Ask a lawyer for advice on any problem and you'll usually get an equivocating answer like, "It depends." I used to think such vacillation must be taught in law school, but recently I've come to the conclusion that lawyers' brains are like everyone else'sthey can't help being ambiguous because that's what each of us is wired to be.
The brain's "law" of simultaneous contrast states that the appearance of an object surrounded by a different shade or hue will change in a manner that reflects the opposite of its surroundings. For instance, the small gray diamond in the images above looks lighter when surrounded by black but darker when surrounded by a lighter gray. The same gray diamond looks slightly magenta when surrounded by green, the complement of magenta. Complementary colors are those that produce white when mixed together.
This law of contrast holds in all casesexcept when it doesn't. Consider the stars in the center of the figures at right. The gray star on the left (with a black inner border) looks darker than its neighbor, which has a light border. Such violations of the law of contrast are called assimilation: Under special conditions, an object will assimilate qualities of its surroundings.
Whether assimilation truly violates neurological statutes turns on your interpretation of the law of contrast. Notice that immediately surrounding the inner borders of the two stars in Experiment 1 are contrasting patterns that may cause everything contained within them, including the stars, to appear darker and lighter. Below, see what happens when the surrounding patterns are removed.
Although the jury's still out on whether or not the neural underpinnings of contrast and assimilation are the same, there clearly is a close relationship. What color do you see in the spaces separating the red line segments in the illustration at right? Cover the red lines and you'll discover that there is nothing in the white spaces except the illusory "spreading" of red from the short line segments. Such spreading, first documented by Dario Varin of the University of Milan in 1971, is highly dependent on what surrounds the short lines whose color appears to spread (in the image below, observe the same red lines with no surrounding color or pattern). The most powerful effects occur when these surrounds are lines that pass seamlessly "through" the line segments and have hues that are complementary to that of the line segments.
Further research into the complicated relationship between contrast and assimilation/spreading may eventually lead to the formulation of more robust laws that explain how the visual system works. In the meantime, ask a neuroscientist a pointed question about that relationship and you'll get a familiar answer: "It depends."
See more contrast and assimilation illusions: www.physics.utoledo.edu/~lsa/_color/mod28.htm
A review of research on illusionary color spreading: www.psy.unipd.it/~pbressan/NCSreview97.pdf