Abstract
Stereopsis relies on precise binocular alignment to compute binocular disparity and infer 3D depth. When humans tilt their heads towards the shoulder, the two eyes rotate around the lines of sight in the opposite direction of head tilt. This ocular counter roll (OCR) only partially compensates for the head tilt. The torsion induced during OCR results in a misalignment of the horizontal meridians of the two eyes, which leads to vertical disparities between the retinas. The current work sought to investigate the effect of retinal image rotation due to OCR on stereoacuity while upright. We hypothesized that these vertical disparities will result in decreased stereoacuity. To investigate this research question, we recruited 8 participants to view stereoscopic random dot ring stimuli (spanning 2° to 3.5° peripherally, duration of 200 ms) with the use of a haploscope. Subjects reported whether a stimulus with crossed and uncrossed disparities of 0.1. 0.3, 0.5, 0.7, and 0.9 arcmins appeared in front or behind a fixation target with zero disparity. The stimulus rings were rotated by ±0°, 5°, 10°, and 30° to simulate OCR. Results revealed that stereoscopic thresholds during the 30° stimulus rotation were significantly worse than the 0° stimulus rotation thresholds (t(7) = 3.00, p=0.02). The reduction in stereoacuity at the 30° stimulus rotation was not worse than what is predicted by the reduction in horizontal disparity alone (p=0.31). Stimulus rotations of 0°, 5°, and 10° were not different from one another (p>0.66). Taken together, these results indicate that the limited amount of OCR (typically less than 10° in humans for any head tilt) may be optimized for stereopsis: with more torsion than is natural for the human body, stereoacuity gets worse, while modest amounts of torsion are tolerable for stereopsis.