Four month old babies can tell whether a speaker has switched to a different language from visual cues alone and those who grow up in a bilingual home seem to hold on to the ability. This research comes from a joint study at the University of British Columbia and McGill University in Canada.

Researchers showed babies silent video clips of three bilingual French-English speakers, who recited sentences first in English or French, and then switched to the other language.

Without hearing any spoken words, babies noticed a difference in the language being spoken simply by watching the shapes and rhythm of the speaker's mouth and face movements.

Researchers aren't sure why babies detect the difference but co-author Athena Vouloumanos says, "English and French are actually in different rhythmical classes - English is a stress-based language, while French is syllable-based."

Both monolingual and bilingual babies at four to six months of age watching the silent video clips noticed a change. They paid closer attention to the video and spent a longer time watching once the speaker switched languages.

By eight months, only babies from a bilingual French-English home were able to tell them apart visually.

Researcher Whitney Weikum says, "This suggests that by eight months, only babies learning more than one language need to maintain this ability. Babies who only hear and see one language don't need this ability, and their sensitivity to visual language information from other languages declines."

The study will be published Friday in the journal Science.