Take a look at this amazing illusion created by Arthur Shapiro (you’ll need the latest version of Flash Player to see it):
You’re looking at two donut-shaped figures whose “holes” are gradually changing color from black to white and back again. It appears that the holes are changing in an opposite pattern — when one is light, the other is dark, and so on. But if you click to remove the surrounding donuts, you’ll see that the two holes are actually changing together.
If you’re still not convinced, get a friend to help. One of you looks at the light donut and the other looks at the dark donut. Then each of you says “light” when your donut hole turns light. You’ll soon be saying “light” simultaneously!
Shapiro calls this the Contrast Asynchrony illusion, and he argues that it can tell us a lot about how the visual system works. Below is an interactive version of the illusion. You can manipulate all sorts of variables to change the way the illusion appears. There’s even more than one way to eliminate the appearance of the illusion entirely. Can you figure out what you need to do to make the illusion disappear?
Of course you can click the “antiphase” button to make the donut holes appear to change in sync, but this too is an illusion. If you remove the surrounds, you can see that the donut holes are out of sync with each other. One way to remove the appearance of the illusion entirely even when the centers are in-phase is to move either of the Color Angle Θ or Φ sliders. The amount you need to move the slider will depend on your computer monitor, but for most people about a 90-degree difference will make the centers again appear to be in-phase — as they really are.
Shapiro showed this illusion to four volunteers and systematically varied the color angle of the donuts, then allowed these observers to adjust the donut-holes themselves to nullify the illusion. With few exceptions, the observers set the center at a 90-degree shift from the surround.
But what does a “90-degree shift” in color angle mean?
It’s a reference to a way of describing the colors that the human eye is capable of seeing. In the 1980’s, psychologists and neuroscientists began to realize that they could describe colors not in terms of the literal wavelength of light, but in terms of how our eye actually responds to those colors. Color vision is made possible by the fact that most people have three different types of cones in their eye. The cones respond differently to different wavelengths of light. While you may have heard the cones described as “red, green, and blue,” they are really firing in response to long, medium, and short wavelengths of light. In addition, they fire more when the light they see is brighter, or has a higher luminance. If you combine the firing rates of the long and medium cones and then plot them against the short cones and luminance, you end up with a three-dimensional solid representing all the colors the human eye can see. Here’s a two-dimensional representation of the solid — think of it as “slices” through the solid block of colors:
The donuts in this version of the illusion are always 180 degrees apart; when the color angle of the center of the donuts is about 90 degrees from those colors, then most people don’t see the illusion.
This simple relationship between the colors used to create the illusion and the perception of the illusion has helped Shapiro come up with a model of how we perceive it. We see the illusion because our visual system relies not just on color information, but also on contrast information to make judgments about what we are seeing. When color values are plugged into Shapiro’s mathematical model, when the difference between the color angle of the surround and center equals 90 degrees, then the plot of contrast for the left (red) and the right (blue) disk becomes identical, so the centers are seen changing together, while for all other values we see them changing out of phase.
I mentioned that there’s another way to make the illusion disappear — that has to do with frequency. Set the color plane for both the centers and the surrounds to LM-S, set the angles equal, and change the amplitude to about 45 percent. If you then adjust the frequency to 3 Hz (oscillations per second), you’ll see the illusion quite readily. But if you set the frequency to 1 Hz, most people will see the disks oscillating together (especially if you’ve been staring at the illusion for a long time). The illusion peaks in effectiveness at about 3 Hz. At extremely fast oscillations the illusion becomes difficult to see just because of the speed.
Notice, too, that the illusion does not rely on a sharp edge — a blurred edge will cause the effect as well. A look at Figure 10 from Shapiro’s paper demonstrates that the effect doesn’t rely on a circle and disk configuration — you can also see it with vertical bars.
Shapiro argues that this shows the visual system relies heavily on contrast to judge colors, in addition the wavelength of the light itself. Far from being a “minor side-effect” of color vision, contrast is fundamental to the process of vision itself.
If you enjoyed this illusion, you should also take the time to visit Arthur Shapiro’s lab web site. There are several different variations of this illusion, as well as a bunch of other fascinating effects.
Shapiro, A.G. (2008). Separating color from color contrast. Journal of Vision, 8(1), 1-18.