Visual Awareness in Binocular Rivalry Modulates Induced Pupil Fluctuations

Abstract

When a visual stimulus oscillates in luminance, pupil size follows this oscillation. Recently, it has been demonstrated that such induced pupil oscillations can be used to tag which stimulus is covertly attended. Here we ask whether this “pupil frequency tagging” approach can be extended to visual awareness, specifically to inferring perceptual dominance in Binocular Rivalry between complex stimuli. We presented two distinct stimuli, a face and a house, to each eye and modulated their luminance at 1.7 Hz either in counter-phase (180° phase shift), with a 90° phase shift or in phase (0° control). In some conditions, we additionally asked observers to attend either of the stimuli. The luminance modulation was sufficiently subtle that rivalry dynamics did not differ among these conditions, and was also indistinguishable from unmodulated presentation of the stimuli. For the 180° and the 90° phase-shifted stimuli, we found that the phase of the pupil response relative to the stimuli was modulated by perceptual dominance; that is, the relative phase depended on the stimulus the observer was aware of. In turn, this perceptually dominant stimulus could be decoded from the phase of the pupil response significantly above chance. Neither percept dependence of the phase nor significant decoding was found for the 0° control condition. Our results show that visual awareness modulates pupil responses and provide proof of principle that dominance in rivalry for complex stimuli can be inferred from induced pupil fluctuations.

Impact of conscious awareness on pupillary response to faces

“…In summary, the current study sheds light on the mechanism of pupillary responses and conscious modulation. On the one hand, we found that pupillary responses required conscious representation of visual content, which is in line with previous findings showing that they are sensitive to conscious awareness (Fahle et al, 2011; Naber et al, 2011; Schütz et al, 2018; Sperandio et al, 2018). Critically, we also found that pupillary responses reflect different processing between upright and inverted faces before the face is consciously perceived.…”

“…In binocular rivalry, the conscious representation of a stimulus usually fluctuates across time, given the retina inputs are constant. Findings show that pupillary response corresponds to the dominant percept in binocular rivalry, in which the pupil constricts when the brighter percept is dominant and dilates when the darker percept is (Fahle, Stemmler, Spang 2011;Naber, Frassle, & Einhäuser, 2011;Schütz, Busch, Gorka, Einhäuser, 2018). Furthermore, pupillary light responses are inhibited when observers are unaware of luminance changes, although the physical luminance input to the retina indeed changes during the suppression phase (Barany & Hallden, 1948;Lorber, Zuber, & Stark, 1965).…”