It's not unusual for optical illusions to create afterimages, ghostly reflections that show up after staring at something too long. This particular afterimage is a bit different. For one thing, it looks nothing like the original image. Also, it moves.
Discovered by game designer Hjalmar Snoep, this optical illusion of lots of tiny interlocking spirals somehow adds up to a single rotating spiral as its afterimage. These are artifacts of looking at something long enough to overstimulate the eye's photoreceptors. To deal with the onrush of information, the eye's affected cones and rods simply stop responding altogether, so that the sudden absence of the image causes the other photoreceptors to start firing normally. The disparity in response between the different photoreceptors is what creates this negative afterimage.
None of that explains how these swirls somehow combine to create one giant moving spiral. As New Scientist reports, Kyushu University's Hiroyuki Ito is currently studying such moving afterimages. His working hypothesis is that the spiral is essentially the averaged together result of different retinal cells getting tired over the length of the clip. He suggests this could be tested by figuring out the average luminosity of each pixel in the video, and how that corresponds to the ultimate afterimage.
Whatever is going on, this represents something of a new frontier for optical illusions. And, as Snoep warns on his YouTube page, this particular frontier is a dizzying one - it's best to remain safely seated for a little while after gazing upon these particular spirals.
Via New Scientist.