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developmental stages. For example, in IDS with young infants (<12 months), communicating emotion is often more prominent than clarifying linguistic structures. In IDS with older children (>12 months), the reverse occurs, such that highlighting linguistic structure is often more prominent than communicating emotion and building social bonds. No doubt, IDS is best understood in the con- text of infant/caregiver interaction and when the needs of the child and the intentions of the caregiver are identified.
Contingency and Synchrony
Are Fundamental
IDS is recognized to be dynamic and actively shaped by
both the infant and caregiver. Contingent and synchro- nized responding between mother and infant is a core feature of IDS. Although an IDS speaking style can be simulated by an adult, IDS production is facilitated by the presence of a baby. The salience of caregiver respon- siveness is demonstrated by the finding that adults can readily identify audio recordings of IDS recorded with and without an infant present (Trehub et al., 1997).
Saint-Georges and colleagues (2013) proposed that IDS creates an interactive communication loop connecting the infant and the caregiver in a synergistic way. This idea has motivated researchers to search for physi- ological markers of enhanced synchrony during IDS. Synchronous activity has been observed in heart rate and respiration measures (McFarland et al., 2019) and gaze patterns (Santamaria et al., 2020) recorded during parent/infant interactions where IDS is commonly used.
The powerful role of dynamic social interaction is also reinforced by research showing that infants can readily learn to discriminate consonants from a foreign language in a live interaction involving IDS but not from audio- visual recordings (Kuhl et al., 2003). It is also intriguing to consider how the musical quality of IDS (which is enhanced in infant-directed singing) shapes this parent- infant synchrony, given that early music exposure affects infant brain development (Zhao and Kuhl, 2020).
The critical role of IDS contingency and synchrony is also supported by evidence that challenges on each side of the interactional loop affect the synergistic connection created via IDS. For example, from the caregiver side, mothers with depression tend to include less affective information and have smaller pitch variations when speaking to their
infants (Kaplan et al., 2001). Infants’ learning is affected when maternal depression persists over an extended period (Kaplan et al., 2011). However, infants of depressed mothers remain responsive to IDS from nondepressed fathers and the quality of IDS is soon improved when the mother’s depression is lifted (Kaplan et al., 2004). On the infant side, the preference for IDS is absent or reduced among children with autism spectrum disorder, presum- ably reflecting difficulties in processing the heightened emotional content of IDS (Kuhl et al., 2005).
New Directions
Going forward, research is moving quickly to expand our knowledge of IDS. Although we have learned a great deal about the acoustic properties of IDS, we need to learn more about the speech movements that give rise to IDS signals. This type of work is technically challeng- ing but critical for understanding exactly what caregivers are doing when they adapt their speech for their infant, especially with respect to vocal resonance properties.
Future research will also continue to build a more com- plete understanding of the social, emotional, cognitive, and linguistic benefits of IDS for the developing child. Research exploring the physiological responses of interact- ing caregivers and infants will play a central role by helping us identify and understand the contingent and synchro- nous processes that are mediated by IDS. Each new finding pushes our curiosity to a higher level. We are confident that IDS will hold the interest of infants, caregivers, and scientists for a long time and can help us understand con- ditions that compromise parent/infant connection and identify new ways to optimize infant development.
We thank Claire Ying Ying Liu for preparing Figure 2 and Sandra Trehub and the ManyBabies Consortium for sharing infant-directed speech samples.
Benders, T. (2013). Mommy is only happy! Dutch mothers’ realisa- tion of speech sounds in infant-directed speech expresses emotion, not didactic intent. Infant Behavior and Development 36, 847-862.
Bradlow, A. R., Torretta, G. M., and Pisoni, D. B. (1996). Intelligibility of normal speech I: Global and fine-grained acoustic-phonetic talker characteristics. Speech Communication 20, 255-272.
Burnham, D., Kitamura, C. M., and Vollmer-Conna, U. (2002). What’s new pussycat? On talking to babies and animals. Science 296, 1435.
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