Grid illusion

[1] The illusion is characterized by "ghostlike" grey blobs perceived at the intersections of a white (or light-colored) grid on a black background.

[2][3] It is constructed by superimposing white discs on the intersections of orthogonal gray bars on a black background.

[4] The effect of both optical illusions is often explained by a neural process called lateral inhibition.

Lateral inhibition theory also can not account for the fact that the Hermann grid illusion is perceived over a range of bar widths.

One alternative explanation is that the illusion is due to S1 type simple cells in the visual cortex.