What to notice: You are looking at two rectangles that change from dark blue to bright yellow and back again. The colors of the rectangles are always identical to each other; that is, they blink together "yellow-blue-yellow-blue, etc." But the rectangles look as if they are not modulating together; rather, they appear to "wink" asynchronously.
If you wish to convince yourself that the two rectangles have the same color, click on and drag one rectangle to place it next to the other rectangle.
Click on the button to add thin bars on the top and bottom of the rectangles. When the bars are present, the perception of alternation disappears—the rectangles appear to modulate together.
Comments: In my first post on this blog, I presented an illusion similar to this one (the contrast asynchrony), so—you may be wondering—how is this illusion different from that one? The important point from this post is to show how little it takes to kill the perception of alternation.
The rectangles appear to modulate in phase as soon as the thin bars are added to the top and bottom of the rectangles. The lever on the side of the display allows you to adjust the length of the thin bars. See how small you can make the bars and still have the rectangles appear to modulate together. In the experiments conducted in my lab (link to abstract), the rectangles always appeared to modulate together once the bars were greater than 10 minutes of visual angle, with some effect occurring at 1 minute of visual angle (that is pretty small).
The illusion “works” because it juxtaposes two sources of information: the color of the rectangles, and the color contrast of the rectangles relative to the background. The color information modulates together, but the color contrast alternates. The effect is surprising because we don’t always take into account that our visual system can process both color and color contrast. The fact that thin edges can effectively shut down the perception of contrast is curious and will be the subject of some future posts.
Lastly, I would like to point out that the contrast asynchrony display is fundamentally different from “dynamic simultaneous contrast” displays, where the appearance of the center patch changes due to induction from the surround field. I will leave this as an open topic to return to later, but I discuss the differences between the two types of displays in the introduction to my 2008 paper “Separating color from color contrast.”