Pupil responses to near and far stimuli at varying fixation depths in virtual reality

The eye’s pupil typically constricts in response to onsets of stimuli, especially if the stimulus is brighter than its background. How the pupil responses to stimuli presented at different depths relative to fixation is unknown. Here we tested for the effects of the depth location of stimulus relative to fixation on pupil responses. In our experiment, observers viewed a sequence of flickering light stimuli on a head-mounted display through stereoscopic virtual reality goggles while their pupil sizes were recorded. Each stimulus was presented for 5 seconds at a different angle and eccentricity. Importantly, stimuli were presented at a fixed, virtual distance per block. We independently varied stimulus distance and fixation distance across blocks using vergence manipulations, while the visual area that the stimulus covered remain fixed across distances. The results showed that pupils responded strongest to stimuli presented in the same depth plane as fixation. Pupil responses weakened when we presented stimuli nearer or farther in depth from fixation. Pupil responses were also enhanced for some participants when fixation depth was near as compared to far, but only when stimuli were shown at the same depth as fixation. Based on these findings we conclude that stimuli that are viewed in focus in stereoscopic goggles, where distance is determined by vergence (i.e., by nasal versus temporal displacements; not by accommodation), produce strongest pupil light responses. This suggests that when the retinal position of a stimulus overlaps across eyes, interocular summation enhances pupil responses. The effect of fixation nearness on pupil responsiveness requires further exploration, while taking into account the angle of displays inside the stereoscopic goggles.