The Hermann Grid
What Are Photoreceptors?
What Is Lateral Inhibition?
How Does The Hermann Grid Work?
How Do Mach Bands Work?
What Are Light Contrasts?
How Does White's Illusion Work?
When looking at the Hermann Grid (named after Ludimar Hermann) you probably have two questions: 1) Why are they are irridesent gray spots located at the intersections between black squares? and 2) Why do those spots disappear when you look directly at them?
The answer to those questions depends on where certain receptive fields are located and the amount of light being detected by the photoreceptors within those fields.
Using ON-Center receptive fields as an example, when your eyes are focused on the grid, except at an intersection, the dark surrounding photoreceptors counteract the activity of the light center photoreceptors.
The image below shows an example of two receptive fields placed within the grid. Within the receptive fields located at the intersection
(e.g., field A), the light photoreceptors in the center are detecting light and, therefore, are becoming active. At the
same time, some of dark photoreceptors within the surround are detecting light while others are not.
This is causing an Inhibitory Reaction (Lateral Inhibition). Light detected by the dark portion of a receptive field reduces the receptive field’s activity and because there are more dark photoreceptors detecting light, they are counteracting the signals of the light photoreceptors and are, therefore, making the area appear gray.
With the receptive fields located in between the squares, but away from the intersections (e.g., field B), there is less interference from the dark photoreceptors causing you to see the white correctly.
But why do the spots disappear when you look directly at them?
Receptive fields vary in size according to their location on the retina and gradually become smaller toward the center, or fovea, of the
eye. So, when you look directly at an intersection, the receptive fields (e.g., Field B to the left) are small enough to
completely fit between the black squares allowing the light photoreceptors to become active without the dark photoreceptors interfering
and thus eliminateing the gray dots. The gray dots only exist on the outside portions of your eye where your receptive fields
are larger.
A similar phenomenon can explain the Mach Bands illusion.