Reverse Phi Motion

Warning: this is a subtle effect. What to observe There are 4 panels: 2 bar sequences a the top, and 2 motorcycle sequences at the bottom. Ask yourself in which you perceive (nearly) smooth motion. Possible observations On the left, there is a fairly smooth motion percept for the motorcycle. The right one obviously only switches between 2 positions. Comment Let’s describe the motorcycle ride, the principles apply also to the bars. In the left row, there are only 2 different positions of the motor cycle, these form the first 2 frames. The remaining 2 frames (there are only 4) are comprised of image negatives of the first 2 frames. The left bars show a never ending, if strange rightwards motion. The right bars show a (somewhat jumpy) rightwards motion (phi phenomenon), with an obvious jump back from the last to the first bar. An improved version of this won George Mather the second place in the Illusion Contest 2005.   Sources Anstis S (1970) Phi movement as a subtraction process. Vision Res 10:1411–1430 Anstis SM, Rogers BJ (1975) Illusory reversals of visual depth and movement during changes in contrast. Vision Res 15:957–961 Anstis SM, Rogers BJ (1986) Illusory continuous motion from oscillating positive-negative patterns: implications for motion perception. Perception 15:627–640 George Mather (from there I extracted the motorcycle and simplified it further. Thanks!) These pages demonstrate visual phenomena, and »optical« or »visual illusions«. The latter is more appropriate, because most effects have their basis in the visual pathway, not in the optics of the eye. When I find the time I will expand the explanations, to the degree that these phe­nomena are really understood; any nice and thoughtful comment welcome. You may be glad to know: There are no tasteless surprises here; you know, these infantile pranks where you scrutinize a picture and suddenly an ugly face (or the opposite end) screams at you… Don’t distress yourself if you don’t see the effect described, even if trying carefully. For many il­lusions, there is a small percentage of people with perfectly normal vision who just don’t see it, for reasons currently unknown.