This is probably one of the weirdest optical illusions I have seen recently, (full explanation on this page).
Go and stare at these two grids for a few minutes. Yes, a few minutes, don't rush this, it's worth it.
When you're done staring at the grids come back here.
If you look at the black-and-white grid again, you should notice a green haze around the horizontal lines, and a magenta haze around the vertical lines. The intensity of this effect varies between individuals. If you don't see this, go gaze at the colored grids for a while longer.
I know what you are thinking: this is a simple afterimage effect. If you think so, walk away from your terminal until you think the after image should be gone. Go home and try it in the morning. Then take a look. Or better, simply rotate the image. I love this effect because it's not just your eyes getting exhausted by one colour but rather something actually taking place in the brain:
The effect typically lasts for hours, or even overnight. The duration can be changed by the consumption of coffee and other psychoactive drugs. One paper found that it is stronger in extroverts than introverts, and might be a reliable test for extroversion.
The precise cause of the effect is unknown, and currently under investigation. It is not a simple case of fatigued neurons: there are neurotransmitters involved and appear to be responsible for the long-lasting nature of the effect. It probably takes place in the V1 processing stage of visual information. This is the first image processing after the signal leaves the retina in the eye. The edge detection circuits somehow become associated with the color. At this stage the processing is monocular: the images from the two eyes have not been combined. I first looked at the grid about a week ago and can still see the haze. Weird.
