Illusory Facial Expressions Caused by Lighting Direction.

Our daily interactions draw on a shared language of what facial expressions mean, but accurate perception of these signals may be subject to the same challenges that characterize visual perception in general. One such challenge is that faces vary in their appearance with the context, partly due to the interaction between environmental lighting and the characteristic geometry of the human face. Here, we examine how asymmetries in lighting across the horizontal and vertical axes of the face influence the perception of facial expressions in human observers. In Experiment 1, we find that faces with neutral expression appear to bear a negatively valenced expression and appear higher in emotional arousal when lit from below—an illusion of facial expression where none really exists. In Experiment 2, we find that faces performing common emotional expressions are more often miscategorized when lit from below compared to when lit from above, specifically for angry and neutral expressions. These data show that changes in facial appearance related to illumination direction can modify visual cues relevant to social communication—and suggest that facial expression recognition in humans is partially adapted to (naturalistic) environments in which light arrives predominately from overhead. Public Significance Statement: The phenomenon that faces can appear sinister or uncanny when lit from below is one that resonates widely, traditionally exploited by children telling a ghost story around the campfire and by film makers when depicting a villain. Here, we investigate systematically how lighting direction contributes to the (apparent) emotional character of the human face. We find that illusory differences in facial expression can be perceived when comparing faces lit from above and below, and that certain facial expressions are less accurately recognized when lit from below. These results speak to the visual processes that underlie our ability to recognize another person's facial expression and the theory that our perceptual system is optimized for environments characterized by a tendency toward overhead lighting. [ABSTRACT FROM AUTHOR]