Babies who are given dummies are at a greater risk of delayed speech development, according to new research.
The findings, which were published in the Proceedings of the National Academy of Sciences, claims to be the first to discover a direct link between oral-motor movements and speech perception.
Babies in the study were unable to make distinctions between letters when they had a dummy in their mouth, which led the researchers to conclude babies need to move their tongues freely to help develop speech.
Professor Janet Werker, of British Columbia University in Canada, said according to the Telegraph: "The freedom to make small gestures with their tongue and other articulators may be an important factor in babies’ perception of sounds."
In the study, teething toys were placed in the mouths of English-learning six-month-old babies while they listened to speech sounds.
They listened to the different Hindi 'd' sounds - there are two different 'd' sounds in the Hindi language.
When the babies had dummies in their mouths which restricted movements of the tip of the tongue, they were unable to tell the 'd' sounds apart.
When their tongues were free to move, they were able to make the distinction between the two sounds.
Nicola Lathey, speech therapist and author of Small Talk, said she was unsurprised by the study's results.
She told HuffPost UK Parents: "If the children in the study were being asked to repeat the sounds, it stands to reason that the child wouldn't be able to articulate it because of an obstacle restricting the movement of the tongue, being the dummy.
"In order to articulate a 'd', you have to move the tip of your tongue, and you can't move it if a dummy is in your mouth."
Lathey said it could be argued it's not that the child couldn't discriminate the two 'd' sounds, but they can't articulate them because they are restricted.
However, she added that sucking dummies continuously can gradually affect speech in the long term.
"The difference between dummy suckers and thumb suckers, is children who thumb suck take their thumbs out of their mouths to talk, dummy suckers keep their dummies in their mouths," she explained.
"The tongue in the mouth gets into a bad habit - including the jaw, tongue and lips. When the dummy is in the child's mouth, none of these move very well so when the dummy is out, the articulator won't work very well.
"So when a child who has been sucking on a dummy a lot takes the dummy out, they continue to articulate sounds with the middle section of their tongue rather than the tip, because it has become used to being immobile due to sucking a dummy.
"Obviously, for a 'd' sound, the tip of the tongue is needed."
An NHS leaflet on speech and language therapy for children highlights the difficulties children can face.
It states: "A dummy prevents the tongue from moving around - tongue movements are vital for speech sounds.
"A dummy therefore may affect your child's speech and language development."
The advice states to wean a child off a dummy when they reach one-year-old.
The Public Health Agency agrees with the above. In a guidance document on speech and language therapy for children, it states dummies can reduce babbling and experimental sounds for a baby.
"When a baby or young child has a dummy in his mouth, he is less likely to copy sounds adults make or to attempt to babble and play with sounds himself. This is important in the development of speech skills."
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