Below is the scintillating grid illusion, most people will see white dots turn black and then turning white again very fast.
How many black dots can you count in the above grid?
Why do I see black dots where none exist?
The effect of the optical illusion is explained by a neural process called lateral inhibition. The intensity at a point in the visual system is not simply the result of a single receptor, but the result of a group of receptors called a receptive field. In the center of the receptive field, the receptors act excitatory on the resulting signal, and the receptors in the surrounding area act inhibitory on the signal. Thus, since a point at an intersection is surrounded by more intensity than a point at the middle of a line, the intersection appears darker. In a person's eyes, the nerve cells of the retina associate and interact with each other, which results in the illusion that there are dots, when there really aren't.
This explanation is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Grid illusion".