A grid illusion is any kind of

grid Grid, The Grid, or GRID may refer to: Common usage * Cattle grid or stock grid, a type of obstacle is used to prevent livestock from crossing the road * Grid reference, used to define a location on a map Arts, entertainment, and media * News g ...

that deceives a person's vision. The two most common types of grid illusions are the Hermann grid illusion and the scintillating grid illusion.

Hermann grid illusion

The Hermann grid illusion is an

optical illusion Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual perception, percept that arguably appears to differ from reality. Illusions come in a wide v ...

reported by

Ludimar Hermann Ludimar Hermann (October 31, 1838 – June 5, 1914) was a German physiologist and speech scientist who used the Edison phonograph to test theories of vowel production, particularly those of Robert Willis and Charles Wheatstone. He coined the ...

in 1870. The illusion is characterized 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.

Scintillating grid illusion

The scintillating grid illusion is an

, discovered by E. and B. Lingelbach and M. Schrauf in 1994. It is often considered a variation of the Hermann grid illusion but possesses different properties. 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 to or too far from the image.

Differences between the scintillating and Hermann grid illusions

The difference between the Scintillating Grid Illusion and the Hermann Grid Illusion is that the former has dots already in place at the intersections, which is not the case for the latter. Since, at first sight, the graphs appear similar, the two illusions are occasionally confused. But the scintillating illusion does not occur with an isolated intersection, as is the case for the Hermann grid; observations suggest that a minimum of 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.

Theories

The effect of both optical illusions is often explained by a neural process called

lateral inhibition In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition disables the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction ...

. The intensity at a point in the visual system is not simply the result of a single

receptor Receptor may refer to: * Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ...

, but the result of a group of receptors which respond to the presentation of stimuli in what is called a

receptive field The receptive field, or sensory space, is a delimited medium where some physiological stimuli can evoke a sensory neuronal response in specific organisms. Complexity of the receptive field ranges from the unidimensional chemical structure of od ...

. A

retinal ganglion cell A retinal ganglion cell (RGC) is a type of neuron located near the inner surface (the ganglion cell layer) of the retina of the human eye, eye. It receives visual information from photoreceptor cell, photoreceptors via two intermediate neuron typ ...

pools the inputs of several photoreceptors over an area of the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

; the area in physical space to which the photoreceptors respond is the ganglion cell's "receptive field". In the center of a so-called on-center 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 areas of 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. For example, making the lines of the grid wavy rather than straight eliminates both the Hermann grid and scintillating grid illusions. The Baumgartner / RGC theory does not predict this outcome. Lateral inhibition theory also can not account for the fact that the Hermann grid illusion is perceived over a range of bar widths. Lateral inhibition theory would predict that decreasing the size of the grid (and therefore decreasing the amount of inhibition at the intersection) would eradicate the illusory effect. One alternative explanation is that the illusion is due to S1 type

simple cell A simple cell in the visual cortex, primary visual cortex is a cell that responds primarily to oriented edges and gratings (bars of particular orientations). These cells were discovered by Torsten Wiesel and David Hubel in the late 1950s. Such ...

s in the visual cortex.

References

External links