Encouraged by parents and others, babies gradually learn to identify which of the millions of sounds they hear are actually words. They learn, for example, that when they hear someone say “pretty baby,” pretty is a word and baby is a word, but ty-ba is not a word.

Saffran has looked into how babies do this by exposing them to made-up words, such as golabu and daropi, and repeating them over and over. She has found that babies compute, unconsciously, the probabilities that certain sounds will be paired together. “It’s statistical learning,” she says. “They learn how often they hear pre before ty and ba before by.” If the sounds come up together often enough, the babies hear them as distinct words.

Petitto has begun to home in on the part of the brain that controls babbling and the early development of language. In a study reported in Science a year ago, she and her colleagues videotaped the mouths of babbling babies. They found that the babies were opening the right sides of their mouths wider than the left. Given that the left side of the brain controls the right side of the body, this suggests that babbling is mainly a left-brain activity.


 

Startfish left

Starfish right

Speech left

Speech right

When infants in the Dartmouth study were shown a moving picture of a starfish, the left and right hemispheres of their brains lit up (dark red indicates high brain activity; dark blue, low activity)—but not as much as when researchers spoke to them, saying: “Hello, baby. Are you a good baby?” The left hemisphere, which neuroscientists believe controls early speech production, lit up most brightly.

Images courtesy of Laura-Ann Petito/Dartmouth

Within the left brain, Petitto has her sights set on the planum temporale, a piece of the superior temporal gyrus, which is a chunk of brain about the size and shape of an index finger that curves over the top of the ear. The superior temporal gyrus is known to be part of the broad neural network that adults use in listening to and producing language. In studies of adults, Petitto has found that both hearing and deaf adults use the planum temporale—mainly on the left side—to process syllables, whether signing or speaking aloud.

The beauty of near-infrared spectroscopy is that it enables Petitto to see how babies’ brains operate while they’re awake and learning to talk. MRI scans don’t work because the babies would have to lie perfectly still. Petitto’s machine, made by Hitachi, uses weak infrared light from a laser diode, which shines through the skull and then about an inch farther into the brain. The amount of light reflected back from each region is determined by how much blood and oxygen the brain is using in that area. The more oxygen being used, the harder the brain is working.

As the machine probes Rebecca’s brain, she sits on her mother’s lap and the room goes quiet and dark. On a video screen, a young woman silently holds her right palm up flat like a traffic cop, then rhythmically rotates it—palm, back of the hand, palm, back of the hand—every second and a half.

Rebecca watches for less than a minute before starting to sigh, fidget, and kick her feet. But in that time a computer has recorded how her brain operates. The planum temporale “was clearly the part of the brain that was activated,” Petitto says, and the same was true for the 10 babies who were examined before Rebecca. So far, she says, “the data are gorgeous.”

Petitto wants to scan at least 100 babies before reaching any conclusions. Then she wants to use near-infrared spectroscopy on babies who are in the act of babbling. “I want to crack the code,” she says.

Five Stages of Baby Talk

1 PHONATION (0 to 2 months): Babies make their first sounds other than crying, often without opening their mouths. Example: a staccato hmm, hmm, timed with exhalations.

 2 PRIMITIVE ARTICULATION (1 to 4 months): Babies use their tongue and jaw to form new sounds. Examples: gleh, glechh.

 3 EXPANSION (3 to 8 months): Babies squeal, yell, or whisper, as if exploring the range of sounds, pitch, and amplitude the mouth can manage. Examples: shrieks, growls, Bronx cheers.

 4 BABBLING (5 to 10 months): Babies begin to form their first syllables. Examples: ba, ba, ba, ba or da, da, da, da, da.

 5 SOPHISTICATED BABBLING (9 to 18 months): Babies combine syllables such as ba, da, ga, mix in real words such as dada or mama, and string together meaningless sounds with the rhythm and pacing of a real sentence.

Source: D. Kimbrough Oller, University of Memphis