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.

Keywords: Attention; Consciousness; Eye movements; Vision; Visual perception.

Figure 1

Stimuli and luminance modulations used. Each row represents one presentation cycle (360° or 2π, corresponding to 0.588s at 1.7 Hz in steps of 30° (π/6)). In this example, the face image always represents the left and the house image the right eye. Note that the luminance modulation depicted here is only approximate depending on screen/printer calibration; in the actual experiment modulation was adjusted to be sinusoidal in luminance by correcting for display Gamma (γ = 2.26). Top: phase shift of 180° (π; counterphase). Middle: phase shift of 90° (π/2). Bottom: same phase (control).

Figure 2

Analysis of individual presentation cycles. Example data from one observer (luminance modulated condition at 180° phase difference; observer #3, block 2). Top: luminance modulation presented to the left (blue) and right eye (red). Middle: Raw z-standardized pupil trace. Colors indicate the percept reported by button press (blue: image in left eye, red: right eye). Bottom: F1 Fourier component of pupil signal at 1.7 Hz. Colors as in middle panel.

Figure 3

Pupillary response at 1.7 Hz (F1 component) over the duration of one presentation cycle, averaged over participants for all luminance modulation conditions and same phase control. Colors indicate reported percept (blue: left eye image, red: right eye image), shaded bands indicate +/–1 SEM.

Figure 4

Complex plane representation of F1 component, averaged across presentation cycles for each participant and luminance modulation condition. Blue circles indicate reported percept of left eye image, red circles report of right eye image. Lines connect opposing perceptual reports within each individual.