Abstract

A theory is presented to explain the difference between the true motion of a figure and its apparent motion, as in the Ames trapezoid illusion. Of central importance are the changes in geometric relationships between the boundaries of a figure as they project on the retina. The changes in retinal image that accompany rotation of the figure have been analyzed by use of a unique picture-plane model, to which the dimension of depth is added. The only assumption necessary to predict the perceived effect from the geometry of the illusion is that the observer will be most affected by whatever element of the retinal image is changing at the greatest rate. Apparent size, displacement, and rates of change are quantified. The interrelationship of the horizontal and vertical edges are shown. The projection of the edge of the figure farthest from the observer recedes in an opposite direction and at varying speed and size from the true edge. The lack of a perfect one-to-one relationship between the physical and psychological stimulus is determined by the nature of the projection of the physical stimulus. While other theories base their explanations on past experience, this theory designates the mechanisms underlying the illusion.

© 1966 Optical Society of America

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