Cognitive Daily

We’ve been away from the internet for a while now, but finally have a slow connection here in rainy Prague. While in Pisa, however, we were able to avail ourselves of a unique opportunity. The favorite trick of the amateur photographers here is to create the “illusion” of their friend/family member “saving” the tower from falling down, like this, admittedly poor attempt:


I’ve neglected to instruct Nora on the proper angle to hold her fingers, and I should probably have held the camera lower.

But a different illusion is both easier to create and, I think, more impressive. A similar photo won Best visual illusion for 2007. All it requires is placing two identical photos of the leaning tower side by side. The tower on the right appears to be leaning further:


I built the illusion from a single photo I took myself with no Photoshop trickerations. Any guesses as to why this illusion is so effective? Let us know in the comments.


  1. #1 Andy
    July 10, 2007

    The left frame of the photo is our reference for the left tower, while the right side of the left tower (which slopes up to the left!) is our reference for the right tower.

  2. #2 G Barnett
    July 10, 2007

    It would have to do with the larger expanse of blue between the two pictures. I’d guess that ye olde brain is comparing angle based on relative amount of “free space” on either side of each tower image, so that when the left border of one is merged with the right border of the other, it confuses that comparison. Instead of the brain catching the faint darker vertical line that’s the true border, it gauges based on tower-to-tower and makes the right one look far closer to its right border than the left one appears, thus the lean.

    I’d bet that even 1/8″ of white space between the two would be enough to break the illusion.

  3. #3 Michael Chermside
    July 10, 2007

    G Barnett: You lose that bet. Visit the link referenced in the article (or this) to see the illusion work with whitespace in the middle.

    I’m quite impressed with this illusion. It took me several minutes before I realized what I was seeing. For the longest time, I was trying to figure out why two pictures of the tower with slightly different rotations of the camera qualified as an optical illusion. Only later did I realize it was the SAME picture twice!

  4. #4 roseindigo
    July 10, 2007

    Fascinating what our eyes do or refuse to do. Since I’m an artist I’m very well acquainted with “negative space” and looking at each picture individually as well as comparing the negative spaces in each picture I can logically see they are identical; yet the illusion remains.

  5. #5 Les Posen
    July 11, 2007

    I used this visual illusion in a presentation I gave June 6 in Montreal at the 3rd World Congress on Fear of Flying.

    I was attempting to show the virtues of using Virtual reality in treating fear of flying, as opposed to cognitive therapy which appears ineffective for substantial proportion of patients. Can you guess why I used this illusion (amongst others)?

    BTW, the same day of my presentation the Montreal Gazette published the same photo with a story about its constructors, based at McGill U.

  6. #6 lori
    July 12, 2007

    The funniest photo I’ve seen of the Tower of Pisa didn’t actually have the Tower in the photo, which was published in Photography Magazine back in the ’90s. It showed a bunch of people scattered on a lawn, seemingly doing Tai Chi moves or some such. It was just a photo taken at the Tower, with the Tower not in the frame, of a bunch of tourists who happened to be striking the “I’m holding up the Tower” pose at that moment.

  7. #7 peter
    July 12, 2007

    interestingly, the traditional design of a schooner style sailboat has the rear mast raked slightly more than the forward mast just to counteract this illusion…
    some examples for those unfamiliar with the schooner.

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