Unconscious Pupillometry: An Effect of "Attentional Contagion" in the Absence of Visual Awareness

When looking at other people, we can readily tell how attentive (or distracted) they are. Some cues to this are fairly obvious (as when someone stares intensely at you), but others seem more subtle. For example, increased cognitive load or emotional arousal causes one's pupils to dilate. This phenomenon is frequently employed as a physiological measure of arousal, in studies of pupillometry. Here, in contrast, we employ it as a stimulus for social perception. Might the human visual system be naturally and automatically engaging in "unconscious pupillometry"? We demonstrate that faces rendered invisible (through continuous flash suppression) enter awareness faster when their pupils are dilated. This cannot be explained by appeal to differential contrast, differential attractiveness, or spatial attentional biases, and the effect vanishes when the identical stimuli are presented in socially meaningless ways (e.g., as shirt buttons or facial moles). These results demonstrate that pupil dilation is prioritized in visual processing even outside the focus of conscious awareness, in a form of unconscious "attentional contagion."