A grid illusion is any kind of grid that deceives a person's vision. The two most common types of grid illusions are the Hermann grid illusion and the scintillating grid illusion.

The image above comes from a 2000 study on the Hermann grid published in the journal Perceptions. Scientists from the Laboratory of Statistical Physics at the University of Oregon explain that we can’t see the 12 dots at the same time because of a phenomenon that occurs in our eyes.

When the white disks in a scintillating grid are reduced in size, and outlined in black, they tend to disappear. One sees only a few of them at a time, in clusters that move erratically on the page. Where they are not seen, the grey alleys seem to be continuous, generating grey crossings that are not actually present. Some black sparkling can be seen at those crossings where no disk is seen.

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