Lightness and contrast illusions help illuminate the visual system

Posted by Dave on May 25, 2010 | 3 Comments

You may not have noticed that the banner artwork for this month’s topic is itself an illusion, based on a design by Stuart Anstis:

Each diamond is identical, yet each row of diamonds appears to be darker than the row above it.

I can construct a simpler version of the illusion with just two squares:

Again, the squares are identical, but the square on the left looks darker (although this illusion isn’t as strong as the diamond illusion). Why?

The reason is that our visual system actually has to do a lot of processing to create a coherent picture of the world. In order to do it needs to make a lot of assumptions. The well-known checkershadow illusion can help you see why:

This illusion, designed by Edward Adelson, shows how our perception of lightness is affected by context. The squares marked A and B are identical, yet appear to be dramatically different. Why? Because our visual system is doing something much more complex than simply comparing absolute color values. In fact, it automatically adjusts for the amount of light it guesses must be falling on an object in order to come to a better assessment of where one object ends and the next one begins.

Unless you’re solving optical puzzles, the important thing to note about the image above is not that A and B are actually the same shade of gray, but that the whole image represents two objects: a green cylinder and a gray-and-white checkerboard. Our visual system automatically accounts for the fact that dark square A is illuminated and light square B is in a shadow, so as to build a coherent representation of the object. In everyday life, it doesn’t help us to know that the luminance of the two squares is the same.

But perceiving a “shadow” and “light” aren’t the only things we use to judge the “real” colors we see. Take a look at this image, known as the Koffka ring:

The image on the right is simply formed by splitting the left-hand image apart. The right half of the “donut” suddenly seems darker than the left half, even though its luminance is the same, and even though we don’t see it as a “real” object.

Now take a look at this:

The effect is even more dramatic, perhaps because we group the right half of the ring with the dark left-background, while grouping the left half with the lighter background on the right.

Vision scientists have systematically manipulated images like this to determine what parts of an image are more likely to be combined by the visual system. Corners and other junctions are critical, perhaps because in three-dimensional objects they often represent portions of an object that are likely to be lit differently.

By combining known effects, truly dramatic illusions can be created:

Amazingly, the four small tilted rectangles in this figure are all identical! Once again, it’s not necessary to see the object as three-dimensional, though some people do — the illusion persists either way. It can all be explained as a side effect of the way our visual system processes everyday objects. Nearly all the time, the system functions perfectly, and the real world corresponds to what we think we’re seeing. But often, an illusion can say more about how the visual system works than an accurate perception of the world.

For a really nice presentation of these and other illusions, be sure to try the demo on Edward Adelson’s web page.

Adelson, E.H. (2000). Lightness perception and lightness illusions. In The New Cognitive Neurosciences, 2nd. Ed., M. Gazzaniga, ed. Cambridge, MA: MIT Press, 339-351.

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