The black dots in the corners of the black boxes ar so clearly visible that you could almost believe that they are printed on paper. The illusion was constructed in 1870 of the German physiologist Ludimar Herrman. He had been thinking about the small shadows he had seen between words and lines of text while he was reading.

In this case it is not the brain but the photoreceptor cells in the eye that creates the illusion.

In the retina there are numerous photoreceptors side-by-side. Each cell measures light intensity in a specific dot in the field of vision. Like all other human sensory cells photoreceptors has a tendency to reduce their output signal when they are activated in the same time as their neighbors. For instance, a single cell will reduce their activity if you look at a white surface. If not, the massive signal from all cells would overload the brain.

The cells that "sees" the white dot in the intersection of the gray lines are on four sides surrounded by cells that sees a gray line. That makes them reduce their activity and send a weaker signal to the brain than the cells that see the gray lines, that on two sides are surrounded with cells that see the black squares. When the brain receives a weaker signal of white light will they actually be interpreted as darker than the gray line.

The effect is strongest in the peripheral vision. When you try to focus on a black dot it will turn white again.