Learning to understand speech isn’t just about hearing. It may also be about movement, according to a new study from the University of British Columbia. 

Writing in the journal PNAS, audiologists and psychologists find that when infants can’t move their tongues, they aren’t able to distinguish between speech-related sounds. The 24 six-month-old infants in the study listened to two different “d” sounds used in Hindi, and were supposed to distinguish between them. (Experts settled on Hindi because previous research has found that infants as young as one month old can distinguish speech sounds, and the English-learning infants would not have learned these sounds before.)

All the infants used teething toys while they were in the lab, but half of these toys blocked the babies from touching their tongues to their palates in the way that would form a “d” sound, and half did not. The researchers assessed whether the babies distinguished between the different sounds based on how long they spent looking at a virtual checkerboard when the alternating sounds played or when just one sound played. If they stared for longer during the alternating sounds, the researchers interpreted that to mean that they could recognize the difference. 

In several different tests, babies who used a teething toy that blocked their tongue movement were not able to distinguish between the “d” sounds, while babies who could move their tongues heard the difference. This indicates that learning to understand words isn’t just an auditory phenomenon, and that a child’s motor development also impacts how well they hear speech. 

This finding may have implications for kids with a cleft palate or other features that impede their ability to move their tongues in speech-related patterns, and should lead to further research on how much time kids need to spend without teething toys or other impediments with an eye towards freeing tongue movement.