Unveiling Infant Surprise in a Social Context: A Multidimensional Analysis of Physiological, Behavioural, and Facial Expression Markers.

Surprise is a cornerstone of early learning, yet identifying it in preverbal infants is challenging because many of its markers are confounded with attention or social engagement. We aimed to identify indicators specific to surprise using behavioral and physiological measures. Ninety-nine infants aged 14–22 months observed five tool-use demonstrations presented either neutrally or surprisingly. We measured social gazes, facial expressions of surprise, freezing behavior, heart rate and electrodermal activity at 1- and 3-seconds post-stimulus. Bayesian mixed-effects models and factorial analyses were used to distinguish surprise from sustained attention or affect. Gaze duration toward the experimenter was the most reliable indicator of surprise: infants looked longer at the experimenter in the surprising than in the neutral condition, especially after the first demonstration. Facial expressions of surprise and freezing were higher in the surprising than in the neutral condition, but their persistence across demonstrations suggests that they indexed sustained attention rather than surprise per se. Heart rate emerged as a correlate of surprise-related arousal, mirroring the pattern observed for the duration of social gazes, whereas electrodermal activity followed a distinct profile, with larger responses in the neutral condition and associations with reduced gaze responses, interpreted as an index of frustration or fatigue. Factorial analyses provided converging evidence for this interpretation by isolating distinct clusters of measures linked to surprise and to sustained attention. Together, these findings illustrate how a multidimensional approach can disentangle surprise responses from longer-lasting attentional and affective processes, providing a robust framework for assessing emotion in infancy.