Contributions of absolute binocular disparity, motion parallax and angular declination to absolute target localization

One accurately localizes a target in the full-cue environment using external depth cues and the ground surface. However, in the dark, a target beyond 3-4 m is often perceived at the intersection between its projection line from the eyes and an implicit curved surface (intrinsic bias). Thus, perceived target distance increases as the target’s angular declination decreases. Yet, it is unknown how angular declination information interacts with absolute binocular disparity and absolute motion parallax to determine perceived target location in the dark. We employed the blind walking-gesturing paradigm to measure judged location of a 0.2 degree target in four conditions (monocular-static, binocular-static, monocular-motion-parallax & binocular-motion-parallax). Motion-parallax was initiated by the observer laterally displacing their head and body by 0.4 m (2 cycles) while judging the target. Twelve target locations [4 distances (1.5, 3.25, 5.75, and 7.0 m) x 3 heights (0.14, 0.74 m, and eye level)] were tested. The average results (n=8) reveal judged target locations in the monocular-static condition transcribed a curvilinear profile, reflecting the intrinsic bias. In the other three testing conditions that carried binocular disparity and/or motion parallax cues, the targets at or nearer than 3.25 m were judged significantly nearer to their physical distances (more accurately) than at the intrinsic bias, whereas targets at or farther than 5.75 m were not judged significantly farther beyond the intrinsic bias. For example, for the targets at eye level, the two nearer ones (1.5 & 3.25 m) were perceived significantly nearer (more accurately) in the binocular-motion parallax than in the monocular-static condition (p<0.001 & p<0.05); whereas perceived distances of the two farther targets (5.75 & 7.0 m) were similar. This indicates absolute binocular disparity and motion parallax are effective depth cues within a 3.25 m distance range in the dark, and beyond that, angular declination influences target localization.