Grid illusion

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

Hermann grid illusion

The Hermann grid illusion is an optical illusion reported by Ludimar Hermann in 1870 while, incidentally, reading John Tyndall's "Sound". The illusion is characterised by "ghostlike" grey blobs perceived at the intersections of a white (or light-colored) grid on a black background. The grey blobs disappear when looking directly at an intersection.

cintillating grid illusion

The scintillating grid illusion is an optical illusion discovered by E. Lingelbach in 1994, that is usually considered a variation of the Hermann grid illusion (see section below).

It is constructed by superimposing white discs on the intersections of orthogonal gray bars on a black background. Dark dots seem to appear and disappear rapidly at random intersections, hence the label “scintillating”. When a person keeps his or her eyes directly on a single intersection, the dark dot does not appear. The dark dots disappear if one is too close or too far from the image.

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. Since they are so similar, the two names are commonly used interchangeably. But the scintillating illusion does not occur with an isolated intersection, as in the case of the Hermann grid; observations suggest that a minimum of 3 × 3 evenly spaced intersections with superimposed discs are required to produce the effect. This requirement suggests the participation of global processes of the kind proposed for the linking and grouping of features in an image, in addition to local processes.

The cause of both scintillating and Hermann grid illusions

The effect of both optical illusions is commonly (and possibly falsely) 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 which respond to the presentation of stimuli in what is called a receptive field.

A retinal ganglion cell pools the inputs of several photoreceptors over an area of retina, the area in physical space to which the photoreceptors respond is the ganglion cells "receptive field". In the center of the receptive field the individual photoreceptors "excite" the ganglion cell when they detect increased luminance. The photoreceptors in the surrounding area "inhibit" the ganglion cell. 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 due to the increased inhibition.

There is strong evidence that the retinal ganglion cell theory is untenable. One alternative explanation is that the illusion is due to S1 type simple cells in the visual cortex.

References

Chaderjian, M., Price, J. M., Parks, T. E.. (2001). A global factor in the Hermann grid illusion or an artifact?. Psychonomic Bulletin & Review, 8 (1), 70-72.

Lavin E., Costal A.. (1978). Detection thresholds of the Hermann grid illusion. Vision Research, 18, 1061-1062.

Levine J., Spillmann L., Wolf E.. (1980). Saturation enhancement in colored Hermann grids varying only in chroma. Vision Research, 20, 307-313.

Oehler R., Spillman L.. (1981). Illusory color changes in Hermann grids varying only in hue. Vision Research, 21, 527-541.

Schiller, P. H., Carvey, C. E.. (2005) The Hermann grid illusion revisited. Perception, 34, 1375-1397.

Wolfe, J. M.. (1984). Global factors in the Hermann grid illusion. Perception, 13, 33-40.

ee also

* Spatial summation
* Cornsweet illusion
* Lateral inhibition
* Mach bands

External links

* [http://www.michaelbach.de/ot/lum_herGrid/index.html Refutation of classical explanation of Hermann Grid Illusion]
* [http://mathworld.wolfram.com/ScintillatingGridIllusion.html Scintillating Grid Illusion – Mathworld]
* [http://www.josef.org/scintillating.html Giant grid]
* [http://eefoof.com/image/10131 Large Hermann Grid Illusion]