Optical origins of opposing facial expression actions.

Darwin theorized that emotional expressions originated as opposing functional adaptations for the expresser, not as distinct categories of social signals. Given that two thirds of the eye's refractive power comes from the cornea, we examined whether opposing expressive behaviors that widen the eyes (e.g., fear) or narrow the eyes (e.g., disgust) may have served as an optical trade-off, enhancing either sensitivity or acuity, thereby promoting stimulus localization ("where") or stimulus discrimination ("what"), respectively. An optical model based on eye apertures of posed fear and disgust expressions supported this functional trade-off. We then tested the model using standardized optometric measures of sensitivity and acuity. We demonstrated that eye widening enhanced stimulus detection, whereas eye narrowing enhanced discrimination, each at the expense of the other. Opposing expressive actions around the eye may thus reflect origins in an optical principle, shaping visual encoding at its earliest stage-how light is cast onto the retina.