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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.
Illusions such as these, and others, provide a window onto the way the eyes and the brain work together in creating perception. Scientists attempt to peer through this window when they propose hypotheses about how perception is accomplished. Illusions can also help us realize that our own perceptions may be limited or different from those of another person viewing the same thing.

Hermann grid illusions
Hermann grid illusions
A Hermann grid illusion. Shape position and color contrast converge
to produce the illusion of gray blobs at the intersections.
The Hermann grid illusion was first reported by Ludimar Hermann in 1870, who discovered the illusion while reading John Tyndall's On Sound. In 1872, Ewald Hering observed that inverse colors (a black grid on a white background) produced similar results. Because of this, the Hermann grid is often referred to as the "Hermann-Hering" grid.
Scintillating grid illusions
Scintillating grid illusions
An example of the scintillating grid illusion.
Dark dots seem to appear and disappear at intersections.
The scintillating grid illusion is an optical illusion discovered by Elke Lingelbach in 1994, and is usually considered a variation of the Hermann grid illusion. Lingelbach and colleagues published their findings in a 1995 article entitled "The Hermann grid and the scintillation effect".
 
 

Description of the Hermann grid illusion
The Hermann grid illusion is created with a grid of black squares upon a white background. "Ghostlike" gray figures are perceived at the intersections of the white lines. These figures disappear when one looks directly at an intersection.

Description of the Scintillating grid illusions
The scintillating grid illusion is similar in construction. Instead of white bars, however, there are gray bars with white discs inserted at each intersection. When viewers moves their eyes around the image, black dots seem to appear and disappear. The illusion is enhanced by eye movement, and decreased by moving too close or too far away from the image..

Explanation
The effect of both optical illusions is commonly explained by a neural process called lateral inhibition. Retinal cells in the eye function as light receptors. If only a single receptor is illuminated, it perceives a larger amount of light than it does when neighboring receptors are also illuminated. The illumination of receptors inhibits the firing of nearby receptors, and the effect is transmitted laterally. In the case of the Hermann grid illusion, the setup of the white bands creates a situation where there is more light surrounding the intersections than there is along the bands between intersections. Thus the region of the intersection is more inhibited, and darker spots appear. The effect is greater when the grid is viewed peripherally, since lateral inhibition works over greater distances in peripheral vision.

Scientists at MIT's Schiller Lab have disputed this widely accepted theory, proposing an alternate type of retinal functioning as an explanation for the illusion. Advocates of such alternative theories argue that the Hermann grid effect is not size dependent, works equally well with contrast reversal, and that there are misconceptions in the function of retinal cells that are assumed by the lateral inhibition theory. The proposed alternate theory, called the "S1 simple-cell theory," suggests that the illusion results from reactions within the cortex, not the retinal cells.

Both types of grid illusions, as well as related illusions, are excellent tools that help further the study of neuroscience. Scientists can use anomalies like perceptual illusions to try to understand more precisely the processes involved in vision and perception.
 New World Encyclopedia

 
 

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.

  
 
Hermann grid illusions


The classical explanation

Why do we see the dark patches?
Look at the left part of the left diagram and assume an on-center retinal ganglion cell. Its receptive field is indicated by the reddish disk. When the ganglion cell is, by chance, looking at the grating so that its centre ('+') is positioned at a crossing (left-top), there are 4 bright patches in the inhibitory surround. A ganglion cell looking at a street (left-bottom) however only gets 2 inhibitory patches, so it will have a higher spike rate then the one at the crossings. This was measured by Baumgartner (1960) in Freiburg, see picture on the right.
Why don't we see the patches when we look right at them?
Because then we direct the fovea at the crossings, and in the fovea the receptive fields are much smaller (see the small reddish disks on the right of the left figure). With such small receptive fields it obviously does not matter whether they are at the crossings or not.

 
 
The effect occurs when contrast is reversed
 
Color of smudges is defined by which set of lines is in front. On the left color lines are in front and smudges have the same color.
On the right the gray lines are in front and the smudges are darker gray.
Scintillating grid illusions
Hermann Grid, curving
When the grid lines are straight, dark patches appear in the street crossings, except the ones which you are directly looking at.
When the streets are curving, the dark patches vanish.
 
"Lighting grid illusion"
Illusory colored rays appear to run obliquely
Copyright Keizo Shimizu 2004
 
  Optical Illusions REFERENCES:
• "Hermann Grid" 1997. Illusion Works. Retrieved October 30, 2007.
• "The Neural Control of Vision" MIT. Retrieved October 30, 2007.
• "The Neural Control of Vision" MIT. Retrieved October 30, 2007.
• Hermann L (1870). "Eine Erscheinung simultanen Contrastes.". Pflügers Archiv für die gesamte Physiologie. 3: 13–15.
• Alexander, D.M.; Van Leeuwen, C. (2010). "Mapping of contextual modulation in the population response of primary visual cortex".
• Baumgartner G (1960). "Indirekte Größenbestimmung der rezeptiven Felder der Retina beim Menschen mittels der Hermannschen Gittertäuschung."
• Geier J, Bernáth L, Hudák M, Séra L (2008). "Straightness as the main factor of the Hermann grid illusion". Perception. 37 (5): 651–665.
• Geier, János (2008). "Stopping the Hermann grid illusion by sine distortion".
• Schiller, P. H., & Carvey, C. E. (2005). The Hermann grid illusion revisited. Perception-London, 34(11), 1375-1398.
• Schiller PH, Carvey CE (2005). "The Hermann grid illusion revisited.". Perception. 34 (11): 1375–97. doi:10.1068/p5447.
 
 
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