Grid illusion
Encyclopedia
Grid (spatial index)
In the context of a spatial index, a grid is a regular tessellation of a manifold or 2-D surface that divides it into a series of contiguous cells, which can then be assigned unique identifiers and used for spatial indexing purposes...
that deceives a person's vision. The two most common types of grid illusions are Hermann grid illusion and the Scintillating grid illusion.
Hermann grid illusion
The Hermann grid illusion is an optical illusionOptical illusion
An optical illusion is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source...
reported by Ludimar Hermann
Ludimar Hermann
Ludimar Hermann 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 word formant, a term of importance in modern acoustic phonetics...
in 1870. 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.
Scintillating grid illusion
Optical illusion
An optical illusion is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source...
, 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 to 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.Theories
The effect of both optical illusions is often explained by a neural process called lateral inhibitionLateral inhibition
In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition sharpens the spatial profile of excitation in response to a localized stimulus.-Sensory inhibition:...
. The intensity at a point in the visual system is not simply the result of a single receptor
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
, but the result of a group of receptors which respond to the presentation of stimuli in what is called a receptive field
Receptive field
The receptive field of a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron. Receptive fields have been identified for neurons of the auditory system, the somatosensory system, and the visual system....
.
A retinal ganglion cell pools the inputs of several photoreceptors over an area of retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
, the area in physical space to which the photoreceptors respond is the ganglion cell's "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. 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. One alternative explanation is that the illusion is due to S1 type simple cells in the visual cortex.
See also
- Spatial summation
- Cornsweet illusionCornsweet illusionThe Cornsweet illusion, also known as Craik–O'Brien–Cornsweet illusion and Craik–Cornsweet illusion, is an optical illusion that was described in detail by Tom Cornsweet in the late 1960s...
- Lateral inhibitionLateral inhibitionIn neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition sharpens the spatial profile of excitation in response to a localized stimulus.-Sensory inhibition:...
- Mach bandsMach bandsMach bands is an optical illusion named after the physicist Ernst Mach. The illusion consists of light or dark stripes that are perceived next to the boundary between two regions of an image that have different lightness gradients .-Explanation:The Mach bands effect is due to the spatial...
