Enjoyment smiles are more often associated with the simultaneous presence of the Cheek raiser and Lip corner puller action units, and these units' activation is more often symmetric. Research on the judgment of smiles indicated that individuals are sensitive to these types of indices, but it also suggested that their ability to perceive these specific indices might be limited. The goal of the current study was to examine perceptual-attentional processing of smiles by using eye movement recording in a smile judgment task. Participants were presented with three types of smiles: a symmetric Duchenne, a non-Duchenne, and an asymmetric smile. Results revealed that the Duchenne smiles were judged happier than those with characteristics of non-enjoyment. Asymmetric smiles were also judged happier than the non-Duchenne smiles. Participants were as effective in judging the latter smiles as not really happy as they were in judging the symmetric Duchenne smiles as happy. Furthermore, they did not spend more time looking at the eyes or mouth regardless of types of smiles. While participants made more saccades between each side of the face for the asymmetric smiles than the symmetric ones, they judged the asymmetric smiles more often as really happy than not really happy. Thus, processing of these indices do not seem limited to perceptual-attentional difficulties as reflected in viewing behavior.

The perceptual-attentional limitation hypothesis posits that the confusion between emotional facial expressions of fear and surprise may be due to their visual similarity, with shared muscle movements. In Experiment 1 full face images of fear and surprise varying as a function of distinctiveness (mouth index, brow index, or both indices) were displayed in a gender oddball task. Experiment 2, in a similar task, directed attention toward the eye or mouth region with a blurring technique. The current two studies used response time and event-related potentials (ERP) to test the perceptual-attentional limitation hypothesis. While ERP results for Experiment 1 suggested that individuals may not have perceived a difference between the emotional expressions in any of the conditions, response time results suggested that individuals processed a difference between fear and surprise when a distinctive cue was in the mouth. With directed attention in Experiment 2, ERP results indicated that individuals were capable of detecting a difference in all the conditions. In effect, the current two experiments suggest that participants display difficulty in distinguishing the prototypes of fear and surprise with the eye region, which may be due to a lack of attention to that region, providing support for the attentional limitation hypothesis.

When participants are asked to identify expressed emotions from pictures, fear is often confused with surprise. The present study explored this confusion by utilizing one prototype of surprise and three prototypes of fear varying as a function of distinctive cues in the fear prototype (cue in the eyebrows, in the mouth or both zones). Participants were presented with equal numbers of pictures expressing surprise and fear. Eye movements were monitored when they were deciding if the picture was fear or surprise. Following each trial, explicit knowledge was assessed by asking the importance (yes vs. no) of five regions (mouth, nose, eyebrows, eyes, cheeks) in recognizing the expression. Results revealed that fear with both distinctive cues was recognized more accurately, followed by the prototype of surprise and fear with a distinctive cue in the mouth at a similar level. Finally, fear with a distinctive cue in the eyebrows was the least accurately recognized. Explicit knowledge discriminability results revealed that participants were aware of the relevant areas for each prototype but not equally so for all prototypes. Specifically, participants judged the eyebrow area as more important when the distinctive cue was in the eyebrows (fear–eyebrow) than when the cue was in the mouth (fear–mouth) or when both cues were present (fear–both). Results are discussed considering the attentional-perceptual and explicit knowledge limitation hypothesis.