Direction-of-motion discrimination with complex patterns: further observations.

Moving one component of a stimulus comprising two sinusoidal gratings of the same orientation sometimes results in mistaken judgments of the direction of motion. If the component with the higher spatial frequency moves and the stimulus is presented briefly, observers report motion in the direction opposite that which actually occurs. The illusory, or backward, motion appears whether the movement producing it occurs smoothly or as a discrete jump at the midpoint of the stimulus presentation. At durations at which motion appears reversed, smooth and discrete motion are indistinguishable. Measurement of the speed of the illusory motion by a cancellation technique permits comparison with results from classical induced-motion paradigms; the classical effect, obtained with spatially separated components, is smaller but in the same direction as the errors in perceived direction of motion that we measure. We suggest that the errors in judging the direction of motion may result from interactions among motion detectors tuned to the different spatial-frequency components of the stimulus.