EVEN before they can talk, infants as young as seven months who grow up in bilingual homes acquire a special ability to distinguish between languages, researchers have found.

Scientists are still baffled by the mechanics of language learning, and how bilingual infants master their mother tongues as efficiently as monolinguals do.

A new study to shed light on the subject revealed that children who learn two languages at the same time develop the ability, which monolinguals do not, to identify a language through the duration and pitch of words, and their position in a sentence.

The study, reported in the journal Nature Neuroscience, examined seven-month-old children who grow up with two languages with inverse word orders - such as English and Japanese.

In languages like English, Italian or Spanish, prepositions and articles typically precede "content words" like nouns (to London/the house) while in other tongues they follow them (Tokyo ni - 'Tokyo to' in Japanese, or etxe bat - 'house a' in Basque), and the pitch of the content word is higher.

The test involved familiarising the infants, through repetition, to one of two fake languages - one with the rhythm of the English group of languages, the other resembling the Japanese group.

"Phrases" were then played for the kids over hidden speakers, both in the language they learned and the one they did not, measuring the time they looked in the direction of each.

The bilingual babies "looked several seconds longer" in the direction of the language they had been exposed to - a scientific measure of recognition, study co-author Janet Werker told AFP.

When the same test was done with monolingual infants growing up in English homes, they made no distinction.

"Our study showed... that infants growing up bilingual in languages that have a different word order are able to use these word duration and pitch cues, whereas infants growing up monolingual are not," said Ms Werker.

"If you speak two languages at home, don't be afraid, it's not a zero-sum game. Your baby is very equipped to keep these languages separate and they do so in remarkable ways."

The team said their work demonstrated how studying bilingual infants can reveal mechanisms of language acquisition.

"As the majority of the world's population today is exposed to multiple languages from birth, a better understanding of their early cognitive development might have considerable impact on social and educational policies world-wide," they wrote.

"Further work will be necessary to explore how the beginnings uncovered in our study develop into full-fledged competence."

The study was presented on Thursday at the annual meeting of the American Association for the Advancement of Science in Boston.