A smiling mouth expression modulates gaze appearance by enhancing how the eyes light up.

Research on composite facial expressions has shown that a smiling mouth, when combined with neutral eyes, can influence how the eyes are interpreted. From a gestalt-oriented perspective, we propose that smiling not only affects emotion recognition but also produces a fascinating, compelling visual counterpart: an enhancement of the eyes' apparent luminosity, or 'lighting up'. We tested this hypothesis in two within-subject experiments by collecting ratings of perceived eye brightness and smile intensity for whole faces, as well as for eyes and mouths presented isolated. In Experiment 1, we combined neutral eyes with neutral, slightly smiling, and fully smiling mouths. Results show that, for each stimulus, eye brightness ratings increased as a function of smile intensity. In Experiment 2, we combined neutral and Duchenne-smiling eyes with neutral and smiling mouths. Data analysis reveals that a smiling mouth significantly enhances the luminosity ratings of Duchenne-smiling eyes compared to their own perceptual baseline (individual eyes presented alone). Furthermore, a neutral mouth appears to 'dim' the brightness of Duchenne-smiling eyes. Through further pictorial demonstrations, we suggest that this phenomenon is remarkably robust, occurring in naturalistic photographic portraits, pareidolic faces and paintings. We propose that this subtle yet compelling effect represents a perceptual signature of the gaze, intensifying facial aliveness as the mouth modulates the eyes' appearance. Mirroring the relational logic of the Wollaston illusion, our findings demonstrate that the eyes and mouth operate in synergy, forming a unified expression where the visual system integrates individual features into a harmonious and living whole.