Theories
The effect of both optical illusions is often 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 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.
Read more about this topic: Grid Illusion
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