Motion parallax and absolute distance.

The accuracy of absolute distance estimation based on monocular motion parallax was determined both before and after specific training. With the usual distance information eliminated, subjects either held their heads stationary, or rhythmically rotated their heads from side to side, while judging the distance of stimuli placed 4-15 ft. away. Before training, head movement produced more accurate judgments than head fixed. After only 10 training trials, accurate judgments based on motion parallax were obtained. Results with a textured background were better than results with a white background, only when the subjects were not given any direct information about motion parallax. Good results were also obtained for motion parallax relative to a near reference object (2.5 ft. away). The results indicate that motion parallax can be useful for absolute distance estimation, and they suggest that motion parallax would be useful in the underwater environment, where the usual sources of distance information are absent or distorted.