Attention in Rivalrous Perception: Novel Insights from Pupillometry

In binocular and inter-ocular grouping (IOG) rivalry, perceptual alternations are generated by competition between eyes and between patterns. Existing models predict that competition occurs at different levels of the visual system for the two types of rivalry (monocular and binocular respectively), which may be differentially affected by top-down factors. To test this, we measured the effect of attention on the two types of rivalry and we applied pupillometry, which previous work suggests to reliably index both perceptual switches and attention shifts. 38 participants tracked the perceptual dynamics of binocular and IOG rivalry, while we measured pupil diameter with an Eyelink1000. Stimuli were 3-deg white or black disks, seen through a mirror stereoscope; for binocular rivalry, each eye saw one of the disks; for IOG rivalry, each eye saw a half-white half-black disk. Both types of rivalry were tested in three conditions: no-cue and black/white-cued, where attention was to be endogenously directed to the black/white disk. Perceptual reports indicate that endogenously directing attention to either percept produced the expected dominance increment. Unexpectedly, we find that the amount of modulation is the same for binocular and IOG rivalry. Pupil diameter reliably tracked the alternation of percepts in both types of rivalry, with a systematic pupil size difference between white- and black-disk dominant phases. However, this difference was not modulated by attention – implying that the bright/dark stimulus became no stronger (brighter/darker) when it was attended, yet it stood a better chance to win the competition for visual awareness. We conclude that attention biases the competition between eyes and between stimuli in a similar way - if these competitions occur at different stages, they are equally affected by attention. Pupillometry results indicate that, surprisingly, attention can bias the competition without affecting stimulus strength, confuting a key tenet in rivalry research.