Those coos of early infancy start changing around age 6 months, growing into the syllables of the baby's native language until the first word emerges, usually just before age 1.

A lot of research has centered on the audio side. That sing-song speech that parents intuitively use? Scientists know the pitch attracts babies' attention, and the rhythm exaggerates key sounds. Other studies have shown that babies who are best at distinguishing between vowel sounds like "ah" and "ee" shortly before their first birthday wind up with better vocabularies and pre-reading skills by kindergarten.

But 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.

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, groups of them at ages 4, 6, 8, 10 and 12 months.

How? 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.

It makes sense that at 6 months, babies begin observing lip movement, Lewkowicz says, because that's about the time babies' brains gain the ability to control their attention rather than automatically look toward noise.

But what happened 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.

But the continued lip-reading shows the 1-year-olds clearly still "are primed for learning," McMurray says.

Babies are so hard to study that this is "a fairly heroic data set," says Duke University cognitive neuroscientist Greg Appelbaum, who found the research so compelling that he wants to know more.

Are the babies who start to shift their gaze back to the eyes a bit earlier better learners, or impatient to their own detriment? What happens with a foreign language after 12 months?

Lewkowicz is continuing his studies of typically developing babies. He theorizes that there may be different patterns in children at risk of autism, something autism experts caution would be hard to prove.