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Construction Zone

"Reality": Construction, Deconstruction, and Reconstruction

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

  • Can you see the road sign in four different ways? If not, click here.
  • How many different ways are there to see the road sign?
  • Can you change how you see the road sign at will?
  • Which of the ways that you see the road sign is "real"?
  • Is the road sign "deceptive"?
  • What are the advantages of giving up a "reality"?


More resources
Ambiguous Quartet and accompanying article

Posted by Laura Cyckowski and Paul Grobstein 29 March 2008.



Sister 's picture

This illusion snatched my wig

This illusion snatched my wig !

Serendip Visitor's picture

I cant make both go clockwise

I cant make both go clockwise or counter clockwise, they always seem to go in opposite directions.

Phreak's picture


at first I seen it as left and right lights jumping up and down and it took a bit of effort changing my perception of them.

but a little bit of time later and I could change them with ease, I also tried seeing them as independent lights turning on an off but it just appeared as changing diagonally.

it was quite fun changing both signs to work in various ways though initially it appeared as if they were going diagonally with the right sign as a mirror image of the left sign.

James Samsell's picture

Construction Zone

At first I could see only one option. After reading the copy I could, after a few minutes of practice, could totally control the direction of the flashers. Ideation leads perception, wow.

Muhammed Ali's picture

Changing the lights

I can change the lights at my will, in anyway I want,

Light A
Light B

Corner 1 2 3 and 4

If LA begins from C1, and LB is at C3,

I can make it go any direction from there, in any order, changing it at will, and I do notice a little blip in vision each time I do that, I think thats because the brain swithces from one part to another.

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