Absolute Distance Discrimination in the Intermediate Distance Mediated by Angular Declination/Elevation and Binocular Disparity Cues

Purpose: Absolute distance discrimination between two midair targets in the intermediate distance range relies on binocular disparity and angular declination/elevation depth cues. We investigated how these cues integrated relative to the background surface. Methods: Paired targets (0.2°, 12-805 ms) were presented from two target arrays between 4.50 m and 5.75 m in a 9 × 3 m light-tight room. These arrays produced paired targets with predetermined angular declination/elevation (-0.6° to +0.6°) and binocular disparity (3.01', 6.02', or 9.84'). Targets were presented in the dark or presence of a visible texture background, on the floor or ceiling. Observers (n = 13) viewed monocularly or binocularly, and reported in a two-interval forced choice procedure the interval with the nearer or lower target. Results: Both binocular disparity and angular declination/elevation cues contributed to absolute distance discrimination, although the angular cue was less used during binocular viewing. During binocular viewing, the angular declination/elevation cue's influence was smaller when a background surface was visible than in the dark. Absolute distance discrimination performance with binocular viewing improved with increased stimulus duration. The angular declination cue influenced absolute distance and height discriminations at short duration, although it was less effective for discriminating distance due to its dependence on the background surface. Conclusions: Weighted integration between the binocular disparity and angular declination/elevation cues determined absolute distance discrimination of targets in midair. Sole reliance on angular cue could cause an absolute distance discrimination error, which was corrected by binocular disparity cue. In turn, both cues rely on a well-represented background surface to function well.