Early caregiver mirroring predicts the emergence of action-unit-based facial representations in the infant brain.

Early caregiver-infant interactions play a crucial role in the development of facial expression understanding, but little is known about how early experience shapes the neural representation of facial expression information. In the adult brain, facial expressions are represented componentially, with neural activity encoding constituent facial muscle configurations, or action units (AUs), independently of global emotion categories. This may provide important functional advantages, but the developmental emergence of such a representational format is unclear. We examined whether neural representations of facial expressions become increasingly organised around AU structure across early development, and whether caregiver mirroring predicts this process. Electroencephalography (EEG) data were analysed from samples of nine-month-olds and 30-month-olds, who viewed short video clips of dynamic facial expressions. Nine-month-olds had also been observed during mother-infant interactions at two months of age. Time-resolved representational similarity analysis was used to characterise the dynamic geometry of neural representations. At 30 months, neural activity reliably encoded AU structure, whereas group-level AU encoding was not observed at nine months. However, individual differences in AU encoding at nine months were predicted by maternal mirroring of infant facial expressions at two months. This relationship was specific to direct mirroring responses, and was not explained by more general maternal responding. These findings suggest that neural representations of facial expressions become increasingly organised around constituent facial movements across early development, and that caregiver mirroring is associated with the emergence of componential facial representations. More generally, they provide initial evidence linking early embodied social interaction to the development of facial representational organisation.