If I presented you with a silent video of someone speaking – do you think that you'd be able to tell if they switched from English to French? Remarkably, between the ages of four and six months, babies can tell. However, it's a skill they lose by the age of eight months, unless they are raised in a bilingual environment.
Whitney Weikum and colleagues played 36 babies the same silent video of a person saying one sentence in one language (English or French), over and over again, until the babies grew bored of it and stopped looking at it so much.
Next they played them a test sentence which featured the same speaker uttering a new sentence in a new language. They compared how long the babies looked at this video with how long they looked at a control video that featured the same speaker, a new sentence, but the same language as in the video they'd grown bored of earlier. If they looked longer when the language had switched than at the control video, this would strongly suggest the babies had registered something different was going on when a new language was spoken – even though there was no sound.
And that's exactly what the researchers found: 4-month-old and 6-month-old babies looked longer at the new silent videos when the language being spoken had changed. Eight-month-old babies didn't register the change in language, unless - a second experiment showed - they were being raised as bilingual.
This study provides the latest example of a discriminatory ability that we start off life with, but then lose, as we adapt to our environment. Other examples include the ability to discriminate consonant and vowel sounds from foreign languages, to discriminate rhythms from other cultures' music, and to distinguish between the faces of individuals within a given animal species.
Weikum, W.M., Vouloumanos, A., Navarra, J., Soto-Faraco, S., Sebastian-Galles, N. & Werker, J.F. (2007). Visual language discrimination in infancy. Science, 316, 1159.
Link to example of English silent video
Link to example of French silent video