Here’s a fun experiment. Cut out a square of bright colored paper about two or three inches across.
Hold the square about four inches from your eyes. Let some light shine on the side of the square you’re looking at, so that the color appears vibrant. Now, keeping both eyes open, look off to a scene in the distance.
You may notice that the square appears transparent in the middle but opaque at the edges. Here’s the effect simulated in Photoshop.
The reason, according to vision scientist Dr. Margaret Livingstone of Harvard, is that our color receptors respond to color borders, but they tend to disregard flat, homogenous areas of color.
Our color system codes a color area by establishing its color contrast at the edges of the area, and then fills in perceptually. Even though a tomato is colored red equally at its edges and at its center, we only get information about its color from the edges. We don’t get any direct information from the red in the center at all.
The color spills into the center of a form from the edges, an effect called "color assimilation."
The phenomenon is also illustrated by the “watercolor illusion” by Lothar Spillmann, where colors along the border of a shape seem to leak into the shape, and we perceive a tint of color throughout the shape, even if it’s not really there.
In our experiment with the colored squares, the information from the far-seeing eye takes over where the near-seeing eye is getting no data, giving preference to the scene. At the edges of the square, the color border information from the near-seeing eye takes precedence, making the square seem opaque.
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“Watercolor illusion” Lothar Spillman on Journal of Vision
Vision and Art: The Biology of Seeing by Margaret Livingstone
