Sit and face the world: ontogenetic adaptation in infant vocal production and visual attention during the transition to independent sitting.

Background: Motor milestones are not only indicators of developmental progress, but they also open up new opportunities for infants to interact with the environment and social partners, as the development of motor, social, and language skills is tightly interconnected in infancy. This study will investigate how the transition to independent sitting relates to key areas of socio-communicative development in infancy: vocal production and visual attention.

Methods: This study addresses the relationship between sitting acquisition and social cognition skills in infancy. It will allow for comparing if infant motor development, vocalizations, and visual attention undergo developmental changes in parallel or whether they have intertwined trajectories. We will conduct a longitudinal study using a milestone-based approach to account for individual differences in relation to the timing of motor milestone acquisition. We will invite parent-infant dyads to the lab when infants are at different stages of independent sitting acquisition: non-sitting, attempting-sitting and expert-sitting. Infants' attention toward faces and toys will be measured with a wearable eye-tracker during free-flowing dyadic interactions with their caregivers. During the same interactions, infant vocalizations will also be recorded and analyzed. Additionally, screen-based eye-tracking will be used to precisely assess changes in infants' attention to the mouth area of the speaker.

Discussion: Altogether, this study will provide a unique dataset that tracks the cross-dependence of motor, visual and vocal developmental trajectories. It will have the potential to inform future studies of neurodevelopmental disorders such as autism that are characterized by socio-communicative challenges.