What they're picking up on is the musicality of the language they've grown accustomed to hearing during their nine-month gestation. If you listen to a person talking through a wall, what you hear are rhythms and intonation patterns. In order to hear the voices, the newborns had to suck on their pacifiers. Even when the words were electronically filtered to muffle pronunciation, the infants were still more interested in the melody of their mother tongue. Despite a baby's innate ear for language, learning to understand and use words requires specific input from parents.
Babies are particularly attuned to the form of speech known as motherese.
When you raise your pitch slightly, draw out vowels, and pronounce words in a sing-song voice, you're speaking your baby's favorite dialect. And the more time your baby spends listening to you, the more time she's cultivating her speech development. Fortunately, motherese comes naturally to parents, not to mention most admiring strangers. Another way parents can foster their babies' language development is through something called shared joint attention. When parents point to an object while naming it, they're engaging in a special kind of interaction that psychologists call social referencing.