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Clara Colombatto, Brian Scholl; Unconscious pupillometry: Faces with dilated pupils gain preferential access to visual awareness.. Journal of Vision 2019;19(10):218. doi: https://doi.org/10.1167/19.10.218.
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Of all the information we extract when viewing others’ faces, one of the most telling is their attentional state: at a glance, we can readily determine both where others are attending, and whether they are attentive (vs. distracted) in the first place. Some of the specific cues to others’ attention are relatively obvious (such as a turned head), but others are more visually subtle. For example, attentional engagement (e.g. in the form of heightened vigilance, increased cognitive load, or emotional arousal) can cause one’s pupils to dilate. Of course, the difference between seeing someone with dilated vs. constricted pupils is visually subtle, since such stimuli differ by just a fraction of a degree of visual angle. But given that dilated pupils are meaningful signals of others’ attentional states, we wondered whether such cues might be prioritized in visual processing — even outside of conscious awareness. To find out, we used continuous flash suppression (CFS) to render invisible faces with either dilated or constricted pupils, and then we measured the time that such stimuli took to break through interocular suppression. Faces with dilated pupils broke into conscious awareness faster than did faces with constricted pupils that were otherwise equated — and a monocular control experiment ruled out response-based interpretations that did not involve visual awareness. Another experiment demonstrated that such stimulus differences only drive visual awareness when represented as pupils, per se: when the identical stimuli were presented instead as (smaller vs. larger) buttons on the actors’ shirts, there was no difference in breakthrough times (with a significant interaction). These results collectively demonstrate that pupil dilation facilitates the entry of faces into visual awareness — a case study of how visually subtle stimuli can be prioritized in visual processing when they signal socially powerful properties such as the attention of other agents.
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