When you look at this design, ghostly black dots appear in some of the intersections. They are not random, but always appear where you are not looking. They appear and disappear as you move your eyes over the image.

If you let your eyes rest on one dot you can make it and the nearest dots fade to white. But as soon as you move your eyes again they reappear.

As our light receptors cope with this high contrast image they are constantly switching between two states: used/excited and unused/resting. Excited receptors work as they should. But after each use, groggy receptors continue to send signals to the brain. These weak signals are received by the brain as an afterimage in the opposite color. The black dots disappear when you look at them because receptors are efficient and always ready to wake up. They love to be put to use.

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