Motion-Orientation-Depth (MOD) Visual Illusion
Volodymyr V. Ivanchenko, University of Rochester, NY (2005)

The display for MOD illusion consists of two frontoparallel transparent planes separated by a depth gap. Planes are represented by an array of oblique lines with a constant tilt and the tilt difference between planes is 90° (see fig. 1). To see the MOD illusion one needs to watch the display stereoscopically and to track the target moving in between two planes. When the target moves to the right, the near plane appears to move upward, and when the target moves to the left, the near plane appears to move downward. When the target moves upward, the near plane appears to move to the right, and when the target moves downward, the near plane appears to move to the left. The function of the target is simply to induce a certain pattern of eye movements; the target can be eliminated provided the eyes move consistently in a certain direction. With shutter glasses one can also perceive the motion of a far plane.

Explanation: The following explanation is given on the algorithmic level and cannot be regarded as a physiologically plausible mechanism. When the eyes move, the images of the planes on retinas move in the opposite direction (e.g. when the eyes move to the left, the images move to the right). We can represent this motion by a vector and assume that its exact orientation is defined by the aperture problem (that is perpendicular to a line segment, see fig. 1).Then the direction of the plane's illusory motion is simply a sum of two vectors: the vector resulting from the aperture motion in the near plane and the negative of the vector resulting from the aperture motion in the far plane. This rule can be applied to any orientations of lines or directions of motion.

Figure 1. The case of downward apparent motion. A tracked target (star) moves to the left causing the retinal image of planes to move to the right. Near plane is depicted by bold lines and far plane by regular lines; circles represent apertures and the vectors show motion signals from lines. The bold vector depicts apparent motion.

A peculiarity of the MOD illusion is that it is based on a vector sum, which suggests motion integration; in contrast, the inversion of the signal from a far plane is typical for motion segregation. Whether it is due to the failure of our perceptual system to handle multiple motions or some other phenomena remains unclear. All components of MOD illusion -- motion, orientation and a depth gap -- are indispensable (this was verified on 7 experimental subjects). This effect can be applied to studying information integration in motion perception and stereo vision.