Friday, 25 March 2011

Are Perceptual illusions always necessarily illusions?

I'm sure that all of us are astounded that the squares A and B are actually the same colour. It is the shadow cast over B by the cylinder which makes us think otherwise. What this suggests is a quite incredible illusion.

However I think there is a pervasive naivety about the nature of perception. Most of us doubtless feel that we see the external world directly. But we emphatically do not.

Consider a red rose. We think of a red rose as being the same colour throughout the day. However the light from the Sun reaching the Earth varies throughout the day. When the Sun is low in the sky, lots of blue light gets scattered away since the sunlight has to travel through a greater quantity of air. So if we were to passively see colours "as they really are", then the colour of our red rose would change throughout the day. Indeed the colour of all objects would change throughout the day. But in fact our rose seems to stay pretty much the same colour throughout the daylight hours. Why is this?

The answer lies in the fact that we do not in fact simply passively see what is out there. Rather the brain performs certain operations on the data coming through our senses and presents it to our consciousness in a form that we can make sense of. Everything we ever see is in fact a hypothesis about how the world is. Thus we have an implicit theory about the external world that it contains objects which have specific intrinsic colours. Hence the brain will perform those operations which ensure that objects do indeed appear to be the same colour throughout daylight hours.

This applies not just to colours, but everything we perceive through our 5 senses. In a way then everything we ever perceive is an illusion. But I think this is misleading.

Let's consider the "illusion" above again. If this were a real 3D object and we were to approach it and view it from various angles, then we would see that squares A and B are very different colours. Indeed their intrinsic colours would be precisely as we perceive them in the illusion above.

But in that case what justifies us in labelling it as an illusion? If this were a real object that we are seeing, then squares A and B are very different colours. Our senses are not deceiving us. Indeed if someone claimed to see the squares as being precisely the same colour, then it is doubtful that he could proficiently visually apprehend his environment.

This is not to say we never perceive illusions. Sometimes we seem to see something, but which on closer inspection turns out to be something else entirely. Or sometimes what we seem to visually see is not consistent with our other senses.

Edited to add:  I have a follow up to this post
 written many years later:  The Checker-Shadow "illusion" Part 2.


  1. Read the Medieval Guide first - only because it's further back in history, and it's the first one he wrote. But they're both good.



    1. OK thanks Mary! Wondered what the heck you were talking about at first!

  2. Yes, the idea that something is "really this or that colour" is nonsense. It may have such-and-such a pigment combination if we perform some type of analysis, but the colour it "is" - is the colour we experience, surely?

    But we are usually ignorant of how we come to be in a world of things which persist, rather than shapes which continually mutate as we move about.

    Sometimes I think we can catch the process in action. Usually, it's as if we have a vast 'perceptual space' in which objects are experienced. There are no objects out there. Rather, objects are concepts are assembled and learned and then experienced. Sometimes, though, we can have the 'raw experience' before it snaps into object form.

    For instance, looking at a painting and just seeing circles and lines - then suddenly we see: it's a portrait of a person in an office. In real life, this tends to arise from perspective confusion - say, a reflection off part of a surface seeming to be separate and nearer than the rest of the shape, then suddenly we realise what we are seeing and it 'snaps'.

    Ropes and snakes!

    If we spend a bit of time, we can actually directly experience that we are having an indirect sort of "dream inspired by the [assumed] senses". For instance, we can do experiments to direct our attention to "where we are looking out of" and we find there is a gap, and this leads us to experience the background space of our awareness. We can actually experience ourselves living in a George Berkeley landscape.

    The only problem is, this means we have to accept that if there is an "underlying world" then it is inaccessible to us, and therefore doesn't exist except as a principle of faith that there is a "coherence" to experience. For instance, if we hold our hand up in front of us, that isn't our "real hand", it's just an image in awareness. The root of the experience, our "real hand" (if we assume such exists), isn't even "over there" it's somewhere else parallel and completely out-with the space we are experiencing.

    So, does this mean that our daily life is totally an illusion - as in, not as we think it is, in a solid world?

  3. Anonymous, SHUT THE FUCK UP. you just explained solipcism here.
    we do experience everything DIRECTLY.

    There is a definitive color in the objects, what changes is if someone has different cones they will perceive color wrong. Color is not just "wavelengths" without appearance. (the coloured object has the appearance of color, that's why we see appearance of color) , without the color appearance existing in the object first, you would never see it later, that's how it works, any place which reflects light, will reflect the whole image already with color (with or without eyes to catch and brains to "perceive") If you have a disease in the brain's part of perception of color, you'll never perceive/understand the color that eyes are seeing.
    Color, is caught by the eyes (and then perceived in the brain. The brain does nothing, just perceives/processes. I'm studying this area and I know a lot about it, I'm ready to get my PhD. and really upsets me when someone says about color/wavelenghts and objective world, without really understanding it. and knowing the difference about perception/reality.
    Obviously color do exist in the objective world, without color existing first we would never see the appearance of color later. It's all dark and electricity in the brain, there are no "images" in the brain, the brain can't create something from "nothing", It NEEDS the objective reality and all appearances.
    We need the objective world to create a subjective one, Our minds don't create if not by the objective reality (blind as an example), for we can not fathom a new color, or imagine a completely new and alien sound, unless without experiencing it first. The world is full of subjective illusions, yet the objective world itself isn't. We see exactly how it is.
    There is clearly only ONE color in every object. and if you have all of your cones/cells working properly you will see the world as it is. If you have a slight difference (in your biology) you must see different contrasts in your surroundings, but, the color still there. (blue gold dress as an example)
    Cerebral achromatopsia is another proof that color IS NOT created in the brain, people with this disease has a problem in the part of the brain responsible to process color, and do you know what happens? NOTHING, they will continue to see color, but they won't be able distinguish or understand that RED is RED, and BLUE is BLUE (qualia)

    Other proofs that color exists independent of "brains"
    what about colour pigmentation?
    What about objects absorbing light and then reflecting what hasn't been absorbed as a certain colour?
    What about Jellyfish who has only eyes (don't have a brain) and can see color, and changes color to attract their prey

    Of course color is objective, and no discussion about that, but how we perceive it (qualia) is entirely dependent of the brain, but the appearance of color is ALWAYS in the object.


  4. You are confusing "wavelength of light" (a physical measurement) with its subjective experience (called "color"). The point of the illusion above is that squares A and B are (roughly) the same wavelengths of light, but our perception of their "color" is altered by other visual context clues: the fact that our visual system thinks there is a shadow falling on square B modifies the experience of that color.

    Color is the perception, wavelength is the rough physical analogue. They are generally correlated, but not always (as this illusion helpfully reveals).

    1. I can assure you I am the very last person to conflate the wavelength of light on the one hand, and the experience of colour it precipitates on the other.

      Apart from that everything you say is spot on. However, it certainly doesn't contradict anything I've said.

  5. Yes, it contradicts your first sentence when you say "squares A and B are the same color." No, same wavelengths, different perceived color courtesy of context clues (actually, courtesy of lateral inhibition processes, color constancy, brightness constancy, 2D/3D scene perception, etc. -- obviously what the brain is doing behind the scenes is complex and interactive).

    Go back through your post, and every time you use the word "color", just replace it with either "wavelength of light" or "perceived color" as appropriate to make the distinctions between the physical and perceptual processes. Then re-read your post carefully. You will see how your discussion towards the end of the post makes little sense regarding the nature of illusions and "intrinsic colors" and other such muddled talk.

  6. Let's try an example of this replacement process: "If this were a real 3D object and we were to approach it and view it from various angles, then we would see that squares A and B are very different colours [WHICH MEANING DID YOU INTEND HERE?]. Indeed their intrinsic colours [NO SUCH THING AS INTRINSIC COLORS] would be precisely as we perceive them in the illusion above."

    "If this were a real object that we are seeing, then squares A and B are very different colours [PERCEIVED COLORS, BUT SAME WAVELENGTHS -- this is the illusion]. Our senses are not deceiving us [YEP they are, they are stating different color, but wavelengths say otherwise]. Indeed if someone claimed to see the squares as being precisely the same colour [PERCEPTUALLY], then it is doubtful that he could proficiently visually apprehend his environment." [WILD UNSUBSTANTIATED SPECULATION]

    Try digging into a textbook on visual color perception. There's over a hundred years of experimental research into this topic, you should familiarize yourself at least with the basics.

  7. This comment has been removed by the author.

  8. (For those who might be interested the particular “anonymous here taking issue with my blog entry is someone called Steve who is a cognitive neuroscientist who specialized in VISION and teaches about perceptual illusions).

    Steve, I'm trying to convey my ideas as simplistically as possible. Scientifically educated people generally tend to draw a distinction between colour as perceived and what they consider to be the "real colour" i.e wavelength of light. Now I don't buy into that. However, it facilitates my message to assume their position in order to show that it leads to an untenable position.

    So I grant you that if we assume the real colour is the wavelength of light, then the above illusion is indeed an illusion. But then *so is everything we ever perceive* and the word “illusion” becomes redundant. That is to say that everything we ever see is in fact an illusion, and we can no longer distinguish what I would regard as genuine illusions from our normal perceptions which accurately depict what is out there. That is the untenable position.

    An illusion has to mean we are somehow not seeing what is out there. That we are actually perceiving something different from what is out there. But this is not the case with the checkerboard “illusion” since if it were a 3D object and we looked at it from different perspectives, we would indeed still perceive squares A and B as being the colours that we initially perceived.

    All that exists are perceived colours, which we can refer to as "actual colours" in maximum light and with normal vision. And colour is colour, it is not something else like a wavelength of light. That is scientists redefining words (as they frequently do). Also, talking about wavelengths of light introduces certain suppositions regarding the nature of reality i.e there is a world out there independently of our perceptions and that our scientific theories depict a literal state of affairs (eg see my I’m not saying that there isn’t a world out there populated by such entities/things/processes, but bear in mind it’s a metaphysical hypothesis. And even if we assume such a hypothesis, it couldn’t be the case that wavelengths of light are *literally* colour.

    Edited to add: Another couple of posts from you. I think you must have seen the response I deleted. My modified response actually addresses your belief that wavelengths of light are literally colours where as what we perceive is not the actual colour.

    You say:

    “Try digging into a textbook on visual color perception. There's over a hundred years of experimental research into this topic, you should familiarize yourself at least with the basics”.

    This is a *philosophical* issue, not a scientific one. I assume that textbooks on colour perception will assume certain background metaphysical beliefs, namely that colours as perceived are a creation of the mind, and actual colours are wavelengths of light. I would argue that for all sorts of reasons this is an untenable position. I also think you might find another a blog entry of mine useful


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