Babies' brains rehearse speech mechanics months before they utter their first words, a new study has found.
Infants can tell the difference between sounds of all languages until about 8 months of age when their brains start to focus only on the sounds they hear around them.
However, it's been unclear how this transition occurs.
The study by the University of Washington researchers in 7- and 11-month-old infants shows that speech sounds stimulate areas of the brain that coordinate and plan motor movements for speech.
The research suggests that baby brains start laying down the groundwork of how to form words long before they actually begin to speak, and this may affect the developmental transition.
"Most babies babble by 7 months, but don't utter their first words until after their first birthdays," said lead author Patricia Kuhl, who is the co-director of the UW's Institute for Learning and Brain Sciences.
"Finding activation in motor areas of the brain when infants are simply listening is significant, because it means the baby brain is engaged in trying to talk back right from the start and suggests that 7-month-olds' brains are already trying to figure out how to make the right movements that will produce words," said Kuhl.
Kuhl and her research team believe this practice at motor planning contributes to the transition when infants become more sensitive to their native language.
The results emphasise the importance of talking to kids during social interactions even if they aren't talking back yet.
"Infants' brains are preparing them to act on the world by practicing how to speak before they actually say a word," said Kuhl.
In the experiment, infants sat in a brain scanner that measures brain activation through a noninvasive technique called magnetoencephalography.
Nicknamed MEG, the brain scanner resembles an egg-shaped vintage hair dryer and is completely safe for infants.
The 57 babies each listened to a series of native and foreign language syllables such as "da" and "ta" as researchers recorded brain responses. They listened to sounds from English and Spanish.
Researchers observed brain activity in an auditory area of the brain called the superior temporal gyrus, as well as in Broca's area and the cerebellum, cortical regions responsible for planning the motor movements required for producing speech.
This pattern of brain activation occurred for sounds in the 7-month-olds' native language (English) as well as in a non-native language (Spanish), showing that at this early age infants are responding to all speech sounds, whether or not they have heard the sounds before.
In the older infants, brain activation was different.
The study was published in the journal PNAS.
