Tuesday, December 07, 2004

"Filling in" for Presentation

Why don't we notice our blind spot? Shouldn't there be a corresponding gap in our visual field? Conventional wisdom has it that our brains somehow "fill in" the gap, but Dennett, in Consciousness Explained, disputes this. He argues, first of all, that once a cognitive discrimination has been made, we do not need to re-'present' this information in some internal theatre, as if for the benefit of some ghostly audience. "That would be a waste of time and (shall we say?) paint. The judgment is already in, so the brain can get on with other tasks!" [p.128, original emphasis.]

As an example, Dennett invites us to consider tiled wallpaper, displaying hundreds of identical Marilyn Monroe portraits. Upon seeing such a wall, you would immediately see all the images in reasonable detail. Yet, as Dennett points out, "you could foveate only one or two Marilyns in the time it takes you to jump to the conclusion and thereupon to see hundreds of identical Marilyns." (p.354) We could not possibly have discriminated them all, for our parafoveal vision is very poor. What we should see, if based only on the raw visual data, is one or two Marilyns "surrounded by various indistinct Marilyn-shaped blobs."

So how are we to explain this clarity of vision? The proponent of "filling in" might suggest that we make a great many mental 'photocopies' of the more detailed image, and plaster them all over our theatre walls. But Dennett suggests a much more economical alternative: your brain simply judges that the rest are the same, and "labels the whole region 'more Marilyns' without any further rendering of Marilyn at all." (p.355)

We can then apply this insight to understanding the blind spot:
The brain doesn't have to "fill in" for the blind spot, since the region in which the blind spot falls is already labeled (e.g., "plaid" or "Marilyns" or just "more of the same"). If the brain received contradictory evidence from some region, it would abandon or adjust its generalization, but not getting any evidence from the blind spot region is not the same as getting contradictory evidence. [...] [T]he brain has no precedent of getting information from that gap of the retina, [...] so when no information arrives from those sources, no one complains. The area is simply neglected. In other words, all normally sighted people "suffer" from a tiny bit of "anosognosia." We are unaware of our "deficit" - of the fact that we are receiving no visual information from our blind spots. [...] The fundamental flaw in the idea of "filling in" is that it suggests that the brain is providing something when in fact the brain is ignoring something. (p.355-6)

13 comments:

  1. I was trying ot explain my blind spot at work the other day to a collegue - he had one of the blind foundation things that showed it as a fuzzy dot and the other explination was that the brain just fills in the gap - ACTUALLY it is neither - the blind spot just isn't there.
    harder for you guys to see because your blind spot is not exactly what you are looking at but you get a weird thing where even though you know you are not seeing somthing it realy just isnt there - you can try to explain it as a fuzzy dot or your brain filling in the gap - but that is all post event analysis - you actually just dont see it your brain doesnt record hte absence of information as "black" it doesn't record it at all.
    It is almost like looking through a lense that is curved so as to make a certain thing impossible to see. (note these blind spots do alsomake it a little harder to see the things around the blind spot - although this is probably accentuated by habit)
    That means I see the data in its relitively raw form even though I would remember it in its processed form (for example if you asked me later and I had not been concentrating I might guess what i should have seen).
    In the light of this I am not sure if you do SEE lots of Monroes - actually you see a few clear ones and a lot of fuzzy ones but you REMEMBER seeing hundreds of them. After the fact processing involves an automated proceedure which will fill in gaps and whatever else finally resulting in a memory being recorded - but instantanious perception if fairly raw. 

    Posted by GeniusNZ

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  2. I would question to what degree you actually see thousands of identical Monroes. The experiment could be easily countered by a wallpaper with superficially similar but subtly different images, which you would still undoubtedly see as being identical.

    Clearly much of our perception involves "filling in" from the brain. This includes temporally - the brain often alters our visual memory based on what it "knows".

    The difference between "ignoring" and "filling in" is slightly wrong, because when probed about our recollections we will give filled-in responses, rather than being unable to answer. This is another experiment that could easily be done. 

    Posted by Tennessee Leeuwenburg

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  3. Perhaps I didn't explain it very well. I was using "see" to refer to the contents of our visual consciousness, i.e. what we seem to see. So Genius' point about memory seems misplaced - the point is that it really does seem to us that we (presently) see a whole bunch of detailed Marilyns. As Tennessee notes, that does not imply that what we see is accurate. We could easily be fooled into "seeing" identical Marilyns when some are actually (slightly) different. But that's of no relevance here - we already know that consciousness can mislead us.

    As for "filling in", we need to be careful how we use that phrase. It's true that cognitive processing involves many important inferences, etc, which add to the raw visual data. Judgements are made; nobody is denying that. Rather, what Dennett disputes is the further claim that our brain then goes and gives a constructive 'performance' based on these judgements. It doesn't go and create some mental 'paint' to fill in the blind spot; it simply judges that there is more of the same. It doesn't reconstruct that 'sameness'. There is no "filling in", in that sense. 

    Posted by Richard

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  4. I dont think you do see 1000 marylins. You see basically what is there ( adjusted for the capacity and methods of our eyes.
    Then it is possible that you will upon questioning recall seeing one to the left or one to the right - and if oyu like you can define seeing as such - but I doubt you would "recall" seeing 1000 of them since I doubt you could not hold that image in your head.
    Of course one day you may well recall an entirely different image You may recall it as being stars or richard nixons for that matter.

    I think our definition of "seeing 1000 marylins" is somthing that needs a little analysis because unless the rest of you are totally different from me it is surely impossible for you to see 1000 marilyns simultaniously. and when looking at them you would be fully aware of that would you not? But you might say "there are 1000 marylins within my field of vision" by using logic but the difference between the one you are looking at and can determine is marylin beyond reasonable doubt is totally different to the ones you see with your pheripheral vision and only know are marylin by deduction.  

    Posted by GeniusNZ

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  5. Well, no, that's not what the empirical evidence suggests. Rather, when we're faced with a tiled wall like that, we (seem to) see all of them - not just one or two - as identical. That's how it seems to us. We don't see a whole lot of blurry blobs and deliberately infer that the rest much have been the same. The inference is unconscious; what we have conscious access to is the finished product of that judgement, i.e. hundreds of Marilyns.

    (Unless, that is, you've actually carried out an experiment to contradict Dennett on this point? I grant the tiling effect is surprising, but that's exactly why Dennett uses it - his book is full of remarkable and counterintuitive data about consciousness. Dennett's one of the top cognitive scientists around - he knows his stuff. One mightn't agree with all his conclusions, but he would know better than to misrepresent the raw data.) 

    Posted by Richard

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  6. Well if I want to investigate his experiemnt I need to see al lthe methodology. In particular - how does he KNOW that you are seeing al those marylins?
    Memory is quite similar to seeing somthing very vaguely - for example if i remember a painting I saw I would tend to remember it as if I saw it through fog or with my peripheral vision not in enough detail to paint it. 

    Posted by GeniusNZ

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  7. I am looking at the screen now and I don’t "fill in the text around the words I am looking at - they are just foggy stuff to the side then when my eye passes over them (and they become in focus) they become clear. I don’t get any impression that they are identical to the thing I am looking at (in fact I know they are not).
    If you asked me a question about those words I would attempt to infer what they were from the evidence available - which is actually basically the same process as I would use to remember anything. After all you can never be sure your memory is correct so you check it against other things - for example if I remembered you writing something totally out of character I might assume my memory was funny or if I remembered a Marilyn with a moustache I might wonder if my memory was bad - it would take a higher level of proof/certainty for me to say I had seen such things).
    But the process of infering something is rather different from that of “seeing” and if you want to confuse the two you need to include all the other effects that the mind can have upon memories.
    Even more importantly your brain cannot handle a few concepts at a time – so even if you do see a marylin then it is because you took a few basic things into your head then went – ok no more thinking required “it is marylin”. You could then continue to gather information if you wanted but you do not see it as marylin you just lable it as such in your head. Wjhen trying to remember what you saw your memory may take a side track via the marylin area of your brain to try to describe it since htat might be more efficient than trawling around for some hard to find memory – or your brain may have failed to record information altogether because it decided it would never need to know any more than “it is marylin. 

    Posted by GeniusNZ

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  8. "I am looking at the screen now"

    There's no repeated pattern of "tiles" on the screen right now, so I would not expect you to see the tiling effect.

    "Well if I want to investigate his experiemnt I need to see all the methodology."

    I'm afraid I can't help you here - I don't know all the experimental details. But I do know that Dennett is a well-respected expert in the field, so I'm inclined to trust him not to make any such elementary mistakes. 

    Posted by Richard

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  9. After doing a bit of research
    As far as i can tell the example is a thought experiment. His main aim seems to be top support his argument that we dont record information for everything on which I agree totally and that there is no non physical memory/mind (what scientist in their right mind would take up the opposing view?).
    he refers somehow to how poepel can sometimes see things they are not aware of
    This is all part of some wider argument against Qualia. Qualia doesnt sit well with me anyway. 

    Posted by GeniusNZ

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  10. Now going way off topic wikipedia notes
    "Supporters of qualia argue that since we can imagine this happening without contradiction, it follows that we are imagining a change in a property which determines the way things look to us, but which has no physical basis. The argument thus claims that if we find the inverted spectrum plausible, we must admit that qualia exist."
    now the reason why that works is becauseone holds certain assocaitions with colours in ones head constant when swaping hte colours (not that one can really do this thought experiment properly because it is too complex) Thus no need for whatever qualia are. just associations are needed.

    Anyway back to the blindspot thing I think as a thought experiment it is supposed to be a "if ... then" which requires assumptions to prove a point. 

    Posted by GeniusNZ

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  11. or I could be wrong - I just read an article written by him about that marylin stuff and qualia and stuff 

    Posted by GeniusNZ

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  12. It sounds like Dennett's just talking about a thought experiment, but there have been related actual experiments. There's one study that I don't remember the exact details of, but it goes something like this.

    People are shown a screen with two arrays of dots, one in the left part of their field of vision and one in the right portion of their field of vision. In each array, many small solid circles are randomly spread out in a rectangular field. The arrays on the screen change several times, with one side always having more dots than the other. Then, they are shown two arrays with the same number of dots, and they say that they see more dots in one array than in the other. In order to get people to say that the two arrays look like they have the same number of dots, one array has contain significantly more dots than the other.

    So, even though you might think that people are seeing the dots that are in front of their faces, the brain is actually doing some work that alters what you think you're seeing based on what you've seen before. I'm not sure if this result fits better with the "filling in" theory or Dennett's view, but I think that it implies that our brain is mediating the relationship between what is out there and what we seem to see in a surprising way. 

    Posted by Blar

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  13. Yeah, you're right he just presents it informally as a thought experiment, but he seems very confident in the outcome, so I assume he wouldn't say that without good reason. (I suspect he's basing it on other experiments he's come across, like those Blar suggests.) 

    Posted by Richard

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