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Abstract
A series of experiments investigated distance, depth and size perception when motion parallax was isolated as the single source of distance information. Small circles of equal visual angle located on different vertical planes were viewed monocularly during head motion. Motion-parallax failed to produce absolute distance judgments that differed from those given under conditions of monocular-static viewing. With respect to relative distance (depth) sometimes the parallax viewing condition led to reports of the objects being viewed as separated in depth, but the order of the depth was often wrong.
When the display objects were placed at close distances (within the effective range of accommodation) distance judgments did differ significantly from those of monocular-static viewing. However in these conditions and those described above, perceived size was not affected by the introduction of motion-parallax. In all cases, motion of the display objects was reported in the parallax conditions.
A further experiment used such parallax-produced perceived motion to test the perceptual genuineness of the depth reports that did occur. Observers judged the path of an object that was yoked to their head motions in such a way that it moved vertically as they moved their heads side to side. Thus the motion of the object relative to stationary objects was along an oblique path. The prediction was that the vertical path should be perceived if the depth separation between the display objects was perceptually available to the observers but not otherwise. The monocular parallax condition resulted in judgments of the path as oblique while a binocular motion condition produced judgments of a vertical path. This is what is to be expected if the display objects were perceptually equidistant in the parallax condition.
A binocular control condition was used in all experiments. Depth perception was much superior and strikingly different in these binocular conditions to those testing motion parallax. The poor depth achieved, the absence of size effects, and the perceived path of motion were interpreted as meaning that the occasional depth reported under parallax conditions was due to a judgmental process that was cognitive in nature. It was suggested that motion-parallax was used by the system responsible for motion perception and/or position constancy.
