Motion tracking of parents' infant- versus adult-directed actions reveals general and action-specific modulations

Parents tend to modulate their movements when demonstrating actions to their infants. Thus far, these modulations have primarily been quantified by human raters and for entire interactions, thereby possibly overlooking the intricacy of such demonstrations. Using optical motion tracking, the precise modulations of parents' infant-directed actions were quantified and compared to adult-directed actions and between action types. Parents demonstrated four novel objects to their 14-month-old infants and adult confederates. Each object required a specific action to produce a unique effect (e.g. rattling). Parents were asked to demonstrate an object at least once before passing it to their demonstration partner, and they were subsequently free to exchange the object as often as desired. Infants' success at producing the objects' action-effects was coded during the demonstration session and their memory of the action-effects was tested after a several-minute delay. Indicating general modulations across actions, parents repeated demonstrations more often, performed the actions in closer proximity and demonstrated action-effects for longer when interacting with their infant compared to the adults. Meanwhile, modulations of movement size and velocity were specific to certain action-effect pairs. Furthermore, a 'just right' modulation of proximity was detected, since infants' learning, memory, and parents' prior evaluations of their infants' motor abilities, were related to demonstrations that were performed neither too far from nor too close to the infants. Together, these findings indicate that infant-directed action modulations are not solely overall exaggerations but are dependent upon the characteristics of the to-be learned actions, their effects, and the infant learners.