Evidence suggests that facial expressions may have originated from a primitive sensory regulatory function (Susskind et al., 2008). For example, wider eye-opening in fear expressions is associated with a subjectively larger visual field and enhanced peripheral stimulus detection. Here we examined the Functional Action Resonance hypothesis (Susskind et al., 2008), predicting that these benefits for fear expressers are, in parallel, passed on to their observers by enhancing gaze directionality discrimination. To test this hypothesis, we derived schematic eye gazes by averaging across 19 individuals expressing canonical fear and disgust facial actions. Eye aperture was interpolated from wide a??feara?? to its functionally opposite, narrow a??disgusta??, and gaze direction was parametrically modulated from 0 (straight) to 0.25 degrees (left and right). The remainder of the face was removed to examine directly how expression effects on eye aperture influence gaze perception. Participants viewed a pair of eyes and made forced-choice response judgments of left vs. right gaze direction. Logistic regression revealed that accuracy increased with gaze angle and with increased eye aperture characteristic of fear expressions. This effect appears specific to eyes, and not reducible to simple geometric properties, as the discrimination enhancement did not extended to analogous rectangles (matching dimension and proportion) that were not perceived as eyes. In an additional exogenous attentional-cuing experiment, where gaze matched or mismatched the location of a target, participants responded faster to the eccentric targets with increasing eye aperture. This facilitation was correlated with increased visibility of the iris, consistent with how fear physically enhances the gaze signal. In sum, these results support the Functional Action Resonance hypothesis demonstrating links between how emotions are expressed on the face, their functional roles for the expresser, and how they influence their observer's perception and action.
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