Wednesday, January 23, 2013

Anomalous Depth Perception Illusion

One of the most traditional phenomenonological questions is how do I know that what you see is the same thing as what I see. If we both look at a picture, and it has the color red, and we both agree it is red, even so, how do I know that the red that you perceive is the same red as I perceive?

The answer is, you don't know. There is natural variation in the red and green cone sensitivities at the very least, and there are other issues involving how the brain processes the information. And of course there are the various types of color blindness and the tests that have been designed to diagnose them.

This post is about an example of an optical illusion that I see, but that no one else that I have tested it on sees. One reason for posting it on the blog is to see if one of my readers will also see the illusion.  So far, I seem to be a "unique". 

Those of you who know about the perception of illusions may skip the next few paragraphs.

For those of you who have never studied the perception of illusions, here are a few things to know. Although we "see" with our eyes, what we see is perceived by our brain, and there is a lot of interesting circuitry in there to process and make sense of the data. This is also an ongoing research topic.  If you have never studied issues of color perception and optical illusions, when you first encounter what we think we know about it, you will be surprised. It is pretty wacky. We are very good at detecting color differences for example, more than the precise color itself. We have lots of circuitry to see movement on the periphery. The point is, seeing is much more than just reporting what may be "objectively there" through the lens of our eye. It is being highly processed all along the way.

But be warned, once you start down the path of understanding color perception and vision you will find that it goes on and on and on.   And then on and on.   Its not simple.

Many optical illusions are essentially bugs in the perception system, that is, the image is causing the perception circuitry to see something that isn't actually there (e.g. motion when there is no motion, colors, when there are no colors, etc). 

Here is an example of a famous illusion that most people are able to see. Its called the Cafe Wall illusion, because its discoverers first noticed it as an architectural detail on the facade of a cafe.

All the horizontal lines in the above image are straight and parallel with each other but most people see the lines at an angle to each other, and also bending depending on where they direct their eyes.  Everyone seems to be able to see this illusion.

The Special K Cafe

Now, look at the following blurry image.

Founders of Google in China from The Register (

Most people will see this image as completely flat, e.g. no depth.  It is of course a very blurry image.  I on the other hand always see a lenticular depth effect of something like 3/8's of an inch.  

By "lenticular depth effect" I mean the illusion of depth one gets from a lenticular picture or postcard when you hold it in your hand.  These were more popular years ago, they would often be post cards of the United Nations, or of Elvis.   The postcard would appear to be perhaps an inch thick or even more, but you know, because you are holding it, that is just a piece of cardboard.   The same illusion of depth that you get from such  a postcard is what I see above, except of course that it does not change as I move my head.   It is a picture with depth from one point of view.

I spent a day trying to recreate the illusion with other images, and I think I know what is going on that causes the illusion for me.   But I have not yet found anyone else that the illusion works on.  One thing I find interesting is that, for me, this is a robust illusion.  I see it every time I look at the picture.

Depth perception is notoriously individualistic.   There are many people out there who can not see "3D" from stereoscopic projection.  I have heard the number is 10% of the general population, but I have never gotten a formal confirmation of that percentage (e.g. a reference to a study).   Alternatively, there are people who can see what appears to be stereo in circumstances when they are watching a movie projected flat that moves through an environment.  I get that sensation, in particular when the moving imagery covers peripheral vision.  

But the first question I have, does anyone else other than me see depth in this image?


Wikipedia page on the Cafe Wall Illuson

Wikipedia page on Lenticular Printing

Science News article on research into color perception variation:

The following two abstracts discuss research in the molecular and genetic basis of variation in color perception.


  1. Hello, could you tell me where you found your photo of the "Special - K " cafe? I am looking for permission to use the image in a documentary.

    1. Dear Unknown. I wish I knew. I normally do a google search and then select images, and browse them until or if I find something I like. I vaguely recall that this image came from the people who originally discovered the illusion, and you may find it in or associated with the original paper associated with the cafe illusion. And this may indeed be the cafe that they named the illusion for. Good luck! MW