A smile underlies the well-known recognition advantage of prototypical happy faces. However, a smiling mouth also has side effects: It biases a tendency to incorrectly judge as “happy” blended expressions with non-happy eyes (neutral, sad, etc.). This reveals interference with the processing of such mixed-smile expressions, which are otherwise ubiquitous in social settings (hence its practical importance). To account for this effect, we investigated two mechanisms: Perceptual overshadowing driven by the smile visual saliency, and categorical priming driven by the smile diagnostic value. In Experiment 1, we obtained diagnostic values for the mouth and eye regions of facial expressions of emotion. In Experiment 2, facilitation and interference effects of prime mouths on probe eyes were examined as a function of such values. In Experiment 3, overshadowing and priming were compared. Results showed, first, a high diagnostic value of the smiling mouth, followed by disgusted, sad, and angry mouths. Second, in correspondence with such values, the mouth expressions facilitated the recognition of congruent eyes. Importantly, the presence of a smiling mouth especially impaired the accurate recognition of non-happy eyes. This supports the categorical priming hypothesis. And, third, the smiling mouth still caused some (albeit limited) interference with the processing of facial information unrelated to expression (masculine/feminine appearance of the expresser). This is consistent with an overshadowing-inattentional blindness hypothesis. An alternative affective priming hypothesis is discussed.
