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Help: Optical Illusion Insight

That illusion doesn't work for me, either. I spend ages on it the first time I saw it, trying different distances and it didn't change at all. I suspect that may be one of the few illusions that is pretty dependent on the person looking at it.
 
Normal Dude said:
One of my absolute favorite illusions:

The checker shadow illusion:

http://www.hemmy.net/2006/01/20/checker-shadow-illusion/
Click to expand...

It's not an illusion. It is an example of how what we see is processed by our brain/mind, which alters what we actually "see", based on lighting, color, depth and the relationships between all visual elements, as well as our memories and learned perceptions.

Looking at the squares next to each other, we can see they are all the same shade. But when placed in shadow, our mind "knows" that color and intensity change, which is why they seem different.

1105346a2a5f7258d0.jpg


The same thing happens all the time in real life, where objects partly in shadow are altered by the brain/mind to achieve consistency. The same thing occurs with size, as well as brightness and color.

Saying it is an illusion, means almost everything we "see" is an illusion.
 
bellonax said:
That illusion doesn't work for me, either. I spend ages on it the first time I saw it, trying different distances and it didn't change at all. I suspect that may be one of the few illusions that is pretty dependent on the person looking at it.
Click to expand...

The "happy" dude on the right doesn't look happy, no matter how you look at it.
 
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robinson said:
It's not an illusion. It is an example of how what we see is processed by our brain/mind, which alters what we actually "see", based on lighting, color, depth and the relationships between all visual elements, as well as our memories and learned perceptions.

Looking at the squares next to each other, we can see they are all the same shade. But when placed in shadow, our mind "knows" that color and intensity change, which is why they seem different.

[qimg]http://www.internationalskeptics.com/forums/imagehosting/1105346a2a5f7258d0.jpg[/qimg]

The same thing happens all the time in real life, where objects partly in shadow are altered by the brain/mind to achieve consistency. The same thing occurs with size, as well as brightness and color....
Click to expand...

Actually, no "mind" is needed to explain simultaneous contrast, just a couple of healthy retinas.
 
Jeff Corey said:
Actually, no "mind" is needed to explain simultaneous contrast, just a couple of healthy retinas.
Click to expand...

I'm not sure what you are saying here. Surely it is processing in the visual cortex that does that sort of 'taking into account' of percieved shadows etc in order for our perception to 'work out' the consistency of real objects? (sorry that isn't worded well, I have a bad headache).
 
Professor Yaffle said:
I'm not sure what you are saying here. Surely it is processing in the visual cortex that does that sort of 'taking into account' of percieved shadows etc in order for our perception to 'work out' the consistency of real objects? (sorry that isn't worded well, I have a bad headache).
Click to expand...
Lateral inhibition at the retinal level has been used to explain how contrast from surrounding fields can change the activity at the retina.
 
Jeff Corey said:
Lateral inhibition at the retinal level has been used to explain how contrast from surrounding fields can change the activity at the retina.
Click to expand...

Have you got any links? It seems like higher order processing involved in knowing that the changes are supposed to be due to a shadow would be needed to explain that particular illusion. I can see how retinal effects might be responsible for something like the Craik-O'Brien-Cornsweet Illusion, but I don't understand it for the shadow/checkerboard illusion.
 
Professor Yaffle said:
Have you got any links? It seems like higher order processing involved in knowing that the changes are supposed to be due to a shadow would be needed to explain that particular illusion. I can see how retinal effects might be responsible for something like the Craik-O'Brien-Cornsweet Illusion, but I don't understand it for the shadow/checkerboard illusion.
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I'm not sure the changes are due to the shadow. If you take a business card and cover the cylinder and cues to it casting a shadow, the percept stays the same (for me, at least).
A is surrounded by lighter squares and B by darker ones, an ideal situation for the production of simultaneous contrast This produces lateral inhibition in A.
http://yorku.ca/eye/simcont2.htm
http://www.unm.edu/~toolson/simulcntrst.html
 
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Maybe it is a combination of both effects then, since when I do the covering up, you're right there is still a difference, but to me it is much lessened.

Edit: see for example, in this link with an explanation, what happens when the shadow is moved, but the square stays the same shade.

http://www.illusion-optical.com/Optical-Illusions/ChessboardShading.php
 
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Professor Yaffle said:
Maybe it is a combination of both effects then, since when I do the covering up, you're right there is still a difference, but to me it is much lessened.

Edit: see for example, in this link with an explanation, what happens when the shadow is moved, but the square stays the same shade.

http://www.illusion-optical.com/Optical-Illusions/ChessboardShading.php
Click to expand...
Another "mind" explanation which explalns bupkis. The fact that the darker area represents a shadow of the cylinder is irrelevant. It's just darker and produces less lateral inhibition.
Also, did you note the error in the moving cylinder graphic? Moving the cylinder away from its original position should have resulted in all the shadowed squares increasing in luminance, preserving the original light to dark ratios. They didn't, somehow becoming identical.
 
The illusion, IIRC, is due to contrast detection starting at the retina, namely the center-surround antagonistic fields of retinal ganglion cells. The interior of the tile is "filled in" based on the contrast at the edges.

ETA: Whoops, Jeff Corey already explained it. Move along, nothing to see here...

ETA II: While we are on the subject, the Cornsweet Illusion is cool too:

http://en.wikipedia.org/wiki/Cornsweet_illusion
 
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Normal Dude said:
If you would prefer, you can argue that point to the guy who made it. Maybe he will move the image out of the "illusions" section of his website:

http://web.mit.edu/persci/people/adelson/index.html
Click to expand...

Fine idea. Professor Adelson agrees with me, and states, for the record,

"As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view."

Edward H. Adelson
Professor of Vision Science
Dept. of Brain and Cognitive Sciences
Massachusetts Institute of Technology
 
Jeff Corey said:
Actually, no "mind" is needed to explain simultaneous contrast, just a couple of healthy retinas.
Click to expand...

Dead wrong of course. But let the person who created the "illusion" explain it, him being an expert, and way smarter than me on this issue.

Why does the illusion work?

The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray "paint" that belongs to the surface.
Click to expand...
http://web.mit.edu/persci/people/adelson/checkershadow_description.html

This is what I was trying to explain before. Before I read the page in which he explained the process.

robinson said:
It's not an illusion. It is an example of how what we see is processed by our brain/mind, which alters what we actually "see", based on lighting, color, depth and the relationships between all visual elements, as well as our memories and learned perceptions.

Looking at the squares next to each other, we can see they are all the same shade. But when placed in shadow, our mind "knows" that color and intensity change, which is why they seem different.

1105346a2a5f7258d0.jpg


The same thing happens all the time in real life, where objects partly in shadow are altered by the brain/mind to achieve consistency. The same thing occurs with size, as well as brightness and color.
Click to expand...
 
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