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Emily A. Cooper, Johannes Burge, Martin S. Banks; Do People of Different Heights have Different Horopters?. Journal of Vision 2010;10(7):372. doi: 10.1167/10.7.372.
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© 2017 Association for Research in Vision and Ophthalmology.
Accurate perception of depth with respect to the ground is critical for walking. The most precise visual cue to depth is binocular disparity. Depth estimates from disparity are most precise for stimuli near corresponding points, pairs of retinal loci that yield the same perceived direction when stimulated. Rays from corresponding points projected into space intersect at the horopter. It would be adaptive if an upright observer's horopter lay in or near the ground. Interestingly, corresponding points deviate systematically near the retinas' vertical meridians: above the left and right foveas they are shifted rightward and leftward, respectively; below the foveas, the shift is opposite. Because of this horizontal shear, the horopter is pitched top-back. Helmholtz noted that this places the horopter near the ground for an upright observer and thereby could optimize depth perception with respect to the ground.
We asked whether people with different eye heights and separations have different shear angles, and whether those angles place the horopter in the ground for each individual. We used a dichoptic apparent-motion paradigm to measure the positions of corresponding points at different retinal eccentricities. We also measured cyclovergence to control for eye torsion and determined the effect of a structured stimulus like the natural environment on cyclovergence. We found a statistically significant, but modest, correlation between predicted and observed shear angles in 28 observers with heights ranging from 4.3 to 7 feet. Thus, corresponding points in most people place the horopter near the ground when they are standing. However, some observers' data were inconsistent with linear shear; their corresponding points yielded curved horopters that cannot be co-planar with the ground.
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