Eye Of The Beholder

I’ve long been puzzled by ambiguous figures, ever since I saw the famous Necker cube as a boy. What changes in the brain when the perception “flips”? (There had better be something.)

I often have trouble along these lines when I’m looking at photographs of the surface of the Moon or of Mars: depending on the direction of the shading, the craters and rilles often look like bumps rather than depressions. Here’s a good example:

With a little effort (especially if I tilt my head to the left, so the shaded of the crater rim at left is at the “top”), I can make this crater look like a shallow dome with a depressed rim. Sometimes I have to work hard to get these sorts of pictures to “pop” the right way, even when I know what they are supposed to look like.

Here’s another, somewhat different example. In this case, flashing pairs of dots are presented in diagonally opposite corners of a square or rectangle: first northeast/southwest, then northwest/southeast. We tend to see this as movement, in one of two ways: either we see the dots sliding from side to side across the top and bottom, or up and down along the sides. (I strongly favor the latter, it seems.) Try it for yourself here.


  1. Charles says

    I can get the crater photo above to switch with a little concentration, but the jumping dots had me stumped for a few minutes. Like you, I saw them jumping horizontally, and no matter how hard I concentrated I couldn’t get them to switch. Then I hit upon an idea: with my hand, I covered the bottom half of the image and slowly moved my hand up and down so I could only see one half at a time. When I removed my hand, the dots were jumping horizontally. I thought I would be able to concentrate and get them to start jumping vertically again, but I had to resort to the hand trick to do it. I guess it’s because the dots switch position too quickly for me to switch the direction by mere concentration alone.

    Posted July 19, 2008 at 9:36 am | Permalink
  2. Malcolm says

    Hi Charles,

    Did you try the other links on that page, for horizontal and vertical bias?

    Posted July 19, 2008 at 11:26 pm | Permalink
  3. Charles says

    Ah, I did not. That does make it much easier to switch.

    Posted July 20, 2008 at 2:49 am | Permalink
  4. JK says



    Have fun.

    Posted July 23, 2008 at 4:54 pm | Permalink

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