As graphic designers we tend to consider the visual world to be a pretty objective fact: it looks the way it really is, and it looks the same way to everyone, more or less. The problem is that human vision has a built-in problem with ambiguity. From visual data alone, a very large tiger some distance away is identical to a fairly small tiger closer to us. This is a rather critical distinction, so we rely on other data to “fill in the picture”: the sound of tiger roar, the sense of being in our bodies, which is called proprioception by the way, and most importantly our memory of similar situations. It is no exaggeration to state that we see what we think is there.
The Cornsweet illusion, named after Tom Cornsweet who described it in the 1960s, demonstrates very elegantly how past experience overrides present sensation. If we look at the two tiles (above) we see a dark tile above and a white tile below. Specifically, the flat surface of the upper tile is noticeably darker than the flat surface of the lower tile.
In fact the two grays are identical, as can be shown by sampling either one in Photoshop and drawing a brush stroke with that color to connect them both:
It is surprising how the illusion persists even with the connection made. It is caused by the way we perceive changes across visual gradients, like the shaded edges of the tiles, and is a learned interpretation of the retinal data. It’s the kind of thing that can get digital artists really excited. It suggests effects that can be exploited to generate visual interest, or avoided where they might cause ambiguity, for example in information design.
In the words of photographer Diane Arbus, if you don’t believe me, go out and look for yourself. The most important tool for a graphic designer is pair of open eyes.
You can learn more about the Cornsweet effect in this article, http://www.americanscientist.org/issues/num2/2002/3/why-we-see-what-we-do/1 from which the tiles illustration is taken.