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Freezing Rotation Illusion
In the neighbouring demo you see (1) an inner rotating part (the text) placed on (2) a rotating disk made up of coloured dots. The outer disk does not rotate continuously, but ‘swings’ back and forth. What is the motion of the central text? Is is a smooth rotation? What to do If you use the ‘hide’ button (top) it becomes obvious that the central rotation is completely smooth. When the outer disk is visible, it exerts an influence on the perception of the central disk’s rotation. You can experiment with the speed of the inner and outer parts and the frequency of the swing. The buttons on the bottom right allow you to add compensatory motion to the inner part, to what is known as ‘to null’ the effect. For slow and medium speed swings, for me ≈33% counter rotation without phase shift largely nullifies the ‘freezing’. More research is necessary ;-). Comment Max Dürsteler’s lucid comments (→PDF) need no further elaboration. Sources Best Visual Illusion 2006 (and thanks to Susana Martinez-Conde for the drive to inspire us all) Max R Dürsteler’s comment (PDF) Max R Dürsteler’s version in high- and low resolution version
What to see
Or doesn’t it rather ‘freeze’ now and then, depending on the outer rotation?
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.