The Cornsweet illusion

In the Cornsweet illusion, a central perturbation is introduced in an otherwise uniform field: from left to right, it consists of a gently increasing luminance followed by a sudden decrease in luminance, and then a gentle increase back to the original level. When looking at the whole field, what is seen is two more-or-less uniform rectangular fields side-by-side, the one on the left looking brighter and the one on the right darker. Lateral inhibition enhances the sudden transition but has little effect on the more gentle luminance gradient, so that the former dominates the overall perception.