Cornsweet illusion

[2] The original version of the illusion involved a rapidly spinning black-and-white disk, painted in a way that would create the appearance of a gradient effect when in motion.

[3] An equivalent static version of illusion is composed of a gray rectangle where the left half fades to a lighter shade as it approaches a vertical center line, and the right half fades to a darker gray approaching the same line.

A far more convincing and dramatic version of the effect can be seen in the article by Purves, Lotto, and Nundy,[4] where it is presented within a quasi-realistic image of solid, illuminated objects.

[5] These writers give an explanation of this and other illusions, in which the visual system and brain are posited to generate percepts on an empirical basis that is much like a reflex.

In their words, "... [perception] accords not with the features of the retinal stimulus or the properties of the underlying objects, but with what the same or similar stimuli have typically signified in the past.

In an example of the Cornsweet illusion, the whole left half of this rectangle seems to be lighter than the right. In fact they have the same brightness, apart from the gradients in the center.
The same image as above, but the edge in the middle is hidden: the left and right part of the image appear as the same color.
The actual distribution of luminance in the picture, and the typical perception of luminance.