Grid Illusion

A grid illusion is any of a family of grid patterns that deceive the visual system, with the two best known examples being the Hermann grid illusion and the scintillating grid illusion. In the Hermann grid illusion, first documented by the German physiologist Ludimar Hermann in 1870, ghostlike grey blobs appear to flicker at the intersections of a white grid drawn on a black background, but these phantom shapes vanish the moment the viewer looks directly at any one intersection. In the scintillating grid illusion, discovered in 1994, white discs are placed at the intersections of gray bars set against a black background, and dark dots appear to flicker in and out of existence at seemingly random intersections, again disappearing whenever one is looked at directly, or when the pattern is viewed from too close or too far away. Both illusions are commonly attributed to lateral inhibition, a neural process in which a retinal ganglion cell's response to brightness detected at its center is inhibited by activity detected by surrounding receptors, an effect that would make the brighter intersections of the grid appear artificially darkened due to greater surrounding inhibition; more recent evidence has challenged this explanation, however, since making the grid lines wavy instead of straight eliminates both illusions entirely, an outcome the lateral-inhibition theory cannot predict, leading some researchers to propose that simple cells in the visual cortex, rather than the retina alone, are responsible for the effect.