Babies adopted across international borders may not remember the language they heard in their first days, but the words leave a lasting mark on their minds, scientists said Monday.

The study in the Proceedings of the National Academy of Sciences is the first that employs brain imaging scans to show how we processes lost language, even years after it was last heard.

"What is kind of striking is that these traces are there even though they don't really need them anymore," said co-author Denise Klein of the Montreal Neurological Institute and Hospital at McGill University.

"The brain is responding to the information."

The study by Canadian scientists included 48 girls, aged nine to 17.

Some were born and raised in a French family, speaking only French. Some were Chinese-born and adopted into French families, and learned to speak only French. Others were fluent in both Chinese and French.

All three groups listened to Chinese language sounds while magnetic resonance imaging (MRI) scans were taken.

They heard sounds like ma-ma, spoken in slightly different tones. Those who don't speak Mandarin would hear them just as sounds.

However, those with some knowledge of the language would know that depending on the tone, "ma" could mean mother, hemp, horse, or scold.

The children heard three syllable sounds and were asked to press a button to indicate if the final syllables sounded the same or different.

- Linguistic relevance -

All the participants responded with high levels of accuracy to the quiz, but only some showed brain activity that indicated recognition, or what the study described as "linguistic relevance."

The bilingual Chinese-French and the children adopted from China who had long forgotten any Chinese they'd learned as youths showed brain activity in the right and left hemispheres, while the monolingual French children showed brain activity only in the right hemisphere.

This signifies that those who had heard Chinese as babies were able to tell, somehow, that the sounds they were hearing were "language, or meaningfully related," even if they no longer understood them, explained Klein.

The left temporal cortex was the center of activity in the bilingual and Chinese-adopted children, whose average age at adoption had been 12.8 months.

"These regions have consistently been recruited in previous research on tonal processing and are thought to be important for the processing of tone in speakers of tonal languages," said the PNAS study.

So even though they had little if any language ability by the time they were adopted, somehow their brains continued to process the sounds as meaningful an average of 12 years later.

But why? The study did not answer that question, but it intrigues scientists who would like to find out if there is some reason for the brain to retain this kind of recognition.

Klein described the MRI scans as showing that mental templates set up early in life are not overwritten by new pieces of information.

"Like everything in life we have to prune out what is irrelevant and focus on what is relevant," she said.

Previous research has shown that babies initially respond to all languages heard in their environment, but as months go by, they stop responding to foreign tongues and turn their heads mainly when they hear their parents' language.

The latest research goes further, showing the precise area of brain activity at play, and suggesting "a special status for language input obtained during the first year of development," said the study.

Future work will seek to understand if language learning is easier or faster among those who retain these templates from early exposure, compared to those who were never exposed.