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  Figure 4. Example spectrograms of “ee” vowels showing the formant patterns produced by an infant (left) and an adult female (right).
vowels found in infant speech are acoustically distinct from those found in adult female ADS or IDS vowels. That being said, when mothers use IDS, they do their
best to alter each component of speech to approximate or converge with the properties of an infant talker.
Notably, these changes in IDS align very well with what infants like when it comes to speech. Infants not only favor IDS, they also are attracted to infant speech. In listening preference tests, young infants listened longer to vowels sounds produced by an infant over vowel sounds produced by a female adult (Masapollo et al., 2016). To measure this, researchers created vowel sounds that simulate a 6-month-old talker using a special speech synthesizer (examples are shown in Figure 4). This study also showed that infants prefer each component of infant speech; infants prefer the high voice pitch of an infant and also the high-frequency vocal resonances produced by a small-infant vocal tract. Importantly, this means that infants have a distinct preference for infant talkers; they are not simply favoring a high voice pitch that is known to be an attractive property of IDS.
It was also noted in this study (Masapollo et al., 2016) that some infants vocalized and smiled more when they lis- tened to infant vowel sounds compared with adult vowel sounds, suggesting that the strong attraction to infant speech may stimulate and reward vocal exploration in young infants. Another study (Polka et al., 2014) examined the infants’ ability to recognize the same vowel when it is produced by different talkers (e.g., man, woman, child, and infant). Including infant vowel sounds in this task made it more challenging, but it also boosted infant listening times
and recognition performance (Polka et al., 2014). It seems that infant speech sounds grab and hold infant attention in ways that help babies recognize important speech catego- ries. Overall, these findings suggest that vocal convergence in IDS may play a broader role beyond social bonding. Vocal convergence may also help the infant discover that their own vocalizations are part of this vocal social space and motivate them to explore and refine their vocal skills.
Infants’ attraction to infant speech sounds raises new ques- tions. Are mothers instinctively aware of this bias? Does this motivate them to sound more infant-like when using IDS? Then again, maybe mothers are shaping this bias by using vocal convergence? Is there an important con- nection between IDS and infant speech that will help us understand how infants acquire spoken language? These intriguing questions drive current research and promise to shed new light on the role of IDS in infant development.
Multitasking with Infant- Directed Speech
Although we have learned a great deal about the acoustic properties of IDS and how it affects infant speech pro- cessing, we are just beginning to understand how IDS impacts infant development. As noted in Vocal Pitch and Rhythmic Properties of Infant-Directed Speech, Vocal Resonance Properties of Infant-Directed Speech, and Infant-Directed Speech and Infant Speech: An Impor- tant Connection?, a range of functions for IDS has been proposed, including attracting and holding infant atten- tion, highlighting and enhancing linguistic segments and structure, communicating emotion, strengthening infant/ caregiver social bonds, and stimulating vocal exploration (Saint-Georges et al., 2013; Golinkoff et al., 2015).
It is widely agreed that IDS is a powerful multitasking tool that caregivers flexibly adapt to meet the moment- to-moment needs of their infant. This adaptability is ideal for meeting parent and infant needs but presents chal- lenges for scientific investigation. In specific contexts, the diverse functions shaping IDS are often intertwined in complex ways (Saint-Georges et al., 2013). For example, modifications that communicate positive affects can promote social bonding while also facilitating speech processing by enhancing attention.
Moreover, these different functions are not equally prominent in any given interaction or across all ages or
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