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Myron L. Braunstein, Zheng Bian, George J. Andersen; Discriminating between upward and downward 3-D motion from projected velocity. Journal of Vision 2010;10(7):811. doi: 10.1167/10.7.811.
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The direction of motion of an object in a 3-D scene can be ambiguous if only the projected motion path is considered. Specifically, downward motion in the projection can represent either upward or downward motion in the scene. The aim of this study was to determine whether observers could discriminate upward from downward 3-D motion from the projected velocity function alone. The displays consisted of a ball moving towards the observer, below eye level, either against a 3-D scene background or against a uniform background. The projected path of the ball was always downward and was identical across conditions. The average projected speed was also identical across conditions. The projected size changes corresponded to those that would occur for a level path in 3-D, regardless of whether upward or downward motion was simulated. Only the velocity function varied according to the simulated 3-D motion. Two displays, one simulating upward motion and one simulating downward motion, were presented successively in a paired comparison design. The independent variables were the angle between the upward and downward 3-D paths and the type of background–full scene or blank field. To avoid having the ball appear to start from a position on the ground, a cylinder was inserted in the scene and served as a platform from which the ball began its motion. We found that observers were able to discriminate upward from downward 3-D motion with projected trajectories all showing the same downward motion paths. For each background condition, accuracy was determined by the angle between the simulated upward and downward 3-D paths. Accuracy was higher with a scene background than with a uniform background. These results indicate that the projected velocity function is sufficient for discrimination of direction of 3-D motion even with motion paths that are identical in the 2-D projection.
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