Emotion category-modulated interpretation bias in perceiving ambiguous facial expressions

In contrast to prototypical facial expressions, we show less perceptual tolerance in perceiving vague expressions by demonstrating an interpretation bias, such as more frequent perception of anger or happiness when categorizing ambiguous expressions of angry and happy faces that are morphed in different proportions and displayed under high- or low-quality conditions. However, it remains unclear whether this interpretation bias is specific to emotion categories or reflects a general negativity versus positivity bias and whether the degree of this bias is affected by the valence or category of two morphed expressions. These questions were examined in two eye-tracking experiments by systematically manipulating expression ambiguity and image quality in fear- and sad-happiness faces (Experiment 1) and by directly comparing anger-, fear-, sadness-, and disgust-happiness expressions (Experiment 2). We found that increasing expression ambiguity and degrading image quality induced a general negativity versus positivity bias in expression categorization. The degree of negativity bias, the associated reaction time and face-viewing gaze allocation were further manipulated by different expression combinations. It seems that although we show a viewing condition-dependent bias in interpreting vague facial expressions that display valence-contradicting expressive cues, it appears that the perception of these ambiguous expressions is guided by a categorical process similar to that involved in perceiving prototypical expressions.