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## Contrast

Because of inhibition, the brain recognizes contrast in visual information, not  the information itself. Simultaneous Lightness Contrast is the term meaning the appearance of something being affected by the appearance of things surrounding it. There are many popular examples of this demonstrating how the eye can easily be "tricked."

Put your mouse over this image
to make the illusion disappear!
The two inner squares to the left are the same color: they reflect the same amount of light. However due to simultaneous lightness contrast the two squares do not appear the same. The squares have different lightnesses because of their surroundings: the left square is lighter than the black around it so is relatively light; the right square is darker than the white around it so is relatively light. The result is the left square appearing lighter than the right square.

 The most famous example of simultaneous lightness contrast is the Hermann grid, discovered by the popular illusionist L. Hermann in 1870. Look at any intersection in the grid, and gray dots seem to appear at the others. This is virtually the same effect as above. Between the black squares, the white lines are surrounded on two sides by black so appear very light. At the intersections, however, the white is only surrounded by black at corners. Because the contrast is smaller, the intersections appear darker (more grayish) than the lines.

## Edges

Because we can only see contrast, we rely almost entirely upon edges to see. This can easily be demonstrated by the following example:

The illusionists Craik and O'Brien discovered how edges can easily distort perception. To most people, the above image appears to be a light gray box on the left and a dark gray box on the right. Put your finger over the center line and look again. The box has turned into one single colored box!

What's happening? Neither the left nor the right side of this image is actually a single shade box - each is a gradient. The right side slowly gets darker as it gets closer to the center, and the left side slowly gets lighter. The change is so gradual that your eye does not recognize any edge so does not even notice a color change. In the middle there is a wide contrast, and the eye sees the edge. Thinking that there are not more edges, it "fills in" the left and right sides to the shades in the middle. When you put your finger over the line you remove the edge, so you only see one single-shaded box!

Above is an example of the optical system unable to see anything other than contrast. As long as the square is moving it is clearly visible. When it stops it blends into its surroundings.

## Channels

Lightness isn't the only contrast that the brain recognizes. When the brain analyzes visual signals, it notices many different aspects, called channels, of the image. Simultaneous contrast can be seen in all of the channels, just as it can be seen in lightness. In all of the below examples, place your mouse over the image to make the illusion disappear.

Simultaneous Frequency Contrast:

In these shapes, both red sections have the same size lines and the same spacing between lines. Because of the surrounding areas, the spacing of the right set of lines appears to be greater.

 Simultaneous Orientation Contrast: In both of these examples, the inner shapes are made up of vertical lines. The lines don't appear vertical because of the angled backgrounds.
 Movement Contrast: Stare at the center of this moving spiral for 1 minute then stare at another object, such as your hand. No explanation needed :-)

Other types of channels include the red, blue, and green color channels which are described in detail in Color Perception.