Abstract
Identifying the intensity of other people’s emotions is critical for effective social interaction because it substantially influences adaptive responses. Although humans regularly use facial expressions to communicate variations of emotional intensity, it remains unknown what specific face movements convey this fundamental social information. Here, we address this knowledge gap using a data-driven approach that combines a novel face movement generator, reverse correlation and subjective perception to model the specific face movements that drive perceptions of high emotional intensity in the six classic emotions – ‘happy,’ ‘surprise,’ ‘fear,’ ‘disgust,’ ‘anger’ and ‘sad.’ On each experimental trial, we generated a random facial animation by randomly sampling a set of individual face movements called Action Units (AUs) and assigning random temporal dynamics (e.g., acceleration, peak latency) to each AU. Each of 60 participants (Western, 31 female, mean age = 22 years) categorized the facial animation according to one of the six emotions and rated intensity on a 5-point scale ('very weak' to 'very strong'), and completed 2400 such trials. Following the experiment, we measured the relationship between the face movements presented on each trial and the participants emotion and intensity responses using an information-theoretic analysis of Mutual Information. Our results revealed that a specific sub-set of face movements convey emotional intensity across the six emotions, characterized by expansion face movements (e.g., Mouth Stretch, AU27) amongst positive and negative emotions, and contraction face movements (e.g., Nose Wrinkler, AU9) exclusively amongst negative emotions. We further observed this pattern in a broader set of 18 complex emotions (e.g., shame, embarrassed, excited), thereby validating our results. Here, we identify for the first time the specific face movements that convey emotional intensity, and show that this fundamental social information is signaled using a latent expressive pattern of expansion and contraction face movements.