Adult people need to hear a language to make the difference from another. But scientists have discovered that four-month old babies can distinguish between different languages just by looking at the facial movements of the speaker.

Even when babies watched video recording with the sound switched off, they were able to make this difference. The "visual speech" ability could be important in language learning and appeared to be retained longer in babies growing in bi-lingual homes where they learn two languages at once.

Babies possess even from birth the ability to recognize basic human facial features, which are now revealed to be important in language learning.

"Talking faces are among the most dynamic and salient stimuli available to infants, and the facial movements accompanying speech influence adult and infant speech perception," said lead author Whitney Weikum, a doctoral student at the University of British Columbia in Canada.

"Apart from the sounds made by a talking face, babies appear to be able used the movements to determine whether someone has switched from one language to another. 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", said Weikum.

The research was made in monolingual English-speaking and bilingual English-French speaking families on babies of various ages - four, six and eight months.

"The scientists 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," said Weikum.

The young age groups, at four and six months old, belonging to both types of families could make the difference between the languages, just by watching the clips even when the voice was turned off, but the eight months, the monolingual babies had lost this ability. Those from bilingual families could still make the difference based just on the visual clues.

"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," said Weikum.

It seems that "visual speech" is important in the language learning process but this ability disappears when babies get older as the auditory information prevails now. However, keeping this ability helps bilingual babies to distinguish between the two languages they are learning simultaneously.

"Traditionally, visual speech has been regarded as a redundant signal in verbal communication. The present research shows that visual speech information alone is sufficient for language discrimination in infancy," said Weikum.