Hsin-I Liao¹ (hsini.liao@ntt.com), Yung-Hao Yang²; ¹NTT Communication Science Laboratories, NTT, Inc., ²Graduate School of Informatics, Kyoto University

Pupil light response (PLR) is traditionally considered a reflexive function of adjusting retinal luminance. However, recent studies indicate that this low-level response also reflects the subjective conscious perception or attended brightness. This shift means the question of whether the pupil responds to brightness when the stimulus is invisible is no longer straightforward, but rather perplexing. The current study addressed the issue by presenting visual stimuli with different luminance levels under interocular suppression. In Experiment 1, human participants discriminated, or guessed, the luminance (dark or bright) or orientation (clockwise or counterclockwise) of an invisible Gabor patch. Experiment 2 involved presenting two invisible Gabor patches, one black and one white, side by side, while attention was directed to one location using a rapid serial visual presentation (RSVP) task. Experiment 3 featured the simultaneous presentation of one visible and one invisible Gabor patch, with participants judging whether the two patches had the same or different luminance. We adopted both subjective visibility and objective judgment criteria to exclude ineligible trials and participants who showed potential awareness of the suppressed stimuli. The results showed a reliable, but attenuated, PLR to invisible brightness compared to the visible control. Similar amounts of PLR were found regardless of whether the participant focused on luminance or orientation. Crucially, paying attention to the invisible stimulus’s location did not induce PLR, unlike in the visible condition. Finally, when there was competition between the invisible and visible stimuli, invisible brightness affected PLR only when the visible stimulus was bright but not when it was dark. The overall results suggest that PLR is modulated by visual awareness. While PLR to invisible brightness remains intact and resistant to attention modulation, the response varies depending on the visible brightness. This highlights a dissociation between the mechanisms underlying awareness-sensitive and attention-modulated components of the PLR.