Space perception of strabismic observers in the real world environment

Space perception beyond the near distance range (>2 m) is important for target localization, and for directing and guiding a variety of daily activities, including driving and walking. However, it is unclear whether the absolute (egocentric) localization of a single target in the intermediate distance range requires binocular vision, and if so, whether having subnormal stereopsis in strabismus impairs one's ability to localize the target.

We investigated this by measuring the perceived absolute location of a target by observers with normal binocular vision (n = 8; mean age, 24.5 years) and observers with strabismus (n = 8; mean age, 24.9 years) under monocular and binocular conditions. The observers used the blind walking-gesturing task to indicate the judged location of a target located at various viewing distances (2.73-6.93 m) and heights (0, 30, and 90 cm) above the floor. Near stereopsis was assessed with the Randot Stereotest.

Both groups of observers accurately judged the absolute distance of the target on the ground (height = 0 cm) either with monocular or binocular viewing. However, when the target was suspended in midair, the normal observers accurately judged target location with binocular viewing, but not with monocular viewing (mean slant angle, 0.8° ± 0.5° vs. 7.4° ± 1.4°; P < 0.001, with a slant angle of 0° representing accurate localization). In contrast, the strabismic observers with poorer stereo acuity made larger errors in target localization in both viewing conditions, though with fewer errors during binocular viewing (mean slant angle, 2.7° ± 0.4° vs. 9.2° ± 1.3°; P < 0.0025). Further analysis reveals the localization error, that is, slant angle, correlates positively with stereo threshold during binocular viewing (r(2) = 0.479, P < 0.005), but not during monocular viewing (r(2) = 0.0002, P = 0.963).

Locating a single target on the ground is sufficient with monocular depth information, but binocular depth information is required when the target is suspended in midair. Since the absolute binocular disparity information of the single target is weak beyond 2 m, we suggest the visual system localizes the single target using the relative binocular disparity information between the midair target and the visible ground surface. Consequently, strabismic observers with residual stereopsis localize a target more accurately than their counterparts without stereo ability.