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We investigated the relative contribution of (a) perceptual (eyes and mouth visual saliency), (b) conceptual or categorical (eye expression distinctiveness), and (c) affective (rated valence and arousal) factors, and (d) specific morphological facial features (Action Units; AUs), to the recognition of facial happiness. The face stimuli conveyed truly happy expressions with a smiling mouth and happy eyes, or blended expressions with a smile but non-happy eyes (neutral, sad, fearful, disgusted, surprised, or angry). Saliency, distinctiveness, affect, and AUs served as predictors; the probability of judging a face as happy was the criterion. Both for truly happy and for blended expressions, the probability of perceiving happiness increased mainly as a function of positive valence of the facial configuration. In addition, for blended expressions, the probability of being (wrongly) perceived as happy increased as a function of (a) delayed saliency and (b) reduced distinctiveness of the non-happy eyes, and (c) enhanced AU 6 (cheek raiser) or (d) reduced AUs 4, 5, and 9 (brow lowerer, upper lid raiser, and nose wrinkler, respectively). Importantly, the later the eyes become visually salient relative to the smiling mouth, the more likely it is that faces will look happy.

This study investigated facial expression recognition in peripheral relative to central vision, and the factors accounting for the recognition advantage of some expressions in the visual periphery. Whole faces or only the eyes or the mouth regions were presented for 150 ms, either at fixation or extrafoveally (2.5° or 6°), followed by a backward mask and a probe word. Results indicated that (a) all the basic expressions were recognized above chance level, although performance in peripheral vision was less impaired for happy than for non-happy expressions, (b) the happy face advantage remained when only the mouth region was presented, and (c) the smiling mouth was the most visually salient and most distinctive facial feature of all expressions. This suggests that the saliency and the diagnostic value of the smile account for the advantage in happy face recognition in peripheral vision. Because of saliency, the smiling mouth accrues sensory gain and becomes resistant to visual degradation due to stimulus eccentricity, thus remaining accessible extrafoveally. Because of diagnostic value, the smile provides a distinctive single cue of facial happiness, thus bypassing integration of face parts and reducing susceptibility to breakdown of configural processing in peripheral vision.