Even with the sound turned off, babies can tell whether a person speaking on video has switched between English and French, a new study suggests.
The findings are the latest contribution to a growing body of research on the remarkable ability of very young infants to process languages.
The paper also shows that babies growing up in bilingual households are better able to retain that ability to visually perceive a switch to another language, whereas such a skill declines among those raised in unilingual settings.
Conducted at the University of British Columbia, the appears today in the journal Science. It comes as other scientists have documented the great abilities of babies to distinguish vowels, consonants, rhythmic patterns and tonal inflections in languages the infants don't yet speak, even in languages that are not native to them.
Babies from English-speaking families are, for example, able to distinguish between similar Hindi-language sounds that adult English speakers would struggle to tell apart.
But unless they remain exposed to other languages the babies' capacity to pick up phonetic traits of a foreign language diminishes as their brain commits to a native tongue.
"The most intriguing part of this research, to put it in a broader framework, is the fundamental question that motivates everyone: How is it that humans learn language?" said one of the UBC study's co-investigators, Athena Vouloumanos, now a psychology professor at McGill University.
The new UBC research tested babies who were four, six and eight months old. Their reactions were recorded as they watched muted videos of the face of a person speaking French or English.
"It was really exciting because at four months their visual acuity actually is starting to get strong, too. So to see this so early on was just amazing," said the study's lead investigator, Whitney Weikum, a neuroscience and psychology doctoral student at UBC.
"The babies love faces, so a lot of the time, when the faces would come up on the screens, they'd start to laugh and smile and coo."
In a quiet room at UBC's Infant Studies Centre, the babies sat in their parents' laps and were shown muted videos of the face of women reciting excerpts from Antoine de Saint-Exupéry's child novel The Little Prince.
The younger babies' attention would wane after a while but would perk up when the woman switched to another language - even though there was no sound.
Researchers believe that the babies spot the switch either because the speaker's mouth adopts different shapes in a different language, or because French and English are not spoken with the same rhythmic cadence.
By the time the babies were eight months old, their ability to spot a language switch lessened, unless the child came from a bilingual home, suggesting that exposure to another language helped to keep their acuity.
It is normal, researchers say, for the young child to prune out unused linguistic skills, a pruning process that helps limit information overload.
"As babies grow up, it wouldn't really be advantageous to continue to hear sound differences in languages they don't use," said the study's research supervisor, UBC psychology professor Janet Werker, a long-time researcher on how babies perceive speech.
At the same time, Prof. Vouloumanos said the recent findings also made her believe that there is no harm in getting children to learn new languages at the earliest age.
There is no reason that a healthy child should not be exposed to multiple languages that are spoken in their natural environment, she said, emphasizing that she was stating a personal opinion.
"Much research has shown that young infants can do many things with language, and, in many cases, better than adults can."
She and Prof. Werker noted that, outside of North America, the majority of young children grow up in multilingual environments.
"The brain is definitely set up to acquire more than one language," Prof. Werker said.
Tu Thanh Ha