This Optical Illusion With A Gradient And Three Identical Paper Squares Is Breaking Our Brains

There are four pieces of paper involved in this crazy illusion. First, there's a large one with a black-to-white gradient printed on it. Then there's three identical little swatches of gray. 

Check out what happens when you slide one of those swatches from the black side of the gradient to the white side and then back again.

The effect at work here is fairly intuitive to understand (a bright background makes objects in the foreground appear darker, and vice versa.) But if you're curious for a more technical explanation, here are scientists Maria Pereverzeva and Scott O. Murray on the matter:

Accurate perception of surface reflectance poses a significant computational problem for the visual system. The amount of light reflected by a surface is affected by a combination of factors including the surface's reflectance properties and illumination conditions. The latter are not limited by the strength of illuminant but also include the relative placement of the light illuminating the surface, the orientation of the surface and its 3d shape, all of which result in a pattern of luminance gradients across the surface. 

[NCBI]

Here's the same effect, but with Mickey Mouse:

Here's a version with a similar effect, but with color:

And if you want to try this out for yourself, Kitaoka has a printable version on his website. 

[Akiyoshi Kitaoka via Flowing Data]