Action units of facial expressions in emotional contagion

While research on facial expressions has traditionally focused on basic emotions, other spontaneous reactions, especially those elicited through emotional contagion, are far more common in daily social interactions, with facial movements playing a key role in conveying emotions. Here, we explore spontaneous facial expressions of laughter, yawning, and mirror pain through a contagion experiment involving 32 participants. We identified key facial action units and used landmark-based distances as morphometric features to capture their dynamics. Our analysis revealed distinct patterns that separate these expressions from each other and from neutral faces, particularly in the lower face. This work provides a curated set of action units and features for the assessment of laughter, yawning, and mirror pain expressions, offering a valuable resource for future spontaneous facial expression recognition studies.