Pupil dilation reflects the social and motion content of faces
Camille Ricou, Vivien Rabadan, Yassine Mofid, Nadia Aguillon-Hernandez, Claire Wardak · 2024
Human facial features such as the eyes, nose, and mouth, allow us to communicate with others and induce a physiological mobilization indexed by the pupil dilation. Still, the relative influence of human realism, its social content, and the amount of motion on pupillary reactivity has never been elucidated. 30 neurotypical adults aged 18 to 33 years old were recorded with a 250Hz eye tracker. We analyzed the event-related pupil dilation in response to stimuli distributed along a gradient of social salience (non-social to social, going from objects to avatars to real faces) and dynamism (static to micro to macro-motion). Pupil dilation was higher in response to social (faces and avatars) compared to non-social stimuli (objects), with surprisingly a higher response for avatars. Pupil dilation was also higher in response to macro-motion compared to static. After quantifying the amount of motion of each stimulus, we found that the higher the quantity of motion, the higher the pupil dilated, especially for avatars and objects, but only in an additive way. Overall, human realism and its social content, as well as the amount of motion, contributed additionally to physiological mobilization.