A NEW MOTION ILLUSION RELATED TO THE APERTURE PROBLEM

A previously unreported motion illusion is described. Oblique lines th at drift smoothly on the retina in a vertical direction appear to be d isplaced laterally. The effect occurs both for moving lines under fixa tion and for stationary lines under ocular tracking of an external tar get. Orientation, length, and homogeneity of the obliques affect the m agnitude of illusory displacement. We propose that this illusion is as sociated with a misregistration of the direction of displacement occur ring, in lines slanted relative to the axis of their motion, because o f the aperture problem.