Few sounds are more delightful than a baby's sweet voice. But baby tones not only enrapture us. New research suggests other babies enjoy hearing them too, even preferring them to our own high-pitched cooing.
This research extends a unique body of experiments led by Linda Polka, professor at McGill University, Montreal, showing that "infant speech seems to capture and hold infant attention, sometimes prompting positive emotions."
The new study in seven-month-old babies, presented this week at the Meeting of the Acoustical Society of America in Minneapolis, found that babies' faces lit up as they spent longer listening to an infant's vowel sounds than to identical noises by a female adult.
Polka, expert in infant speech perception, believes listening to other babies could help support language development. She says it "may motivate infants to be vocally active and make it easier to evaluate their own vocalisations."
Between four and six months of age, babies start producing simple sounds like "er" with their mouth in a neutral position. They then learn to produce more distinct vowels like "ee", "oo" and "ah" involving muscle coordination. This leads eventually to babbling, when they start producing syllables like "ba ba ba."
To develop speech, babies must learn to monitor and assess their own sounds. Polka explains, "in learning to speak, they have to listen to their own output and determine whether it matches other sounds that they've heard in their environment."
But very little is known about how they perceive other baby sounds with similar acoustic properties to their own.
According to Polka, "an infant's voice is unique to the infant." Because their vocal tracts are small, and their bodies are small, the vibrations their voice creates are resonated in a very small space. Adults can't possibly reproduce that.
Collaborating with Lucie Ménard, linguistics professor and expert in speech production at the University of Quebec in Montreal, Polka's team devised a synthesiser that can create vowel sounds simulating a baby and a female adult.
In the experiments, babies are given control over how long the sounds last – when they look at a checkerboard screen the sound starts, and if they look away for two seconds it stops.
Polka says "the babies figure out fairly quickly that when they're looking at the screen they're turning the sound on, and when they look away they turn it off."
Initial experiments in four- to five-month old babies showed they preferred baby sounds over adult noises at lower pitches, but equally liked baby and adult sounds at high pitches. Polka thinks this is because they hadn't had much experience hearing those sounds – they weren't producing them yet.
However, in the new experiment, older babies who are more vocally active showed "a very distinct preference for those high resonance frequencies of the infant voice."
"So that suggests to us that they're learning about their own vocal abilities and recognising that that sound is something like that," Polka says. And because they're attracted to it, it could "draw their attention in ways that help them process speech."
Polka contemplates that "maybe there is something about babies being together that we're missing – there may be some benefits to peer contact;" that listening to other babies might "promote them to explore and play with their speech abilities."
Should parents keep cooing? "I think speaking to your baby is important," Polka says. "I'm always a bit surprised that people ask me if you should talk to your baby in this kind of interactive, baby-talk way.
"I think the main thing is to be sensitive to how your baby responds. If they like it, then you should do it."