TheBlackCat–you mentioned the illusion where a patch of color in shadow appears different than one in light, even if they are actually the same color. You’re probably thinking of that illusion with a cylinder over a checkerboard, and it appears that a dark square is darker than a light square in shadow. So does that represent a mistake or not?

Imagine that we’ve constructed that scene in real life, but in two different ways. In the first way, it’s as you might expect–you have a light source off to one side, and the cylinder therefore casts a shadow. The dark square is legitimately darker than the light square in terms of pigment.

In the other case, though, the actual lighting environment is completely even (like an overcast day). What appears to be a shadow has actually been manually colored in where the shadow would have been. The two squares have been painted the exact same color value.

Perceptually–that is, given the limitations of our eyes–these two scenes are completely identical. If it’s been constructed accurately, there is simply no way for us to distinguish between the two cases, even in principle, unless you add additional sensory inputs.

So our brains need to make a judgment call based on what is more likely–that the shadow comes from an offset light source, or that the shadow was inserted manually and has nothing to do with the light.

Clearly, the former is going to be more frequent. So that’s the prediction our brains make. And it’s the right decision, even with the existence of constructed illusions, because there is no negative survival value with misinterpreting these illusions, while there is indeed negative survival value with misinterpreting real-world scenarios.

Now, I’ll certainly grant that there are illusions that don’t fall into this category. The optic nerve thing is certainly a fault in our visual system (although our brains still deal with it in an impressive way). But I’d argue that most things that are called illusions are really instances of our brains making the correct (and by “correct” I mean the most likely) prediction in the face of ambiguity, and that this ambiguity is unavoidable.

PS: It’s true that nowhere in the definition of “illusion” does it say “flaw”, which is why I said “implies”. The problem is that it’s not our vision system that’s fooling us; it’s reality itself (a constructed reality in the case of many illusions).

http://en.wikipedia.org/wiki/Synesthesia

Yes, it does, but only in pitch black. During normal light conditions, I do not see colors or see the location from where a sound comes from depicted through colors. When a loud noise happens, I hear it and turn towards it to see it, but do not see any colors associated with the noise. I certainly do not experience other elements of synesthesia — I have met someone before who had it and said that he would see colors and shapes often when certain words were uttered, but I certainly don’t have anything like that.

It appears though, at least from this wikipedia entry, that there are folks researching how the brain mixes the senses, and maybe this is just an artifact of the brain compensating for the loss of one of its senses (sight) by teaming it with another one (sound). I’d be interested to know if others have had the same experience. I probably should start by talking to a few blind people.

Supposedly, if the mechanism proposed was real, then there should be a surplus of stars in circles that had similar formation times. He occasionally elicited the help of his fellow grad students in identifying circles of stars.

It is truly amazing how many circles one sees in an essentially random background of stars in any arbitrary area of the sky: it is also amazing how seldom one maps the same pattern of circles on the same plate (unknowingly) when it is mapped on different days! Even though committed to the original hypothesis, my colleague had to eventually admit defeat and select another thesis topic.

I suppose this is an example where skepticism should be practiced with the same zeal when considering things scientific as it should be with things metaphysical. “That upon which you dwell with the most satisfaction, hold most in suspicion.” (Needs attribution but I can’t remember the original source!)

BTW, I never said “‘illusion’ implies that there’s something wrong with our visual system” however you (TheBlackCat on 26 Feb 2008 at 9:10 am) said “Many optical illusions very much do expose flaws in our visual system.” It’s there for all to see. Are you really denying having said that? If so, grow up.