Implicit Encoding of Social Trait Perceptions: Modeling Eye-Gaze, Pupillary, and Neuronal Responses to Faces

Faces convey a wealth of social information, guiding human interactions and decisions. While extensive research has examined explicit perceptions of social traits from facial appearance, how physiological responses to faces encode social perception processes remains largely unexplored. By modeling eye-gaze, pupillary, and neuronal responses to faces, we demonstrate that spontaneous physiological reactions implicitly reflect face-based social trait perceptions, independent of the ongoing task. Our findings reveal systematic encoding of social perceptions in eye-gaze patterns and pupillary responses, with increased fixation duration on the eyes reflecting perceptions of negative/neutral high-power traits (e.g., frightening) and variations in pupil size representing both valence and power dimensions of social perceptions (Preliminary Study, Study 1). Notably, in socially anxious individuals, pupil size more sensitively reflected valence perceptions, validating the concept of implicit social perception encoding (Study 1). We also demonstrate the encoding of social trait perceptions in neuronal responses. The amygdala primarily encoded valence from the mouth region, with decreased neuronal activity when viewing the mouths of faces perceived as positive (e.g., competent, extraverted) (Study 2). The hippocampus, in contrast, showed a feature-invariant encoding, with its activity increasing in response to negative/neutral high-power trait perceptions and decreasing with positive low/neutral-power trait perceptions, regardless of the facial features fixated upon (Study 2). Additionally, the hippocampus modulated eye movement planning during face evaluations. This research unveils the physiological mechanisms underlying face evaluation, offering a new perspective on how social information is implicitly encoded at multiple domains of physiological response.