Rhythmic entrainment also structures caregivers' own cueing, enhancing their visual display of social-communicative content: caregivers increase wide-eyed positive affect, reduce neutral facial affect, reduce eye motion, and reduce blinking, all in time with the rhythm of their singing and aligned in time with moments when infants increase their eye-looking. Here we show that the simple act of infant-directed singing entrains infant social visual behavior on subsecond timescales, increasing infants' looking to the eyes of a singing caregiver: as early as 2 months of age, and doubling in strength by 6 months, infants synchronize their eye-looking to the rhythm of infant-directed singing. Entrainment is a widely observed physical phenomenon by which diverse physical systems adjust rhythmic activity through interaction. Infant-directed singing is a culturally universal musical phenomenon known to promote the bonding of infants and caregivers. These results highlight the importance of infants' brain and movement coordination to their caregiver's musical presentations, potentially as a function of musical variability. Importantly, neural coordination and rhythmic movement to playsongs were positively related to infants' expressive vocabulary at 20 months. In Study 2, infants showed more rhythmic movement to playsongs than lullabies. In Study 1, we found above-threshold neural tracking of maternal singing, with superior tracking of lullabies than playsongs, and the acoustic features of infant-directed singing modulated tracking. In both studies, we assessed children's vocabulary when they were 20 months old. In Study 2 (n = 30), we coded infant rhythmic movements during the mother's singing. In Study 1 (n = 30), we measured infant EEG and used an encoding approach utilizing ridge regressions to measure neural tracking. In total, 60 mother-infant dyads were observed during two singing conditions (playsong and lullaby). The goal of this study was to examine neural and movement responses to live and dynamic maternal singing in 7-month-old infants and their relation to linguistic development. Infant-directed singing has unique acoustic characteristics that may allow even very young infants to respond to the rhythms carried through the caregiver's voice. Quarter notes dominate-12 within each line-with a longer note (equal in duration to two quarter notes) at the end of the three main phrases (corresponding to the words “are”, “sky”, and “are”) and at the boundary between sub-phrases (corresponding to the words “star”, “high”, and “star”). Note that the first and third lines are identical and that each of the three main phrases has 14 notes. The tune has a simple structure consisting of three main phrases (lines 1, 2, and 3), each of which can be subdivided into two sub-phrases. Figure 1 presents a notated version of Twinkle, Twinkle, Little Star with numbered phrases and bars. Thus it is unclear how many of them rehearsed their songs before the onset of recording. Nevertheless, they were told to take as much time as they needed before beginning their recording. Mothers interacted with their infants prior to the recording of Twinkle, Twinkle, Little Star, sometimes by talking, sometimes by singing other songs. Obviously, conditions were different for the ID and non-ID performances. These latter singers, who remained alone in the test room, started and stopped the recording. They were asked to try, to the best of their ability, to reproduce the casual singing style they would use if they were singing on their own at home. A compari- son sample of non-mothers ( n = 10), consisting of women who were similar in age, musical back- ground, and native language, provided a single sample of Twinkle, Twinkle, Little Star. Only those who sang Twinkle, Twinkle Little Star ( n = 10) were included in the present study. Mothers were instructed to sing in their usual manner of singing to infants. Mother and infant were seated face-to-face, very close to one another, and no observer was present in the room. ID recordings were obtained from a previous study (Nakata & Trehub, 2004) in which mothers sang songs of their choice to their 6-month-old infants in the course of informal interactions under controlled conditions. performances were recorded digitally (Sony TCD-D7 DAT recorder, Sony PC-62 lapel micro- phone) in a double-walled sound-attenuating room (Industrial Acoustics).
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