Above you see a circle of blue-violettish (=magenta) blobs, one of which briefly disappears, circling around.
Let your gaze rest on the central fixation cross, but observe with your “inner eye” the patches just when they disappear. With good fixation, you should see a strong greenish colour whenever the violet patch has disappeared.
When you are fixating well, after a few cycles you will actually see a rotating green spot! If your gaze is really steady, the magenta patches will disappear completely, leaving only a rotating green spot (this is easier if you reduce saturation to, e.g., ≈20%); when you then shift your gaze the magenta circle reappears.
Try the following: Increase the number of gaps from the preset of 1 to, say, 5, Again fixate the center, and observe how the greenish afterimages fades from blob to bolg.
With the colour pickers you can switch to other colours and observe the respective opponent colour. [Remember to fixate for a while because the adaptation to the previous colour subsists for several seconds.] It may surprise you that the complementary colour to red is not green (as mentioned in many textbooks), but blue-green.
The temporal presentation enhances the well-known afterimage in complementary colour.
Jeremy Hinton, the inventor, writes: “The illusion illustrates Troxler fading, complementary colours, negative after-effects, and is capable of showing colours outside the display gamut.”
I have been repeatedly asked to explain this in more detail, so here goes:
Jeremy L Hinton (jeremy dot hinton at bigfoot dot com) invented this illusion in 2005. On 2005-05-22 he sent it to me as a personal communication (in the form of an animated GIF) to be published on my site. Within a few days it was copied widely over the net.
Zaidi Q, Ennis R, Cao D, Lee B (2012) Neural locus of color after-image. Current Biology [PDF]
Robert O’Shea started a pertinent Wikipedia entry
Nice variation by Sebastiaan Mathôt
Nice variation by Blelb