Why does the brain fabricate blurred vision?

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Dorp
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Why does the brain fabricate blurred vision?

Postby Dorp » Tue Sep 27, 2011 12:58 am UTC

I've had this question on my mind for awhile now ever since I started reading about evolutionary psychology. The thing I can't figure out is why, if the images you see of the world are largely fictional representations of that world constructed by your brain out of signals about changes like line stops and ends, are those same images subject to being either blurry or clear depending on whether you're wearing glasses? Logically I would think that once the brain settles on a "this is what things look like" schema, you should see that sort of world whether you have your glasses on or not. But this isn't the case. I have double astigmatism which causes all vertical lines to be blurred and doubled when not corrected for, and yet I can see normal singular lines when I have my glasses on. Why does my brain allow this? Shouldn't it reconstruct a blurry world for me regardless of what kind of lens I put in front of it? And how did my brain know what to do with the corrected signals the first time I got glasses?

I'm not bringing this up to try and disprove established brain theory or anything stupid like that. I'm genuinely curious how this seeming contradiction between "your brain fabricates half of everything you see" and "it also takes for granted that the entire world is subject to immediate and drastic changes when your glasses are removed" came to be and how it is explained by people more knowledgeable in the field than I am.

Thanks for any answers! You guys are always the people I come to with these kinds of questions and I haven't been disappointed yet.

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Re: Why does the brain fabricate blurred vision?

Postby gorcee » Tue Sep 27, 2011 2:49 am UTC

The things you see aren't really fictional representations. They're more like numerically interpreted data.

Your eyes pass a signal to the brain to make sense of. The brain makes sense of this signal through more or less fixed rules; in other words, your visual cortex is a mostly* fixed, but very advanced algorithm. Your brain doesn't look at a vertical line and say, "oh hey, that's a vertical line." It looks at a set of inputs and applies its algorithm to that data.

Disorders like astigmatism are disorders in the eye, not the visual cortex. So it's like a having a faulty speedometer in your car. If your speedometer always reads your speed plus some normally distributed error, you can never exactly identify your speed (but you can get a pretty good estimate, depending on the variation of the noise). Your brain does the best it can, but the blurriness is basically random noise.

Other vision disorders, like amblyopia, are cortical disorders, and can be corrected either by preconditioning your input data (corrective lenses), or by retraining the algorithm (going through therapy to essentially rewire your neurons in your visual cortex). Refractive disorders can't be retrained like this, because the blurriness is basically random noise.

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Re: Why does the brain fabricate blurred vision?

Postby lightvector » Tue Sep 27, 2011 2:58 am UTC

Dorp wrote:Logically I would think that once the brain settles on a "this is what things look like" schema, you should see that sort of world whether you have your glasses on or not. But this isn't the case. I have double astigmatism which causes all vertical lines to be blurred and doubled when not corrected for, and yet I can see normal singular lines when I have my glasses on. Why does my brain allow this?


I don't know much about this topic, so I'll just ask a question. Why would you expect your brain to produce exactly the same perception when you give it a radically different input? (blurred/double light hitting the back of your eye vs focused light hitting the back of your eye). After all, in most cases, when you provide your brain a very different input, you get back a very different subjective perception. I imagine it wouldn't be very adaptive in many cases if you always perceived the exact same thing no matter what you actually sensed.

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Re: Why does the brain fabricate blurred vision?

Postby Angua » Tue Sep 27, 2011 8:53 am UTC

I'll do this is steps so you can work through it. It probably comes off as a bit condescending -sorry about that.

1) - The brain is reconstructing most of what you're seeing. - Well, where does the reconstruction come from?
The eye only gets truly clear vision on a very small spot of it called the fovea, which is less than 5 degrees of arc across. That is a tiny portion of your retina. That is literally all you can see at any given time.
like thisImage The rest of your eye (the blurry bit in the photo) is basically concerned with registering movement (it also does contrast). You can sort of replicate this by forcing yourself to look at one point for any length of time - you'll find the rest of your vision disappearing as your brain 'forgets' what's around it.

2)The sensory system is concerned with salient information how does it know what is salient?
Ok, so as I said, you can only see a small bit at a time. So your brain needs a way of figuring out what it should be looking at. It does this in 2 main ways. The first way is the most basic. It looks for changes in the environment. If a rock has been sitting there all day, it's probably not going to move, so you don't have to pay attention to it. The tiny movement in the corner of your eye could be food, or something hunting you, so you look immediately. This is the same as when you automatically look up when there is an unexpected noise. You are assessing what is new around you. The second way your brain decides is by learning where the important information is. Your eyes are automatically drawn to what you feel is the best features of an object to identify it (eg, if you are looking at a picture of a person, your eyes will spend more time looking at the eyes and mouth (central features of the face), and around the face, only pausing briefly at the rest of the photo (eg clothes, etc).
so - like this
ImageImage

So, basically, your brain is only reconstructing what it sees from what you are seeing at the moment. Your eyes make lots of quick movements (called saccades) every second, so fast that you don't even notice it. It looks at everything around you, and builds up from there. There is no memory bank inside your brain that once it sees an object, just presents that. This is why once you get corrective lenses, your brain is using the correct information to build up the scene. However, you can miss noticing differences if they lie outside of where you normally look, as this circumvents the mechanism of you looking at the most important features of a scene.

I'll be happy to clear up any other questions, or if I haven't explained something very clearly.
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Re: Why does the brain fabricate blurred vision?

Postby TrlstanC » Tue Sep 27, 2011 2:02 pm UTC

Do you have the same kind of astigmatism in both eyes? Because I had this thought - maybe our brain/nervous system is set up to extract visual information (or make predictions) by comparing the signals from each eye against each other, as opposed to within each independent visual field. For example I know people who have had laser eye surgery on one eye only, this lets them have distance vision in that eye, and can still read without reading glasses with the other. So, it could be that the visual pathway from each eye may be hardwired to pick out relatively simple patterns like straight lines (and maybe even things like motion) but to make complex predictions, such as an object's location in 3D space (depth perception) the brain compares the data from both eyes. And the part of the brain doing this more complex comparison may be more elastic and less hard wired.

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Re: Why does the brain fabricate blurred vision?

Postby Angua » Tue Sep 27, 2011 2:08 pm UTC

I'm confused about what you're saying? You do need 2 eyes for 3d vision (though you can still have a good estimate with only one eye, as your brain also works by learning that things further away are smaller, as can be seen by multiple optical illusions). You can learn to use one eye for distance, and one for close vision (this is also done with contacts for people who would otherwise need bifocals), however, depth perception would probably be compromised. However, what you mean about extracting visual information needing 2 eyes is confusing me.
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Re: Why does the brain fabricate blurred vision?

Postby TrlstanC » Tue Sep 27, 2011 3:51 pm UTC

The brain has to put together information from a lot of different sources, and one of the main ways that it can make predictions about the world is by comparing these signals to find patterns. For example, each rod and cone in each eye sends a signal towards the brain. If we just look at each individual signal we can't get a lot of information from it (only the patterns that show up over time), but when comparing the signal but at the point where they all come together it's possible to extract more information from the visual data. For example: finding lines, planes and motion. It would also be possible to compare this information to tactile information to determine that small object (or which kinds of small objects) are further away then big objects.

But we don't have to assume that the brain compares all of the nerves from all of the cones/rods at one point. Instead of thinking about it as a single point with lots of nerves branching out, we can instead imagine that it's a series of forks, with different nerves coming together and the signals they're carrying going through different stages and types of processing. The final stage in this model would be when two streams of data (one from each eyes) come together.

I wondering if this model is more realistic if we see evidence that the processing that's done on the data from each eye is basically hardwired i.e., that part of the visual cortex is dedicated to finding patterns in this 2D image and that it does it in a pretty much fixed and determined way. While the processing that's done to create a 3D experience from the two stream of data is more flexible and plastic?

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Re: Why does the brain fabricate blurred vision?

Postby Qaanol » Tue Sep 27, 2011 4:04 pm UTC

Dorp, have you studied optics in a physics class?

Consider a simple object, like a pencil. Ambient light hitting the tip of the pencil gets scattered in all directions. Also consider a simple light-sensitive object, like a piece of film being exposed. Each point on the film has light striking it from all directions. Without a lens, all the film “sees” (meaning, all that will come out when it’s developed) is a uniform color that’s the average of all light in the scene.

In order to produce an image of the pencil, we need a lens to focus the light that comes onto the film. A lens works by bending light, and when a lens is properly shaped, it bends the light in a particular way. To understand how this works, we will follow the light backwards from the film. For simplicity, we’ll say that we have a black tube or something around the lens and film, to block out the light that hasn’t come from the lens. Thus, the only light that reaches the film is light that has gone through the lens.

Pick a very small region of the film, ideally a single point. Consider light falling on this point. It could be coming from any angle in front of the film. Since we put a black tube around everything, we know light is really only coming from those angles that lead back to the lens. For any particular one of those angles, we can trace the beam of light back to find the point where it came through the lens. The line that the light follows take a bend when it goes through the lens, so coming out the other side it is pointing along a different angle.

Still looking at the same point on the film, consider every angle that light could come in one. That is, all the lines that lead from that point on the film to each point on the lens. These lines form a cone. The lines all bend going through the lens. And if the lens is made well, they all bend enough that they form another cone on the other side of the lens, converging to a point. If there is a part of an object at that point, such as the tip of the pencil, then only the light from that will reach our point on the film.

Now think this through from the other direction. We have a pencil, and we have a lens with a film behind it (and a black tube so that only light coming through the lens ever reaches the film.) Light strikes the tip of the pencil and goes off in all directions on straight lines. Consider only those lines that go from the tip of the pencil onto the lens. These lines form a cone. The lens bends the light to new lines on the other side, forming a cone on the other side. Light in that cone falls on the film. If the film is placed at the correct distance, where the tip of the cone is, then there will be a single point on the film that receives all the light from the tip of the pencil.

For each point on the pencil, there is a cone of light going to the lens, and a bent cone of light coming out the other side. Each of those cones converges to a point at its tip, which falls on the film. This makes a perfectly sharp image of the pencil on the film. Each point on the film gets light from exactly one point on the pencil. When the film is developed, it shows a picture of a pencil.

Suppose that the film were not at the right distance from the lens though. Suppose the film is too close to the lens. So the cones of light coming from the lens do not get to converge all the way to their tips. They get cut off as frustums. Each cone strikes the film on a small circle. That means each point on the pencil provides light to a small circle on the film. So each point on the film gets light from all the small circles that overlap it.

Thinking about it in reverse, the light reaching a single point on the film makes a cone to the lens, which bends to another cone on the far side. That cone gets cut off by the pencil before the cone converges. So the cone makes a small circle on the pencil. The light reaching a single point on the film comes from a small circle on the pencil. That means each point on the film gets hit be the average of the light coming off a small circle of the pencil.

When the film is developed, the image of the pencil is slightly blurry. That means the pencil is out of focus. By moving the film closer or further from the lens, we can make the image more or less blurry, meaning closer or further from being in focus. You can experiment with this yourself, by taking a pair of binoculars and adjusting the focal length. Most good binoculars have a dial on top that lets you move the focal point closer or further.

Your eye works like this simple model of a lens and a film. The front of the eye is the lens, and the back of the eye is the film. When the lens is working correctly, the image that reaches the film is sharp and in focus. But if the lens is shaped incorrectly, the light might be focused in front of or behind the film. Those correspond to near and far-sightedness. However, the lens could also be poorly shaped, so that it does not bend light equally at all points on the lens for all directions of incoming light. This means a cone of light coming in does not produce a perfect cone on the other side, so there is nowhere that actually receives a sharp image.

I mentioned binoculars, and those usually have more than one lens, one in front of the other. That way light coming through one lens can be further adjusted by subsequent lenses. This also allows the focus point of the system to be adjusted by moving the lenses closer and further apart. Your eye only has one lens, so it cannot change focal distance that way. However, you eye can actually change the shape of its lens, by using eye muscles to bend the lens. That way you can adjust the distance at which your eye focuses.

When you put on a pair of glasses, now there is a second lens in front of your eye’s natural lens. This means light from a single point, such as a pencil, can be redirected by the first lens before it reaches your eye. When the glasses lenses are designed correctly, they actually take that incoming light out of focus in exactly the right way. Then when that distorted image strikes your eye, your own astigmatism (or near or far-sightedness) distorts the incoming light again. But your glasses are designed to distort the light just enough so when light coming through them strikes your eyes, the second distortion exactly counteracts the first and you actually get a perfect cone of light aimed at the back of your eye, where the “film” is.
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Re: Why does the brain fabricate blurred vision?

Postby Angua » Tue Sep 27, 2011 5:00 pm UTC

TrlstanC wrote:The brain has to put together information from a lot of different sources, and one of the main ways that it can make predictions about the world is by comparing these signals to find patterns. For example, each rod and cone in each eye sends a signal towards the brain. If we just look at each individual signal we can't get a lot of information from it (only the patterns that show up over time), but when comparing the signal but at the point where they all come together it's possible to extract more information from the visual data. For example: finding lines, planes and motion. It would also be possible to compare this information to tactile information to determine that small object (or which kinds of small objects) are further away then big objects.

But we don't have to assume that the brain compares all of the nerves from all of the cones/rods at one point. Instead of thinking about it as a single point with lots of nerves branching out, we can instead imagine that it's a series of forks, with different nerves coming together and the signals they're carrying going through different stages and types of processing. The final stage in this model would be when two streams of data (one from each eyes) come together.

I wondering if this model is more realistic if we see evidence that the processing that's done on the data from each eye is basically hardwired i.e., that part of the visual cortex is dedicated to finding patterns in this 2D image and that it does it in a pretty much fixed and determined way. While the processing that's done to create a 3D experience from the two stream of data is more flexible and plastic?

I'm still a bit confused about what you want to know. Information about lines and motion starts being encoded at the level of the retina. As is colour. The 2 streams are actually put together fairly earlier in the process (at the level of the Lateral Geniculate Ganglion and V1) - and the rest of the scene is split into components, which are all analyzed separately in different areas of the visual cortex (eg you have one place with a colour map, one place with a motion map, one place with an object shape map, etc) - all of which use information from both eyes. The basic way you do 3d (by looking at the difference in location between the 2 eyes) is done in V1, with neurons that fire preferentially depending on how closely the two images line up. The way we see everything as one visual experience is unknown, and known as the binding problem. Interestingly, information from the visual cortex seems to be directed into 2 pathways - the dorsal pathway (the where pathway) and the ventral pathway (what) - with the where pathway being more concerned with motion, and allowing you to locate things in your environment and coordinate with them (so damage can lead to loss of hand-eye coordination, amongst other things) and the what pathway being more concerned with letting you identify stuff (so damage can lead to an inability to identify something by looking at it, though you might be able to do so via other senses, eg hearing or feeling it).
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Re: Why does the brain fabricate blurred vision?

Postby Technical Ben » Tue Sep 27, 2011 5:40 pm UTC

TrlstanC wrote:The brain has to put together information from a lot of different sources, and one of the main ways that it can make predictions about the world is by comparing these signals to find patterns. For example, each rod and cone in each eye sends a signal towards the brain. If we just look at each individual signal we can't get a lot of information from it (only the patterns that show up over time), but when comparing the signal but at the point where they all come together it's possible to extract more information from the visual data. For example: finding lines, planes and motion. It would also be possible to compare this information to tactile information to determine that small object (or which kinds of small objects) are further away then big objects.

But we don't have to assume that the brain compares all of the nerves from all of the cones/rods at one point. Instead of thinking about it as a single point with lots of nerves branching out, we can instead imagine that it's a series of forks, with different nerves coming together and the signals they're carrying going through different stages and types of processing. The final stage in this model would be when two streams of data (one from each eyes) come together.

I wondering if this model is more realistic if we see evidence that the processing that's done on the data from each eye is basically hardwired i.e., that part of the visual cortex is dedicated to finding patterns in this 2D image and that it does it in a pretty much fixed and determined way. While the processing that's done to create a 3D experience from the two stream of data is more flexible and plastic?


This is something I think has been missed in other posts (like the "extra colour" one). The brain finds patterns and decodes them. It seems to do this regardless of source. So whatever way we wire the eyes, with enough time, the brain should be able to decode the signal. Now, where did I put that spare HDMI cable and those wire cutters... ;)
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Re: Why does the brain fabricate blurred vision?

Postby gorcee » Tue Sep 27, 2011 6:17 pm UTC

Technical Ben wrote:
TrlstanC wrote:The brain has to put together information from a lot of different sources, and one of the main ways that it can make predictions about the world is by comparing these signals to find patterns. For example, each rod and cone in each eye sends a signal towards the brain. If we just look at each individual signal we can't get a lot of information from it (only the patterns that show up over time), but when comparing the signal but at the point where they all come together it's possible to extract more information from the visual data. For example: finding lines, planes and motion. It would also be possible to compare this information to tactile information to determine that small object (or which kinds of small objects) are further away then big objects.

But we don't have to assume that the brain compares all of the nerves from all of the cones/rods at one point. Instead of thinking about it as a single point with lots of nerves branching out, we can instead imagine that it's a series of forks, with different nerves coming together and the signals they're carrying going through different stages and types of processing. The final stage in this model would be when two streams of data (one from each eyes) come together.

I wondering if this model is more realistic if we see evidence that the processing that's done on the data from each eye is basically hardwired i.e., that part of the visual cortex is dedicated to finding patterns in this 2D image and that it does it in a pretty much fixed and determined way. While the processing that's done to create a 3D experience from the two stream of data is more flexible and plastic?


This is something I think has been missed in other posts (like the "extra colour" one). The brain finds patterns and decodes them. It seems to do this regardless of source. So whatever way we wire the eyes, with enough time, the brain should be able to decode the signal. Now, where did I put that spare HDMI cable and those wire cutters... ;)


The brain finding patterns and decoding them is not the same process, if I am not mistaken, as just "seeing". In other words, I look at my desk and I see a lamp. I take off my glasses, and I see a blurry lamp. Even if I didn't know it was a lamp, the shape recognition factors are still there, but the image is blurry. Or another way of putting it is the thing that makes you see a vertical line and focus on it will work (perhaps to varying degrees) regardless of whether that line is blurry or not.

The OP is asking, "if my brain knows it's a line, how come it can't just adapt out the blurriness?"

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Re: Why does the brain fabricate blurred vision?

Postby TrlstanC » Tue Sep 27, 2011 6:33 pm UTC

Angua wrote:The 2 streams are actually put together fairly earlier in the process (at the level of the Lateral Geniculate Ganglion and V1) - and the rest of the scene is split into components, which are all analyzed separately in different areas of the visual cortex (eg you have one place with a colour map, one place with a motion map, one place with an object shape map, etc) - all of which use information from both eyes. The basic way you do 3d (by looking at the difference in location between the 2 eyes) is done in V1, with neurons that fire preferentially depending on how closely the two images line up. The way we see everything as one visual experience is unknown, and known as the binding problem.

That's interesting, that the two streams are put together for a lot of processing early on, but that the creation of a single visual experience happens later. I was assuming that those two parts would happen closer together; that when the two streams came together that would also be the time that the brain creates a single visual experience.

Do we know if while the streams are being processed in the "Lateral Geniculate Ganglionand V1" they are being done mostly in parallel or if there are a lot of comparisons going on as well? For example, 3D information is created in V1 by comparing how similar different parts of the two images are, this processing would only be possible if both streams are being processed together. But we could also imagine that the two streams come together and are processed in parallel in other parts (for color, motion, ect) without there being any comparison. They're just being processed at the same time, but the same processing could be done for just one stream and the results would be the same for that stream i.e., the results don't depend on comparisons.

I assume it's not broken out as cleanly as that, or that exactly what's going on at each step might be a bit beyond us, but it would be interesting to hear what the current theories are.

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Re: Why does the brain fabricate blurred vision?

Postby Turtlewing » Tue Sep 27, 2011 8:45 pm UTC

gorcee wrote:
Technical Ben wrote:
TrlstanC wrote:The brain has to put together information from a lot of different sources, and one of the main ways that it can make predictions about the world is by comparing these signals to find patterns. For example, each rod and cone in each eye sends a signal towards the brain. If we just look at each individual signal we can't get a lot of information from it (only the patterns that show up over time), but when comparing the signal but at the point where they all come together it's possible to extract more information from the visual data. For example: finding lines, planes and motion. It would also be possible to compare this information to tactile information to determine that small object (or which kinds of small objects) are further away then big objects.

But we don't have to assume that the brain compares all of the nerves from all of the cones/rods at one point. Instead of thinking about it as a single point with lots of nerves branching out, we can instead imagine that it's a series of forks, with different nerves coming together and the signals they're carrying going through different stages and types of processing. The final stage in this model would be when two streams of data (one from each eyes) come together.

I wondering if this model is more realistic if we see evidence that the processing that's done on the data from each eye is basically hardwired i.e., that part of the visual cortex is dedicated to finding patterns in this 2D image and that it does it in a pretty much fixed and determined way. While the processing that's done to create a 3D experience from the two stream of data is more flexible and plastic?


This is something I think has been missed in other posts (like the "extra colour" one). The brain finds patterns and decodes them. It seems to do this regardless of source. So whatever way we wire the eyes, with enough time, the brain should be able to decode the signal. Now, where did I put that spare HDMI cable and those wire cutters... ;)


The brain finding patterns and decoding them is not the same process, if I am not mistaken, as just "seeing". In other words, I look at my desk and I see a lamp. I take off my glasses, and I see a blurry lamp. Even if I didn't know it was a lamp, the shape recognition factors are still there, but the image is blurry. Or another way of putting it is the thing that makes you see a vertical line and focus on it will work (perhaps to varying degrees) regardless of whether that line is blurry or not.

The OP is asking, "if my brain knows it's a line, how come it can't just adapt out the blurriness?"


I'm not a neurosceintist but my initial thought would be:

Perhaps your brain doesn't know what kind of line it is and therefore can't "adapt out the blurriness", because there are several subtly different things it could be. It could be a dotted or solid, have a range of thicknesses, perhaps it's not clear what color it is or whether it's a straight line or a sine wave with a really short wavelength, etc.

I'd imagine that there are limits to the amount of information the brain can extract from the eyes, and that eyes with sub standard optical properties would result in the brain loosing more information than optically optimal eyes, and thus more objects get poorly defined in your "conceptual map".

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Re: Why does the brain fabricate blurred vision?

Postby gorcee » Tue Sep 27, 2011 9:00 pm UTC

Turtlewing wrote:
I'm not a neurosceintist but my initial thought would be:

Perhaps your brain doesn't know what kind of line it is and therefore can't "adapt out the blurriness", because there are several subtly different things it could be. It could be a dotted or solid, have a range of thicknesses, perhaps it's not clear what color it is or whether it's a straight line or a sine wave with a really short wavelength, etc.

I'd imagine that there are limits to the amount of information the brain can extract from the eyes, and that eyes with sub standard optical properties would result in the brain loosing more information than optically optimal eyes, and thus more objects get poorly defined in your "conceptual map".


That's really not the case. Recognition of objects is a different process than resolution of images when the resolution flaws are refractive. There is a difference when the resolution is in the visual cortex.

Take a neural network model. If your outputs contain noise, there are two possibilities: 1.) your input data has noise, and that noise is propagated through the network; 2.) the inputs are noiseless, but one or more sub-networks injects noise.

Either outcome will result in output values with noise. The difference is that if the noise is in the hidden layers, you can possibly train around them. If the noise is in the input data, to remove it you need to perform some correction or filtering.

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Re: Why does the brain fabricate blurred vision?

Postby WarDaft » Tue Sep 27, 2011 9:05 pm UTC

A perfectly straight crisp line with low entropy and a furry line with very high entropy are indistinguishable above a certain amount of diffusion. Your brain could only fill in information well for the first (because it's the only image type that can be reverse engineered from a blurry image with any accuracy) but you will encounter far more of the second. That's just how entropy works.
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Re: Why does the brain fabricate blurred vision?

Postby shk » Tue Sep 27, 2011 9:09 pm UTC

So it's like a having a faulty speedometer in your car. If your speedometer always reads your speed plus some normally distributed error, you can never exactly identify your speed


What if you've lost your map and don't have any idea where you are?

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Re: Why does the brain fabricate blurred vision?

Postby gorcee » Tue Sep 27, 2011 9:40 pm UTC

shk wrote:
So it's like a having a faulty speedometer in your car. If your speedometer always reads your speed plus some normally distributed error, you can never exactly identify your speed


What if you've lost your map and don't have any idea where you are?


You'll still be able to determine that you're on the Planet Earth.

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Re: Why does the brain fabricate blurred vision?

Postby Dopefish » Wed Sep 28, 2011 12:18 am UTC

gorcee wrote:
shk wrote:
So it's like a having a faulty speedometer in your car. If your speedometer always reads your speed plus some normally distributed error, you can never exactly identify your speed


What if you've lost your map and don't have any idea where you are?


You'll still be able to determine that you're on the Planet Earth.


You might be in an enclosed box accelerating through space in just the right way to simulate g and thus not even know that. :P

In any case, uncertainty principle jokes are perhaps out of place in this particular thread.

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Re: Why does the brain fabricate blurred vision?

Postby Qaanol » Wed Sep 28, 2011 1:08 am UTC

WarDaft wrote:A perfectly straight crisp line with low entropy and a furry line with very high entropy are indistinguishable above a certain amount of diffusion. Your brain could only fill in information well for the first (because it's the only image type that can be reverse engineered from a blurry image with any accuracy) but you will encounter far more of the second. That's just how entropy works.

This is true, except in the case where someone’s eyes don’t focus properly. When they wear corrective lenses, they can see fine, for obvious reasons. When they take off the glasses, there is a consistent level of blurring that is present all the time. The OP wants to know why the brain doesn’t compensate for that by unblurring the known level of blurriness.

The simple answer is, because it’s hard. It requires, essentially, inverting a convolution operation, otherwise known as deconvolution.

There are some simple tricks that can sharpen an image, such as the Unsharp Mask operation. That has been used in film photography for decades, and amounts to the following:

Take your negative, which is a little bit blurry. Use that negative to expose another negative, but make this one a little out of focus so it’s more blurry than the first, and overexpose it so it gets somewhat washed out. Notice that the second, blurrier negative is actually a positive image, since it’s the negative of a negative.. Now put both negatives one in front of the other. Shine light through them both, and use that to develop the final print. This works because the second negative, which is really a positive, subtracts from the light coming through the original negative. But the positive is also blurrier, so it is in a sense subtracting the blurriness. And it is overexposed so it only subtracts a small amount.

I don’t know “why” the brain doesn’t do that, nor why the eye doesn’t continuously change blurriness levels and subtract one from the other, except that they just don’t as far as I know.
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Re: Why does the brain fabricate blurred vision?

Postby Angua » Wed Sep 28, 2011 6:30 pm UTC

TrlstanC wrote:
Angua wrote:The 2 streams are actually put together fairly earlier in the process (at the level of the Lateral Geniculate Ganglion and V1) - and the rest of the scene is split into components, which are all analyzed separately in different areas of the visual cortex (eg you have one place with a colour map, one place with a motion map, one place with an object shape map, etc) - all of which use information from both eyes. The basic way you do 3d (by looking at the difference in location between the 2 eyes) is done in V1, with neurons that fire preferentially depending on how closely the two images line up. The way we see everything as one visual experience is unknown, and known as the binding problem.

That's interesting, that the two streams are put together for a lot of processing early on, but that the creation of a single visual experience happens later. I was assuming that those two parts would happen closer together; that when the two streams came together that would also be the time that the brain creates a single visual experience.

Do we know if while the streams are being processed in the "Lateral Geniculate Ganglionand V1" they are being done mostly in parallel or if there are a lot of comparisons going on as well? For example, 3D information is created in V1 by comparing how similar different parts of the two images are, this processing would only be possible if both streams are being processed together. But we could also imagine that the two streams come together and are processed in parallel in other parts (for color, motion, ect) without there being any comparison. They're just being processed at the same time, but the same processing could be done for just one stream and the results would be the same for that stream i.e., the results don't depend on comparisons.

I assume it's not broken out as cleanly as that, or that exactly what's going on at each step might be a bit beyond us, but it would be interesting to hear what the current theories are.
You process them together. You have specialised neurons that act as basic 3d interpretters (so fire differently depending on how well the two eyes are matching up) but the information from each eye is laid out so as all the info from the same space in your visual field is together, regardless of which eye it's from. It's not well known amongst laypeople that in the optic chiasm (where the optic nerves meet and cross), actually, only the parts of the nerve carrying information from the lateral side of the eye (as opposed to your nose side) cross over, so that the brain is processing everything from the right side of your body in the left brain - not everything from your right eye.
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