Fixate on the small circle in the center of the figure, while monitoring magenta blobs around it. They are disappearing one at a time, creating a perception of clockwise motion. If you keep looking, you will notice that the gap turns into a green blob. And if you keep looking even longer this green blob will wipe all magenta ones off the screen. Congratulations! Now you see a green blob (which is not where!), while not seeing magenta blobs (which are still on the screen!).

Why do you see a green blob?

Most of the information in your brain is encoded via a push-pull system. For example, you have neurons that code for “magenta-minus-green” color contrast. In other words, they “fire” stonger when the patch is magentier than greener. There is also a complementary “green-minus-magenta” batch that likes patches that are greener than magentier.Accordingly, when you look at a magenta patch for a long time, “magenta-minus-green” neurons keep firing more than “green-minus-magenta” ones and you perceive magenta (or vice versa). And when you look at a grey screen, where magenta and green components are equal, both of these groups of neurons “fire” equally strongly. This tells your visual system that neither of this colors are stronger than the other one and you see grey.

However, having a magenta blob at particular location on your retina, “fatigues” corresponding “magenta-minus-green” neurons. So, when magenta patch disappears and the corresponding part of the screen turns grey, fatigued “magenta-minus-green” fire much less than “green-minus-magenta” ones, making you see green. This aftereffect is short lived, so that when you stop the motion the green blob would dissapear in a few seconds.

Why do magenta blobs disappear?

This is because blurry images that are far away from the point where you are fixating disappear if you keep your eyes still. This effect is called “Troxler’s fading” and is also caused by neural fatigue of “magenta-minus-green” neurons. Their fatigue means that even in the presence of a magenta blob, they can fire only as strongly as the “green-minus-magenta” ones, turning patch into grey. Moving your eyes puts magenta blobs to a different location on your retina, where corresponding neurons are not adapted and the color-contrast representation is again a correct one.

If you want to see more disappearing dots, take a look at motion-induced blindness display!