Check this out. I recently found this on a few feeds in my Google Reader setup. Stare at the 3 dots on the nose of the woman in this image for about 30 seconds. Then look at some sort of white space (a blank piece of paper, or a white background on a computer screen) and blink your eyes really fast.
Negative afterimages are caused when the eye’s photoreceptors, primarily those known as cone cells, adapt from the overstimulation and lose sensitivity. Normally the eye deals with this problem by rapidly moving small amounts (see: microsaccade), the motion later being “filtered out” so it is not noticeable. However if the color image is large enough that the small movements are not enough to change the color under one area of the retina, those cones will eventually tire or adapt and stop responding. The rod cells can also be affected by this.
When the eyes are then diverted to a blank space, the adapted photoreceptors send out a weak signal and those colors remain muted. However, the surrounding cones that were not being excited by that color are still “fresh”, and send out a strong signal. The signal is exactly the same as if looking at the opposite color, which is how the brain interprets it.
I love optical illusions, things that trick your eyes and brain. Like that spinning girl that Steve wrote about a couple of times on Neurologica. If anyone has any good ones to share, post them in the comments.