Young babies use their tongues to help them to hear and make sense of spoken sounds, new research has shown.
Learning to speak, which includes both producing sounds and comprehending the language of others, is arguably one of our most defining traits.
But how the brain unites the input of sounds from the ears with the execution of the complex sequence of movements of the tongue, mouth, throat and chest that are needed to learn a language in the first place isn't well understood.
Now, scientists have shown that babies experiencing sounds for the first time use their tongues to tell apart sounds that are subtly different from each other.
Working with a group of 24 six month old infants growing up in English-speaking households, University of British Columbia researcher Janet Werker and her colleagues presented the babies with a screen to look at and played them two different sounds of the letter "d" as it would be said in Hindi.
One of the sounds - "dah" - is produced by pushing the tongue against the front teeth. The other - "duh" - which is unusual in English, is produced by curling the tongue back on itself and applying the underside to the roof of the mouth. The team used Hindi sounds to ensure that the children would not be familiar with either.
The team knew from previous work that when babies of this age experience something novel they tend to look at the screen for longer.
By playing sequences of sounds containing the occasional, unusual Hindi pronunciation the team could spot that the babies were registering these sounds because they attended to the screen for significantly longer coincident with when the Hindi "duh" sounds were played.
Next, the researchers repeated the experiment but asked the accompanying parent to hold in the babies' mouths a teething toy that extended over the tongue and prevented it from moving.
When this was in place, the ability of the babies to discriminate between the two sounds dropped to no better than chance.
To prove that the babies were not merely being distracted by the presence of the teething toy, the researchers then repeated the experiment using a dummy that did not extend onto the tongue.
With this in place, the sound-discriminating abilities of the infants returned to a level equal to when the experiment was conducted without the toy being present.
This shows, Werker says, that young infants learning language sounds for the first time use their tongues to help them to make sense of the sounds that they are experiencing.
Moving their tongues to the equivalent places in the mouth that might be needed to produce the sounds that they are hearing reinforces the relevant pattern of motor actions, which in turn seems to reinforce sound discrimination.
The research is important on a number of levels. For infants prevented from moving their tongues normally, as occurs in some facial syndromes, or in ill babies on a ventilator, there may be impacts on their ability to learn their native language. Secondly, the results disclose the close relationship between how we comprehend and how we produce speech.
Perhaps, as adults, we are all making small tongue movements to mimic those speaking with us to improve intelligibility.
What the research, published this week in PNAS, doesn't reveal, however, is what every parent wants to know: should I give my baby a dummy? Although Werker suggests that she thinks any effect of a dummy is probably small.
"Babies don't normally have dummies in their mouths all the time. They can take them out and move them around, so there are ample opportunities for them to learn these sound relationships..."