A new motion illusion related to the aperture problem.

A previously unreported motion illusion is described. Oblique lines that drift smoothly on the retina in a vertical direction appear to be displaced laterally. The effect occurs both for moving lines under fixation and for stationary lines under ocular tracking of an external target. Orientation, length, and homogeneity of the obliques affect the magnitude of illusory displacement. We propose that this illusion is associated with a misregistration of the direction of displacement occurring, in lines slanted relative to the axis of their motion, because of the aperture problem.