Babies learn to talk not only by hearing your voice, but they start adopting reading lips at six months, suggests a new research.
Around the age of six months, a baby begins shifting from the intent eye gaze of early infancy to studying mouths when people talk to them.
Their babbling then gradually changes from gibberish into syllables and eventually into those precious first words.
The finding offers more evidence that quality face-time with your tot is very important for speech development - more than, say, turning on the latest baby DVD.
Scientists have long known that babies also look to speakers' faces for important social cues about what they're hearing. Just like adults, they're drawn to the eyes, which convey important nonverbal messages like the emotion connected to words and where to direct attention.
Dr Lewkowicz went a step further, wondering whether babies look to the lips for cues as well, sort of like how adults lip-read to decipher what someone's saying at a noisy party.
So he and doctoral student Amy Hansen-Tift tested nearly 180 babies, at ages 4, 6, 8, 10 and 12 months.
They showed videos of a woman speaking in English or Spanish to babies of English speakers. A gadget mounted on a soft headband tracked where each baby was focusing his or her gaze and for how long.
They found a dramatic shift in attention: When the speaker used English, the 4-month-olds gazed mostly into her eyes. The 6-month-olds spent equal amounts of time looking at the eyes and the mouth. The 8- and 10-month-olds studied mostly the mouth.
At 12 months, attention started shifting back toward the speaker's eyes.
Dr Lewkowicz said six months is the time babies' brains gain the ability to control their attention rather than automatically look toward noise.
When these babies accustomed to English heard Spanish, the 12-month-olds studied the mouth longer, just like younger babies. They needed the extra information to decipher the unfamiliar sounds.
That fits with research into bilingualism that shows babies' brains fine-tune themselves to start distinguishing the sounds of their native language over other languages in the first year of life. That's one reason it's easier for babies to become bilingual than older children or adults.
The findings appeared in this week's issue of the Proceedings of the National Academy of Sciences.