Abstract
The visual system can construct a global representation of the ground surface for use as a reference frame to localize an object according to the relative depth cues between the object and ground surface. The relative depth cues include binocular disparity, motion perspective, cast shadow and nested contact information. We investigated if binocular disparity, which is not commonly considered an effective absolute distance cue beyond 2m (Wu et al, 2004), is nevertheless critical for localizing suspended objects in the full-cue environment. Spherical targets (diameter=0.51deg) were either located on the floor, or suspended 30cm, 67cm, or 90cm above the floor, at various distances (2.73m, 4.15m, 5.51m, 6.93m) from the observer in a lighted hallway. Viewing the targets with either one or both eyes, the observers judged target location using the blind walking-gesturing task. We found that observers with normal stereoacuity judged target locations with better accuracy and precision in the binocular than in the monocular condition. Strabismic observers with poor stereoacuity performed worse than normal observers in the binocular condition, while one observer without stereopsis exhibited no binocular advantage. But regardless of viewing conditions, all observers accurately localized targets on the floor. Overall, that binocular viewing improves performances over monocular viewing of suspended targets reveals that binocular disparity plays a major role in localizing objects with reference to the ground surface. This role of binocular disparity is remarkable given that our observers were situated in a full-cue environment where other relative depth cues were accessible.