At four months, babies may be too young to speak or crawl, but they can certainly tell when a speaker has switched to a different language - with only visual cues.
Researcher Whitney Weikum from the University of British Columbia found that infants are able to discern when a different language is spoken by watching the shapes and rhythm of the speaker's mouth and face movements.
The findings suggest that babies growing up in a bilingual environment advantageously maintain the discrimination abilities needed for separating and learning multiple languages.
As a part of the study Weikum and colleagues tested three groups of infants - ages four, six and eight months - from monolingual English homes and two groups of infants -ages six and eight months - from bilingual homes.
They showed each group silent video clips of three bilingual French-English speakers, who recited sentences first in English or French, and then switched to the other language.
Their findings suggest that visual information alone will prompt the babies at four and six months to pay closer attention and watch the video for a longer period when the speakers switch languages.
'We already know that babies can tell languages apart using auditory cues. But this is the first study to show that young babies are prepared to tell languages apart using only visual information,' says Weikum.
The researchers found that six-month-old babies from both bilingual French-English and monolingual English homes could tell the languages apart visually. These groups would watch the video clips for a significantly longer period if the speaker switched languages.
However, by eight months, only babies from a bilingual French-English home and familiar with both languages were able to tell the languages apart visually.
'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 is published in the May 25 issue of the journal Science.