Some theorists believe infants enter the world with “hard-wired” neurons that are preadapted for both understanding and producing speech. Others believe that speech is learned through experience. Now research reveals how a baby’s speech centers function at five days old, then six months, then a year.

Neuroscientist Patricia Kuhl of the University of Washington, working with colleagues at the University of Helsinki in Finland, used a new technique, magnetoencephalography (MEG), to measure brain activity by sensing the magnetic fields neurons create when they fire. The results lend empirical evidence to the notion that speech is indeed learned.

“When we played three kinds of sounds—pure tones, a three-tone harmonic and the Finnish speech sounds PA and BA—for newborns, we saw activity in their auditory centers,” Kuhl says. This means they heard and could distinguish the sounds. “But there was no activity in the inferior frontal cortex,” where speech production is analyzed and mouth and throat muscles are prepared for talking.

By six months, however, the infants were activating this region when they heard either the harmonic or the speech sounds, and the one-year-olds activated both the auditory and speech-production areas simultaneously, indicating “cross-talk between the areas that hear and produce speech,” Kuhl says. Babies, she explains, need time to experiment: to make sounds, listen to them, and link what they hear to what their speech muscles are doing. Once they have figured out this process, they can start listening to and mimicking other speakers.