Babies use their tongues to listen
Babies use their tongues not just to make sounds but also to help them to listen, scientists have shown this week. Using a teething toy to temporarily prevent young babies from moving their tongues prevented them being able to tell apart two subtly different sounds. The discovery is important because it reveals a crucial way in which our speech and language develops. So should you give your child a dummy? Chris Smith spoke with study author Janet Werker to find out more...
Janet - We brought babies of 6 months of age who were just learning English and we had them sit on their mum's lap while they were listening to sounds and having the opportunity to look at a checkerboard on a computer screen. Babies will look at a display when they hear something that sounds interesting. We had trials were the babies heard the same sound over again: 'ba ba ba' and trials where they heard two different sounds: ' ba da ba da'. And if the babies can hear the difference they look longer at the checkerboard. Now in this case we used two D's that are used in Hindi but not English, one produced by pushing the tongue on the front of the teeth: Da, and one produced by curling the tongue back and flipping the underside on the roof of the mouth: da. Now you as an adult English speaker can’t hear that difference, but it’s obvious to a Hindi adult, and importantly, it’s also obvious to a young English learning infant.
Chris - And how does the constraint of the baby’s tongue movements come into the equation?
Janet - Once we had ascertained that babies of 6 months could indeed discriminate these two Ds that were used in hindi, then we had 2 further groups of babies. We had the moms hold a flat teether in the baby's mouth, and importantly this teether prevented the baby’s tongue from moving. In the other condition we had the mum hold a dummy teether in the baby’s mouth, but it didn’t project back on the baby’s tongue.
Chris - And the rationale here is that if you’ve got something that prevents the tongue moving versus the equivalent oral stimulus - the dummy- which doesn’t prevent the tongue from moving, then they ought to be able to do the discrimination task if it involves moving their tongue around when they’ve got the dummy but when they've got the teether then that should interfere with their ability to do the task if it is important to move their tongue.
Janet - That’s correct, that's indeed what turned out. The flat teether interfered with their ability to discriminate the sounds, whereas the dummy was in their mouth they looked longer at the checkerboard when they heard the two different D sounds.
Chris - What do you think the implications are of this? How do you interpret these results?
Janet - There are two important implications of this results. One, it provides pretty strong evidence that speech perception involves more than just listening in early infancy. It also involves information from the baby’s own motor movements. And the clinical implication is that babies unable to move their tongues, through cleft palate, tongue tie, or perhaps through having ventilators in their mouth, these babies may have a deficit in processing speech and hints in learning their native language.
Chris - What about the question, which I'm sure a lot of parents are considering hearing this, whether or not to give your child a dummy?
Janet - It’s an open question whether having dummies interferes with speech perception. We think it’s unlikely however, because a dummy is not in a baby’s mouth all the time. They take it out, they move it around, they have ample opportunity to listen to speech and language even when that dummy isn’t in their mouth.