The newborns slept peacefully, cradled in the arms of their mothers, while researchers gently attached 128 electrodes to their tiny heads.
Each infant then heard two recordings, one of their mom pronouncing a single syllable and one of a female nurse making the same sound. But the reaction in their brains - as recorded by the electrodes - was completely different, scientists at the University of Montreal discovered.
Mom's voice activated circuitry in the left hemisphere that is associated with language processing as well as the area that controls the movement of the mouth during speech, said University of Montreal psychologist Maryse Lassonde. The nurse's voice activated the right side of the brain, which is associated with voice recognition.
The 16 newborns were tested within 24 hours of their birth, and the distinct way they responded to such similar sounds suggest that mothers jump-start the process of learning language in their babies. The work, published this week in the journal Cerebral Cortex, adds to growing understanding of how infants start to learn language from the minute they are born.
"It is amazing what babies can do," says Dr. Lassonde, who also works at the Saint-Justine University Hospital Research Centre.
Previous research has shown that when babies hear an "A" for example, they will make the mouth shapes needed to imitate the sound, even if it is the first time they have heard it spoken.
"The baby hears the sound and tries to reproduce it," said Dr. Lassonde. This explains why the brain circuitry that controls the mouth and tongue during speech was active when each infant listened to the voice of his or her mother.
Dr. Lassonde and her colleagues are now doing a similar experiment to see if preterm babies respond in the same way. She is also interested in the role of fathers in helping their newborns learn language.
One of the babies who took part in the experiment was her granddaughter, Maika Ptito-Brisson. Her father's voice triggered a huge response in her brain, said Dr. Lassonde, but it will take another experiment to see if this is the case with other infants.
The two recordings the babies heard lasted less than a second, the time it takes to pronounce a short "A" vowel sound. But the response was dramatic, even if the newborns were dozing.
It wasn't painful for them to have the electrodes put on their heads, Dr. Lassonde said. Each one had a small sponge on its end, which became sticky when dipped in lukewarm water.
The babies stayed in the arms of their mothers the entire time, and their heartbeats and temperature were carefully monitored.
The work builds on earlier discoveries that babies have an innate capacity for learning language. Newborns prefer hearing speech to non-speech sounds and can discriminate words that convey meaning on their own - nouns and verbs - from those that don't, such as articles and conjunctions.
At six months, they recognize sounds from many languages, not just the ones spoken by their parents. They lose that ability before they turn one, unless they hear a language consistently.
Just as babies seem built to learn language, mothers and fathers also seem naturally to change the way they speak to their infants.
"Motherese," as it is called, is high-pitched, slow-motion and repetitive, but research suggests it is what babies need to hear.
To make sure the infants in the study weren't simply responding to "motherese," Dr. Lassonde and her colleagues asked a nurse who has children to make the same "A" sound as the mothers. The moms also met the nurse several times before they gave birth, so their infants heard the nurse's voice while they were in the womb.