see “barco,” meaning boat, and hear “dos,” meaning two. The correct response was “barcos.”

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During all this time, the researchers looked at what happened in the brain when the participants had to change a word, rather than simply repeat it. Some trials used the cue “say” in English or “di” in Spanish. In those cases, participants just repeated the word on the screen.

That gave the researchers a clean contrast: moments when the brain had to perform a grammatical operation, and moments when it did not.

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The team tested 23 highly proficient Spanish-English bilinguals. As the participants completed the task, the researchers recorded their brain activity with magnetoencephalography, or MEG, a technique fast enough to track language planning in milliseconds. The signal acquisition speed was rather key to the study because grammar prepares a word almost immediately before speech.

The participants performed the task accurately overall, suggesting they understood it well. But the brain data hid some surprises.

One Network, Different Timing

When participants changed words to fit grammar, activity appeared in a left frontal-temporal network, a set of regions long associated with language. The signal emerged early, roughly within the first 100 milliseconds after the cue.

The same broad pattern appeared for English and Spanish. Computer classifiers trained to recognize the brain pattern for grammatical inflection in one language could recognize the same operation in the other. This suggests the result was not just a vague overlap in “language areas.” The neural pattern itself generalized across languages.

Spanish responses were faster than English responses, and the brain signal for Spanish appeared earlier by about 110 milliseconds. This may reflect differences in the processing efficiency of the languages themselves rather than separate grammar systems. Spanish pluralization is highly regular; English has a less orderly plural system, even though the study carefully used forms that could be matched across the two languages.

The researchers also tested whether the effect was really about grammar, rather than about adding or removing sounds. It was. Words that required a sound change and words that did not still recruited overlapping neural signatures.

Then came the invented words. If grammar depended mainly on stored words, pseudowords should have behaved differently. Instead, the same system handled them too. That suggests the brain was not merely retrieving memorized forms. It was applying a reusable operation.

“The results provide some of the clearest neural evidence to date that grammatical computations are shared across languages in bilingual speakers,” said Blanco-Elorrieta. “More broadly, because the brain appears to use a common neural system across languages, our findings offer new insight into how we communicate and learn new languages.”

What the Study Does, and Doesn’t, Prove

The study does not mean every language uses identical grammar in the brain. The researchers chose English and Spanish partly because their plural systems can be closely matched. Future work will need to test bilingual speakers whose languages differ more dramatically, such as languages with richer case systems, different word order or very different ways of marking number.

The study also involved a small, highly proficient group of bilinguals. It cannot answer how the same process develops in children, late learners or people who use one language much more than the other.

Still, scientists increasingly view bilingual brains not as two language systems competing behind a wall, but as integrated systems that share resources while still keeping languages apart when needed.

The findings appeared in JNeurosci.