I agree with most of what you say; I think there's just a semantic difference here. When I hear "illusion," to me that connotes that our perceptions are being fooled in some way.
Which would require a qualification of what 'fools' means.
All perceptions are, by nature, an interpretation of stimuli. However, to cut corners, assumptions are frequently made. I tend to define illusions as conflicts in these assumptions, depending on which stimuli we use to form a conclusion.
For example, an animated cartoon is an illusion as we conclude there is movement, yet this conflicts with other stimuli that lead us to conclude there is no single object that is changing position, but a sequence of different objects that create a sense of one object moving. The conflict manifests due to the shortcuts used by our brain, such as beta movement.
I don't remember the name, but you're probably familiar with the illusion of a spinning black-and-white pattern on a wheel that appears to be colored. This I would consider a true illusion, since it seems to result from a true flaw in our optical system, and there's no obvious reason why there should be an advantage to seeing things that way.
Benham's Top? Yeah, it's a classic.
It's an interesting illusion that does actually originate in the retina, and seems to be related to lateral inhibition. As such, the nature of how our retina has overcome certain shortcomings to improve our vision (lateral inhibition evolved so we can distinguish shades of the same colour better) yet can inadvertantly produce a sensation that conflicts with other assumptions.
The Necker cube is different, though. There's no inherent reason why it has to be a cube instead of a corner--both results are legitimate interpretations of an ambiguous scene. Most interesting to me is that those seem to be the only two interpretations we see--nobody sees a "flat" object, even though that is also a legitimate interpretation.
Actually, some people do. I'd have to dig through my notes for the details (I give talks on this very topic for Australia's National Science Week each year; combining art and neurology is a topic that always fills seats), but there was research conducted on late-age blindness cures in some impoverished countries like India. This involved older people who had never seen now having sight, and being forced to deal with this new sense. They found that they had to literally learn to see shapes - Necker cubes weren't cubes, for example, but collections of 2D shapes. It was indeed flat. More interesting still was that movement of the lines quickly taught them to 'see' in 3D, so they learned to see shapes in different contexts. In no time they were able to see the necker cube as you and I would.
Would you consider an ordinary photograph an illusion? If you hand a picture to someone and ask what they see, they'll answer "a man", "a house", "the sun", and so on. Nobody answers "I see a multicolored pattern of chemicals imprinted on a flat surface of fibrous cellulose". The latter is the correct answer, but it's not a very good answer. We need these so-called illusions for basic survival.
Photography does involve illusions in some sense - you'll get a sense of dimensionality through occlusion and line angles, even though the photograph is clearly flat.
Illusions don't need to fool you convincingly to be illusions. For instance, you could experience the waterfall effect with an image yet know without a doubt that the picture isn't moving.
Some of the color illusions are even less "illusory" than the common photograph, in my opinion. They reflect a real conclusion about the real world. Take the last demo in the shown videos with the filtered dots. With the covers on, the center dots look the same because there is no way for our brain to distinguish between the two based on the surrounding information. When the covers are removed, the dots suddenly look different. Why? It's because our brains learned a new fact: one of the sets consists of yellow dots with a purple filter, and the other is purple dots with a yellow filter. Those are two different things, and I would therefore say it's almost the opposite of an illusion--our brains have successfully inferred a new fact about the universe.
I think I understand you, and agree we seem to have different understandings of what defines an illusion.
Athon
