The role of temperament and socio-emotional stress regulation in 9-month-old infants’ Frontal Alpha Asymmetry: A Still-Face study

Parent–infant interactions are fundamental for socio-emotional development, yet infants differ widely in how they regulate stress during relational disruptions. This study examined behavioural and neural responses of 9-month-old infants during the Still-Face Procedure (SFP), focusing on Frontal Alpha Asymmetry (FAA) and the moderating role of temperament, assessed through the IBQ-R Very Short Form. Sixty-six mother–infant dyads participated in an EEG hyperscanning session, with infant behaviours coded for both emotionality displays and gaze direction. Repeated-measures analyses confirmed previous findings, showing increased negative emotionality and gaze avoidance during the Still-Face phase, followed by partial recovery in Reunion. Contrary to expectations, FAA did not vary significantly across episodes, nor was it directly associated with negative emotionality. However, temperament moderated the association between gaze avoidance and FAA: infants low in negative affectivity who engaged in more gaze avoidance exhibited greater left frontal activation, consistent with adaptive regulation. These effects were most pronounced during the Still-Face phase. Overall, findings indicate that FAA is not a uniform marker of infant regulation but reflects the interplay between behavioural strategies and dispositional traits, highlighting its role as a flexible, context-dependent measure.