April 06, 2003

Checkershadow "Illusion"

Edward Adelson's Checkershadow Illusion: the most impressive demonstration I've seen of just how *much* cognitive processing is going on in the rear of my cortex before the *I who is writing this* even gets a peak at what is coming in through the visual channel.


Checkershadow Illusion:

A light check in the shadow is the same gray as a dark check outside the shadow.
How does this illusion work?

For more illusions, click here.

©1995, Edward H. Adelson

Posted by DeLong at April 6, 2003 10:27 PM | TrackBack

Comments

Yup. The brain begins at the retina. A scary amount of processing goes on, even at that level, and it just gets more sophisticated as it gets closer to *I who is writing this*. Very nifty.

Posted by: Claire Bickell on April 7, 2003 02:31 AM

I agree completely, its truly impressing,

The illustrated importance of cognitive processing supports a story I heard a propos the problems with realistic sound in "Virtual Reality" applications.

In the real world, you can detect a person a standing behind you, silently observing. You don't hear the person, you don't see the person, yet you feel the person's presence.

Objects in a room interact with sound in a non-trivial way, but basically you can say that furniture and people absorb sound, they have a souhd shadow. This shadow kills the echo when you move furniture into an empty room.

You could thus hear someone silently standing behind you, through a slight softening of the echo of the background sounds.

The fact that you didn't here sound from the sound-shadowing object moving into its position indicates that it is capable of moving itself silently: it's probably a person

The fact that this person doesn't make any sounds of it own: he/she tries to avoid detection, probably in order to secretely observe you.

But the above process makes you feel his/hers eyes behind you. This is tricky in a VR application, basically because soundwaves have wavelenghts about the same size as objects in a room. Light is easy because its wavelength is small compared with this size.

Posted by: Mats on April 7, 2003 03:48 AM

While I'm well aware of how powerful these illusions can be...this one seems to be cheating. At least on my computer screen. I covered all of my screen but a portion of the two relevant squares. With no distracting context, square A was still quite a bit darker.

Posted by: Ben Vollmayr-Lee on April 7, 2003 05:34 AM

I thought that too, and tried checking it out. Try copying the image, opening it in Paint (or any other graphics program), cutting away all but square A and square B (better to cut into the squares than leave any other colors around the edges), and then dragging one into the other. They turned out to be the same color, as advertised.

Posted by: Julian Elson on April 7, 2003 05:52 AM

Not Found
The requested URL /movable_type/checkershadow_description.html was not found on this server.

Apache/1.3.26 Server at www.j-bradford-delong.net Port 80

Posted by: David Lloyd-Jones on April 7, 2003 06:38 AM

it was a funny website

Posted by: http://www.meki.8m.com/ on April 7, 2003 09:13 AM

it was a funny website

Posted by: http://www.meki.8m.com/ on April 7, 2003 09:14 AM

Amazing. I checked it in Photoshop and they are the same.

Posted by: Bernard Yomtov on April 7, 2003 09:30 AM

Yes, amazing. I didn't believe it, when I first read it. I tried to "cut out" the picture, using my hands across the screen. But even though I thought I was cutting out everything, the illusion remained.

But the illusion doesn't fool photo editing software. Cutting with the software, they're clearly the same shade.

Posted by: Mark Bahner on April 7, 2003 09:38 AM

Apologies for the multiple trackback entries, MT playing its own tricks on me. Feel free to delete them!

Posted by: ogged on April 7, 2003 09:54 AM

Apologies for the multiple trackback entries, MT playing its own tricks on me. Feel free to delete them!

Posted by: ogged on April 7, 2003 09:55 AM

oy vey

Posted by: ogged on April 7, 2003 09:56 AM

Yeah, I found them to be the same in when I cropped the pictures also. Wow, impressive!

Posted by: Ben Vollmayr-Lee on April 7, 2003 11:29 AM

This picture is amazing, but not just for the cognitive processing aspect. To me, it makes sense that the mind would try to recognize the pattern. I couldn't tell that the brightness was the same in each of the squares.

But thinking about it, there's no reason why we should not expect a "dark" square in full light to have the similar brightness, on net, as a "light" square in dim light or shadow.

Perhaps the mind needs *more* information to work with--like differences saturation or hue along with differences in brightness--to understand what is going on.

Posted by: Kevin Brancato on April 7, 2003 11:33 AM

Yeah, I found them to be the same in when I cropped the pictures also. Wow, impressive!

Posted by: Ben Vollmayr-Lee on April 7, 2003 11:34 AM

Yeah, I found them to be the same in when I cropped the pictures also. Wow, impressive!

Posted by: Ben Vollmayr-Lee on April 7, 2003 11:39 AM

Yeah, I found them to be the same in when I cropped the pictures also. Wow, impressive!

Posted by: Ben Vollmayr-Lee on April 7, 2003 11:44 AM

I don't see it :(

Posted by: Bobby on April 7, 2003 12:09 PM

bobby - that's the whole point. Your brain won't let you see it, even if you know it's true.

For good or for ill, the brain has an imense capacity to assume a pattern has held true. It's amusing with this visual exercise. It's not amusing when someone has seen a pattern *portrayed* in the media of, say, certain ethnic groups committing crimes more often than certain other ethnic groups.

Is the same principle at work here? (not a rhetorical question. I really am wondering what everyone thinks becasue I can't figure it out).

Posted by: a-ro on April 7, 2003 12:18 PM

Nifty of Prof. DeLong to sneak-in the illusion in the post title; I read "Checkershadow" and my brain saw "Chickenhawk".

(been hanging aroung blogs a wee bit much, maybe?)

Posted by: Stoffel on April 7, 2003 12:36 PM

If you cut small holes in a notecard to screen out the other squares and also screen out the A and B (which are different colors), you can see this without reprocessing it on your computer. If you don't screen out the A and B you still get the illusion.

Posted by: zizka on April 7, 2003 12:46 PM

I still think the B is just a tiny but lighter than the A even with zizka's trick.

a-ro, did you just watch the movie Bowling for Columbine? Not that I think it's untrue. I just watched that movie this weekend, and your comment reminded me of it.

Posted by: Bobby on April 7, 2003 12:54 PM

Damn, damn, damn, didn't believe it.

Tested colors with Fireworks:

Squre A: #6B6B6B

Square B: #6B6B6B

Grrrr. Well, heck, I guess I'll just go watch Fox News now to learn what's happening in the Iraq war... ^_^

Posted by: Bryan on April 7, 2003 05:46 PM

I think the most impressive part of this illusion is that it nicely shows how other spatial information is applied by our brain to help us determine the *relative* shade of the square on the checkerboard that lies in the shadow of the cylinder.

Sadly, of the responses I've noted elsewhere about this illusion, most talk about how nifty the Flash graphic is. Ah, geekdom... ;-)

Posted by: David Wilford on April 7, 2003 06:46 PM

a-ro: "For good or for ill, the brain has an imense capacity to assume a pattern has held true. It's amusing with this visual exercise. It's not amusing when someone has seen a pattern *portrayed* in the media of, say, certain ethnic groups committing crimes more often than certain other ethnic groups.

Is the same principle at work here? (not a rhetorical question. I really am wondering what everyone thinks becasue I can't figure it out)."

Well, sort of. If the principle is 'the brain has limited capacity and so has to process the information that's generally most useful' that is. In the visual illusion, we see an example of this. It's more useful to us to see edges, and local areas of contrast, rather than absolute light intensity. So our brain gets busy at identifying edges and textures and so on, and we can't compare the two areas and say if they're the same colour. But we can see the check pattern and the cylinder, and make sense of it as a description of a 3D world.

In social areas, humans have a number of heuristics to allow us to make decisions quickly, and efficiently in terms of brain power. These decisions are short cuts, and so aren't always right, but they're useful, or right often enough. A lot of those that have been studied relate to how we perceive out groups, that is people who are different from us, versus in groups, those we identify with. They're a major factor in the maintenance of stereotypes, and a huge area of research in it's own right.

Posted by: Claire Bickell on April 8, 2003 01:58 AM

There are all sorts of similar visual tricks, mostly they're known by fine artists who employ them to various ends in their work. I use em in my own work.

Posted by: Charles on April 8, 2003 08:06 AM

I'm glad that Brad DeLong appreciates this kind of thing (I remember when he had the insight that non-spectral colors exist. :-)) What worries me most about some of the undergraduates I teach is that they can look at an illusion like this one, or hear about the Monty Hall problem and have their lives unaffected by the experience. Perhaps things are better at Berkeley in this regard...I sure hope so.

Posted by: Jonathan King on April 8, 2003 12:26 PM

well while u all wasted time with photoshop i simply blurred my eyes up as much as possible n noticed that it is true even though until i thought of doing this i was swearing blind that it was a complete load of bull.
hmm i tihnk thats this could come in handy sumwhere in life, i think!
phil

Posted by: phil on April 8, 2003 01:57 PM

More optical illusiosn here

http://www.purveslab.net/

Posted by: alf on April 9, 2003 11:00 PM

instead of trying to draw a moral from this illusion, just try to imagine a world in which people simply saw things as they really were and the brain did not 'interfere'. a child would not recognize its own mother, if the lighting in the room changed. any takers?

Posted by: vieri on April 13, 2003 06:55 PM
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