Bilingual babies' vocabulary linked to early brain differentiation

Babies and children are whizzes at learning a second language, but that ability begins to fade as early as their first birthdays.

Researchers at the University of Washington's Institute for Learning & Brain Sciences are investigating the brain mechanisms that contribute to infants' prowess at learning languages, with the hope that the findings could boost bilingualism in adults, too.

In a new study, the researchers report that the brains of babies raised in bilingual households show a longer period of being flexible to different languages, especially if they hear a lot of language at home. The researchers also show that the relative amount of each language – English and Spanish – babies were exposed to affected their vocabulary as toddlers.

The study, published online Aug. 17 in Journal of Phonetics, is the first to measure brain activity throughout infancy and relate it to language exposure and speaking ability.

"The bilingual brain is fascinating because it reflects humans' abilities for flexible thinking – bilingual babies learn that objects and events in the world have two names, and flexibly switch between these labels, giving the brain lots of good exercise," said Patricia Kuhl, co-author of the study and co-director of the UW's Institute for Learning & Brain Sciences.

Kuhl's previous studies show that between 8 and 10 months of age, monolingual babies become increasingly able to distinguish speech sounds of their native language, while at the same time their ability to distinguish sounds from a foreign language declines. For instance, between 8 and 10 months of age babies exposed to English become better at detecting the difference between "r" and "l" sounds, which are prevalent in the English language. This is the same age when Japanese babies, who are not exposed to as many "r" and "l" sounds, decline in their ability to detect them.

"The infant brain tunes itself to the sounds of the language during this sensitive period in development, and we're trying to figure out exactly how that happens," said Kuhl, who's also a UW professor of speech and hearing sciences. "But almost nothing is known about how bilingual babies do this for two languages. Knowing how experience sculpts the brain will tell us something that goes way beyond language development."

In the current study, babies from monolingual (English or Spanish) and bilingual (English and Spanish) households wore caps fitted with electrodes to measure brain activity with an electroencephalogram, or EEG, a device that records the flow of energy in the brain. Babies heard background speech sounds in one language, and then a contrasting sound in the other language occurred occasionally.