A grid illusion is any kind of grid that deceives a person's vision. The two most common types of grid illusions are Scintillating grid illusions and Hermann grid illusions.
The Scintillating grid illusion is an optical illusion when dots
seem to appear and disappear at the intersections of two lines crossing each
other vertically and diagonally. When a person keeps his or her eyes directly on
a single intersection, the dot does not appear. A variation of the Scintillating
illusion is the Hermann grid illusion (see section below). In the picture on the
right, a person should see white dots turn black and then turning white again
The Hermann grid illusion is an optical illusion reported by
Ludimar Hermann in 1870 while, incidentally, reading John Tyndall's Sound. It is
very similar to the Scintillating grid illusion.
Differences between the Scintillating and Hermann grid illusions
The difference between the Hermann grid illusion and the Scintillating illusion is that Scintillating illusions have dots already in place at the intersection, whereas there are no dots already in place at the intersections of Hermann grid illusions. However, since they are so similar, the two names are commonly switched around.
The cause of both Scintillating and Hermann grid illusions
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 has recently been challenged by Janos Geier (see the interactive counter example).