Pupillometry and Microsaccade Responses Reveal Unconscious Processing of Face Information Under Interoc-ular Suppression

Abstract

Interocular suppression happens when a monocular stimulus is suppressed by strong competing masks from the other eye. Typically, researchers measure stimuli detection from interocular suppression or aftereffects of suppressed stimuli (e.g., priming and cueing) to reveal the visual processing outside of visual awareness. However, these psychophysical measurements only reveal the end-product of unconscious processing, not the temporal dynamics. In this work, we tested whether pupillary responses and microsaccades could be reliable indicators for revealing the dynamical unconscious processing of visual information. To demonstrate unconscious visual processing, we presented upright and inverted faces under interocular suppression, and participants were asked to press a response key as soon as they detected any part of the face. Meanwhile, their pupillary responses and eye movements were recorded by an infrared eye camera. The detection times showed the typical face inversion effect—upright faces were detected faster than inverted ones. Critically, upright faces evoked stronger pupil constriction than inverted ones roughly one second prior to the behavioral detection. On the other hand, while the microsaccade rate was similar for the upright and inverted faces, it generally decreased 500-ms before face detection. On top of these observations, we speculate that pupil constriction may be associated with a gradual unconscious accumulation of face information, presumably reflecting deeper memory processing and/or emotional arousal. In contrast, the microsaccade rate may reflect a general cognitive effort and/or decision-making process before awareness access, regardless of the content of the visual information. These pupillometry and microsaccade rate measurements represent a methodological improvement for revealing dynamical unconscious processing under interocular suppression.