Effects of Convergence on Binocular and Monocular Perception of Upright in Virtual Reality

When we converge to look at a near target our eyes also typically undergo incyclotorsion where their top poles rotate towards the nose. Tilting the head triggers Ocular Counter-Roll (OCR) which causes both eyes to rotate in a direction opposite to the head. The amount of OCR is <10% of the head tilt, resulting in a tilted retinal image. Studies have reported even smaller OCR with convergence. This interaction reflects a conflict between our evolutionary older balance system and the more recent binocular and stereo system. Here we utilized Virtual Reality (VR) technology to study how our perception of upright is affected by these torsional eye movements. Subjects performed a Subjective Visual Vertical (SVV) task using the FOVE0 VR headset at three head positions: upright, 20 degrees right-ear down and 20 degrees left-ear down. While they fixated on a head-fixed central target, subjects reported whether a line originating from the target, tilted between -12 to 12 degrees, was to the left or right of world-upright. The fixation target was binocularly viewed while the stimulus line was presented either monocularly or binocularly at two vergence distances: 0.25m (near) or 1.5m (far). Near and far conditions were blocked while the other conditions were randomized. We found a mean difference in the perceived upright between the left and right eye viewing conditions across all head positions: -2.7 ± 0.3 degrees for near and -0.7 ± 0.3 degrees for far. That is, perceived upright was tilted more towards the left in the left eye viewing condition. These results show a bias in the monocular perception of upright consistent with the expected incyclotorsion at near. Our findings suggest that we may not fully account for the torsional eye position when assessing the orientation of a stimulus presented monocularly in a VR setting.