|Hofstadter's Road Sign|
"I often used to drive back and forth between Eugene and San Francisco Bay on Interstate 5, usually late at night, and I have the most vivid memory of the flashing warning signs that I would frequently encounter along sections of the highway that were under repair. The signs consisted of four lights at the corner of a square, and they would alternate between two patterns...
I watched these flashing lights mesmerizedly, with the feeling that hidden in this simple-seeming pulsation there was some kind of deep perceptual ambiguity. And indeed, soon enough I realized that I could perceive such a sign in many different ways. I could see it as two lights (an upper and a lower one), each of them jumping back and forth horizontally, but out of phase with each other. I could see it as two lights (one on the left and one on the right), each of them jumping up and down vertically, but once again out of phase with each other. Or else I could it as two lights at diagonal opposite corners of a square, each light chasing the other... round and round the square, always going clockwise. Or I could flip the direction in my mind and see two diagonally opposite lights chasing each other counterclockwise.
And it became a game, or rather a challenge, for me to see how easily I could tame the flashing sign, making it do my bidding..."
Douglas Hofstadter, in Masters of Deception by Al Seckel, New York: Sterling, 2004, pp 5-6