You are looking at a collection of “eggs” that are placed in front of a striped pattern. As the pattern moves from left to right across the screen, the eggs appear to swim up and down, and to change from light to dark.
Press the “Click to toggle background” button to replace the striped pattern with a stationary background. As you can see, the eggs are completely stationary.
The lever on the bottom left of the display allows you to rotate the eggs. Try rotating the eggs by about 45 degrees, and notice how the orientation of the eggs changes the pattern of the motion.
Brief comments: The motion of the eggs is surprising, but it really shouldn’t be. In the real world, we frequently encounter conditions where relative information makes stationary objects appear to move. For instance, when clouds drift by the moon, the moon may look as if it is shifting in the opposite direction (this is called “induced motion”). Also, there are motion aftereffects: if you view motion in one direction and then view a still object, the object will appear to drift in the opposite direction (here is another motion aftereffect).
Motion information from stationary objects can also be created whenever you move your eyes or your head, or whenever you move closer or further away from the stationary object. (Do stationary objects appear stationary when you move your camcorder?)
There are so many sources of motion information in our environment that I often find it surprising that anything ever appears stationary.
The motion in the swimmers illusion seems to arise from the contrast between the eggs and the background. The eggs are shaded light on top and dark on the bottom. When a light portion of the background crosses the egg, the egg appears to move upward; when a dark portion of the background crosses the egg, the egg appears to move downward. Does this type of motion sound familiar? The shift toward the point of low contrast is the same in some illusions in previous posts: the window shade illusion; Lucy in the sky; and the grouping by contrast illusion.
So, why should the contrast motion in the swimmers illusion surprise us? Motion from contrast seems to contradict an object-centered representation of the world. We have many words for objects, but not so many ways to describe the relationship between the objects and their surrounds—I am stuck with “high contrast” and “low contrast.” Perhaps the motion seems surprising because we have an impoverished ability to talk about (and consciously represent) relative stimulus information, compared to our ability to talk about objects.
The swimmers illusion was created when I was investigating effects of combining different sources of gradients in the environment (shadows on shadows). In 2007, this illusion was among the top ten in the third annual Best Illusion of the Year contest and was presented by Emily Knight, then an undergraduate in my lab.
All effects have precedence. The swimmers illusion is, in many respects, a two-dimensional version of Stuart Anstis’s “footstep illusion” (2001). Click here for a .pdf of the original footsteps article, and click here to see a demonstration of the footstep illusion.
Stuart Anstis gives extraordinarily engaging lectures that are both humorous and informative. Here is a link to his lecture entitled “Colours, Faces and Mrs. Thatcher's Bikini,” which was given December 2007 and posted by Cambridge Research Systems. You really should see the lecture. It is an excellent opportunity to watch a maestro at work and to see some absolutely fantastic new illusions.