10 ms·

I disagree with the author's interpretation of this. As a neurologist myself, I was taught it was specifically to simplify visual processing, although there ma

I disagree with the author's interpretation of this. As a neurologist myself, I was taught it was specifically to simplify visual processing, although there may be other theories but this is what I was taught. Like others commenting here, the way the lens works in each eye is by flipping the image onto your retina. If we had only one eye, there would be no issue, the image would appear as a continuous image, just flipped around. However, because we have two eyes, they both individually flip different fields, thus separating the continuity of the image horizontally. If you try drawing out various different ways to try and remedy this problem of binocular vision, the way nature's approach is quite elegant in reuniting the image as well as separating visual processing into left/right. To avoid a large text explanation, this is a simple diagram of the concept how the brain reforms the arrow. https://nba.uth.tmc.edu/neuroscience/s2/images/html5/s2_15_14.jpg https://nba.uth.tmc.edu/neuroscience/s2/images/html5/s2_15_1... The way it works is by separating the left and right fields of each eye, and then crossing them so that the left fields goes to the right half of the brain and the right fields go to the left half of the brain. Each right/left half is now interpreted by one side of the brain and the image is again continuous if you draw it out on the brain. Of course now each side of the brain sees the opposite side, but we remedy this by crossing everything else so it plays well with visual interpretation. Now the right side of the brain sees, senses, and controls the left side of the body and vice versa. When it comes to everything else, there isn't a clear benefit for having processing swapped to opposite hemispheres. But visual processing benefits from it greatly, and so the rest of the nervous system goes along with it.

punnerud 3y ago
Just the swapping of sides in the brain, isn’t there a clear benefit for the stress/weight of the nerves to the rest of the body? If right side was connected right arm the nerves would be pressed “out”. When they cross there is less pressure in the spinal cord?
elcritch 3y ago
Flipping the eyes is the same geometric reasoning the article discusses. The same thing applies to touch and motor control too. Its just a general extrapolation of the same principle. It would also explain why creatures without sight as well like many worms.
- nomel 3y ago
  > It would also explain why creatures without sight as well like many worms This would require that worms evolved the cross crossing independently, after our common ancestor, which doesn’t seem to be clear [1]. All it would take is a few light receptors to get the cross cross party started. [1] https://www.sciencedaily.com/releases/2010/02/100201101905.htm https://www.sciencedaily.com/releases/2010/02/100201101905.h...
  - jaggederest 3y ago
    Those kind of light receptors don't have the lenses that cause our eyes to flip the image. They are omnidirectional and usually not even on the surface of the critter. Just a small bag of rhodopsin that triggers a nerve to depolarize. I think the geometric arguments in the article make sense in that context as well, the same way they would for a chemotactic or pressure sensor.
- 1lint 3y ago
  I think the eye example is in fact different because light passing through the lense is inverted onto the retina as the root comment explains, there's no such inversion for perceiving other senses. I find the explanation intuitively appealing at first, but given connections are criss crossed for far simpler animals such as worms like you mention, I don't think the idea that vision is the primary driver of this phenomenon is correct.
  - walnutclosefarm 3y ago
    It's not different, because the left-right flip by the lens is part of a rotationally symmetric image translation (that also flips up for down, e.g.), not a bilaterally symmetric reflection. With the rotation, there is no change in the relationship of parts of the image one to another, relative to our personal orientation, the way there is in the reflection, so it's trivial for the brain to account for it - up on the retina is always down in the real world, left on the retina is always right in the real world, etc.
    - 3y ago
thevagrant 3y ago
My theory is unlikely perhaps. Neural connections on the left side are potentially protected from damage to the right side of the body and vice versa. If the damage occurs to one side of the body but the brain is protected, would it not make for better chance of repair as the body heals?
- robbiep 3y ago
  Why? I don’t understand the logic of what you’re theorising. If your right side is damaged, why does having your ipsilateral side help? How common would this have been from an evolutionary perspective? And if your right side is damaged, and it’s taken out your right brain, now you’ve got a right side that doesn’t work and a left side you can’t control
  - XorNot 3y ago
    I kind of get the idea: if damage is coming to one side of the body, then the sensory data reporting it would be better handled by putting the brain half dealing with it on the opposite side, to give it a chance to react rather then also be damaged. But, I think the visual processing explanation is more compelling - since it seems unlikely this scenario would arrive in enough non-catastrophic situations to be evolutionary significant.
    - brokenkebaby 3y ago
      But visual processing theory doesn't make any sense with those worms, for instance
- NikkiA 3y ago
  That was my intuitive first thought, but a healthy left brain stuck trying to control a crippled right body is a worse situation than a healthy left brain controlling a healthy left body and merely deadweight on the right.
jwrallie 3y ago
Thanks for the explanation, it makes much more sense than the original article and the attached image was not even necessary for me to "get" the idea while many paragraphs and diagrams just left me confused before reading it.
- kerkeslager 3y ago
  The person you're responding two is wrong, though. Worms don't have retinas or lenses, but they have hemispherically crossed nervous systems.
naikrovek 3y ago
why can't we admit that we don't know? creatures who have photoreceptors only, and not eyes, have this same swapping. we don't know why this happens. period.
- kerkeslager 3y ago
  I agree that we don't know, but we can weight the different possible explanations by plausibility. The article's explanation would explain all the facts we do know, so we can reasonably weight that as a very plausible explanation. I'll also add that the article says, 'Now, this all makes sense mathematically, but it’s important to note that we don’t know for certain that this is truly why our brains and bodies are connected the way they are. There is very little biological research on this intriguing question. The convenient dodge often heard is that the scientific method tells us “what,” not “why.”' The GP's explanation doesn't explain why worms have criss-crossed nervous systems despite not having retinas or lenses, and it incorrectly assumes that other systems don't benefit from a criss-crossed nervous systems, so we can reasonably weight the GP's explanation as highly implausible.
  - naikrovek 3y ago
    > we can [weigh] the different possible explanations by plausibility that's not science. we have no idea what is plausible or not, really, and we should not forget that. we can make up reasons that seem reasonable or which makes sense when combined with other unproven hypotheses but we simply do not know and we should not be offering up any words other than "we don't know" when asked for explanations.
    - kerkeslager 3y ago
      > we have no idea what is plausible or not, really, and we should not forget that. we can make up reasons that seem reasonable or which makes sense when combined with other unproven hypotheses but we simply do not know and we should not be offering up any words other than "we don't know" when asked for explanations. You're saying "we have no idea what is plausible or not" and then go on to describe what plausibility is. The one part of your definition of plausibility that you're leaving out is observation, which is critical. We aren't just comparing hypotheses to other unproven hypotheses, we're comparing hypotheses to what we observe. We observe that criss-crossed nervous systems predate eyes in the phylogenic record, so from that observation, we can fairly conclusively say that criss-crossed nervous systems did not evolve as a result of eyesight. You can say that this is implausible, unreasonable, or doesn't make sense--I don't particularly care which terminology you prefer, but they're all saying basically the same thing: we know (i.e. have high degree of confidence) that a criss-crossed nervous system did not result from eyesight. I'm not sympathetic to Platonic idealist arguments that "it's not possible to know anything" because that's a completely useless way of thinking, which even the people saying it don't believe. If I offer a philosopher the opportunity to get punched in the face, they'll decline my offer, because they know from prior experience that getting punched in the face will be an unpleasant experience. You can't reasonably claim that it's impossible to know anything in conversation when all the choices in your life are based around the things that you know. Platonic idealist epistemology is a convenience argument that people only trot out when they want to disagree with something. Additionally, the idea is self defeating: if it's impossible to know things, then how do you know it's impossible to know things?
hackernewds 3y ago
but the flip is a vertical flip due to the nature of lenses? im having a really tough time interpreting the diagram too. I appreciate you trying to share your understanding though.
- omnicognate 3y ago
  The flip is both vertical and horizontal. Imagine all the rays converging at the focal point and carrying on. Each ray ends up on the opposite side of the centre of the image. Up->down, left->right, etc.
rcme 3y ago
I mean… it’s possible, but this explanation feels incomplete. Why does the brain need to create a contiguous imagine in this way?
- walnutclosefarm 3y ago
  To be mobile and agile in a three dimensional world, a creature needs to be able to process information about its environment such as adjacency, and direction - that is, the relationship of different parts of space, or things in space, to one another, matters a lot to the organism. You could store store each individual perceivable bit of spacial information randomly, but then to get this structural information you'd need an elaborate mapping layer that allowed the brain to compute distance and direction. It's far simpler to rely on actual spatial relationships in the cortex to model the structural information by mapping it to an analogous spatial structure information. Simpler means less energy in construction and operation of the facility, and that wins out in an evolutionary race.
  - rcme 3y ago
    This just seems like conjecture. There is already a mapping layer between the specific nerve activation and spatial information. A random mapping wouldn't be any more or less efficient in that regard. I could see something like going from one to two eyes causing this. When having one eye, you'd have a random nerve mapping. It's advantageous to have two eyes over one, but if the optical inputs for two eyes were to be randomly remapped, then the evolutionary knowledge stored in the single-eye mapping would be lost. So it would advantageous to map the optical nerves from two eyes in a way that mostly fits the single-eye mapping. Obviously, this is just a random theory without any evidence. I offer it only as an example of a logical argument as to why the spatial orientation of an object would affect the spatial orientation of the nerve mapping.
sebosp 3y ago
I only have one eye (retinoblastoma) and have never been able to see through my left eye, you can't imagine the amount of questions that just arrived in my head, do you have some material to explain this a bit more? Specially in regards to the flipping different fields? Thanks so much!
- moremetadata 3y ago
  You might like to read this. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4476552/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4476552/ PirB aka LILRB3 is also a protein involved in Alzheimers, toggling it affects neuroplasticity which includes vision. https://www.nature.com/articles/nrn3616 https://www.nature.com/articles/nrn3616
hackernewds 3y ago
> Of course now each side of the brain sees the opposite side, but we remedy this by crossing everything else so it plays well with visual interpretation. why
kerkeslager 3y ago
I don't think you understood the author's interpretation well enough to say you disagree with it. The problem you describe, of mapping binocular vision is an example of the topological problem described by the author. But it's not the only example: feeling nerves, for example, have the same symmetrical problem with mapping your skin sensations to the physical space, and your hearing also is "binocular" (there's actually a separate word for this, "binaural"). The visual problem you're describing is part of the topological problem. You're describing the same problem, but you're describing a part of the problem. Where you're just wrong is on two points: 1. "When it comes to everything else, there isn't a clear benefit for having processing swapped to opposite hemispheres." Wrong. As mentioned before, binaural hearing also needs to map a 3d topology to a 2d topology from two data collection points, and skin needs to map a 3d topology from many more points (but also symmetrical). Additionally, the effect works on "outputs" as well as "inputs", mapping the 2d space to a 3d space so that you can control symmetrical tools such as your arms and legs means that the swapping is needed when sending signals outward as well. 2. "[V]isual processing benefits from it greatly, and so the rest of the nervous system goes along with it." Vision is not the evolutionary driver here. The criss-crossed neurology predates the existence of vision in our evolutionary heritage. The article mentions worms, for example: how does your hypothesis explain why their nerves crossed hemispherically given they don't have lenses and retinas?

punnerud 3y ago

Just the swapping of sides in the brain, isn’t there a clear benefit for the stress/weight of the nerves to the rest of the body? If right side was connected right arm the nerves would be pressed “out”. When they cross there is less pressure in the spinal cord?

elcritch 3y ago

Flipping the eyes is the same geometric reasoning the article discusses. The same thing applies to touch and motor control too. Its just a general extrapolation of the same principle. It would also explain why creatures without sight as well like many worms.

nomel 3y ago
> It would also explain why creatures without sight as well like many worms This would require that worms evolved the cross crossing independently, after our common ancestor, which doesn’t seem to be clear [1]. All it would take is a few light receptors to get the cross cross party started. [1] https://www.sciencedaily.com/releases/2010/02/100201101905.htm https://www.sciencedaily.com/releases/2010/02/100201101905.h...
- jaggederest 3y ago
  Those kind of light receptors don't have the lenses that cause our eyes to flip the image. They are omnidirectional and usually not even on the surface of the critter. Just a small bag of rhodopsin that triggers a nerve to depolarize. I think the geometric arguments in the article make sense in that context as well, the same way they would for a chemotactic or pressure sensor.
1lint 3y ago
I think the eye example is in fact different because light passing through the lense is inverted onto the retina as the root comment explains, there's no such inversion for perceiving other senses. I find the explanation intuitively appealing at first, but given connections are criss crossed for far simpler animals such as worms like you mention, I don't think the idea that vision is the primary driver of this phenomenon is correct.
- walnutclosefarm 3y ago
  It's not different, because the left-right flip by the lens is part of a rotationally symmetric image translation (that also flips up for down, e.g.), not a bilaterally symmetric reflection. With the rotation, there is no change in the relationship of parts of the image one to another, relative to our personal orientation, the way there is in the reflection, so it's trivial for the brain to account for it - up on the retina is always down in the real world, left on the retina is always right in the real world, etc.
  - 3y ago

thevagrant 3y ago

My theory is unlikely perhaps. Neural connections on the left side are potentially protected from damage to the right side of the body and vice versa. If the damage occurs to one side of the body but the brain is protected, would it not make for better chance of repair as the body heals?

robbiep 3y ago
Why? I don’t understand the logic of what you’re theorising. If your right side is damaged, why does having your ipsilateral side help? How common would this have been from an evolutionary perspective? And if your right side is damaged, and it’s taken out your right brain, now you’ve got a right side that doesn’t work and a left side you can’t control
- XorNot 3y ago
  I kind of get the idea: if damage is coming to one side of the body, then the sensory data reporting it would be better handled by putting the brain half dealing with it on the opposite side, to give it a chance to react rather then also be damaged. But, I think the visual processing explanation is more compelling - since it seems unlikely this scenario would arrive in enough non-catastrophic situations to be evolutionary significant.
  - brokenkebaby 3y ago
    But visual processing theory doesn't make any sense with those worms, for instance
NikkiA 3y ago
That was my intuitive first thought, but a healthy left brain stuck trying to control a crippled right body is a worse situation than a healthy left brain controlling a healthy left body and merely deadweight on the right.

jwrallie 3y ago

Thanks for the explanation, it makes much more sense than the original article and the attached image was not even necessary for me to "get" the idea while many paragraphs and diagrams just left me confused before reading it.

kerkeslager 3y ago
The person you're responding two is wrong, though. Worms don't have retinas or lenses, but they have hemispherically crossed nervous systems.

naikrovek 3y ago

why can't we admit that we don't know? creatures who have photoreceptors only, and not eyes, have this same swapping. we don't know why this happens. period.

kerkeslager 3y ago
I agree that we don't know, but we can weight the different possible explanations by plausibility. The article's explanation would explain all the facts we do know, so we can reasonably weight that as a very plausible explanation. I'll also add that the article says, 'Now, this all makes sense mathematically, but it’s important to note that we don’t know for certain that this is truly why our brains and bodies are connected the way they are. There is very little biological research on this intriguing question. The convenient dodge often heard is that the scientific method tells us “what,” not “why.”' The GP's explanation doesn't explain why worms have criss-crossed nervous systems despite not having retinas or lenses, and it incorrectly assumes that other systems don't benefit from a criss-crossed nervous systems, so we can reasonably weight the GP's explanation as highly implausible.
- naikrovek 3y ago
  > we can [weigh] the different possible explanations by plausibility that's not science. we have no idea what is plausible or not, really, and we should not forget that. we can make up reasons that seem reasonable or which makes sense when combined with other unproven hypotheses but we simply do not know and we should not be offering up any words other than "we don't know" when asked for explanations.
  - kerkeslager 3y ago
    > we have no idea what is plausible or not, really, and we should not forget that. we can make up reasons that seem reasonable or which makes sense when combined with other unproven hypotheses but we simply do not know and we should not be offering up any words other than "we don't know" when asked for explanations. You're saying "we have no idea what is plausible or not" and then go on to describe what plausibility is. The one part of your definition of plausibility that you're leaving out is observation, which is critical. We aren't just comparing hypotheses to other unproven hypotheses, we're comparing hypotheses to what we observe. We observe that criss-crossed nervous systems predate eyes in the phylogenic record, so from that observation, we can fairly conclusively say that criss-crossed nervous systems did not evolve as a result of eyesight. You can say that this is implausible, unreasonable, or doesn't make sense--I don't particularly care which terminology you prefer, but they're all saying basically the same thing: we know (i.e. have high degree of confidence) that a criss-crossed nervous system did not result from eyesight. I'm not sympathetic to Platonic idealist arguments that "it's not possible to know anything" because that's a completely useless way of thinking, which even the people saying it don't believe. If I offer a philosopher the opportunity to get punched in the face, they'll decline my offer, because they know from prior experience that getting punched in the face will be an unpleasant experience. You can't reasonably claim that it's impossible to know anything in conversation when all the choices in your life are based around the things that you know. Platonic idealist epistemology is a convenience argument that people only trot out when they want to disagree with something. Additionally, the idea is self defeating: if it's impossible to know things, then how do you know it's impossible to know things?

hackernewds 3y ago

but the flip is a vertical flip due to the nature of lenses? im having a really tough time interpreting the diagram too. I appreciate you trying to share your understanding though.

omnicognate 3y ago
The flip is both vertical and horizontal. Imagine all the rays converging at the focal point and carrying on. Each ray ends up on the opposite side of the centre of the image. Up->down, left->right, etc.

rcme 3y ago

I mean… it’s possible, but this explanation feels incomplete. Why does the brain need to create a contiguous imagine in this way?

walnutclosefarm 3y ago
To be mobile and agile in a three dimensional world, a creature needs to be able to process information about its environment such as adjacency, and direction - that is, the relationship of different parts of space, or things in space, to one another, matters a lot to the organism. You could store store each individual perceivable bit of spacial information randomly, but then to get this structural information you'd need an elaborate mapping layer that allowed the brain to compute distance and direction. It's far simpler to rely on actual spatial relationships in the cortex to model the structural information by mapping it to an analogous spatial structure information. Simpler means less energy in construction and operation of the facility, and that wins out in an evolutionary race.
- rcme 3y ago
  This just seems like conjecture. There is already a mapping layer between the specific nerve activation and spatial information. A random mapping wouldn't be any more or less efficient in that regard. I could see something like going from one to two eyes causing this. When having one eye, you'd have a random nerve mapping. It's advantageous to have two eyes over one, but if the optical inputs for two eyes were to be randomly remapped, then the evolutionary knowledge stored in the single-eye mapping would be lost. So it would advantageous to map the optical nerves from two eyes in a way that mostly fits the single-eye mapping. Obviously, this is just a random theory without any evidence. I offer it only as an example of a logical argument as to why the spatial orientation of an object would affect the spatial orientation of the nerve mapping.

sebosp 3y ago

I only have one eye (retinoblastoma) and have never been able to see through my left eye, you can't imagine the amount of questions that just arrived in my head, do you have some material to explain this a bit more? Specially in regards to the flipping different fields? Thanks so much!

moremetadata 3y ago
You might like to read this. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4476552/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4476552/ PirB aka LILRB3 is also a protein involved in Alzheimers, toggling it affects neuroplasticity which includes vision. https://www.nature.com/articles/nrn3616 https://www.nature.com/articles/nrn3616

hackernewds 3y ago

> Of course now each side of the brain sees the opposite side, but we remedy this by crossing everything else so it plays well with visual interpretation. why

kerkeslager 3y ago

I don't think you understood the author's interpretation well enough to say you disagree with it. The problem you describe, of mapping binocular vision is an example of the topological problem described by the author. But it's not the only example: feeling nerves, for example, have the same symmetrical problem with mapping your skin sensations to the physical space, and your hearing also is "binocular" (there's actually a separate word for this, "binaural"). The visual problem you're describing is part of the topological problem. You're describing the same problem, but you're describing a part of the problem. Where you're just wrong is on two points: 1. "When it comes to everything else, there isn't a clear benefit for having processing swapped to opposite hemispheres." Wrong. As mentioned before, binaural hearing also needs to map a 3d topology to a 2d topology from two data collection points, and skin needs to map a 3d topology from many more points (but also symmetrical). Additionally, the effect works on "outputs" as well as "inputs", mapping the 2d space to a 3d space so that you can control symmetrical tools such as your arms and legs means that the swapping is needed when sending signals outward as well. 2. "[V]isual processing benefits from it greatly, and so the rest of the nervous system goes along with it." Vision is not the evolutionary driver here. The criss-crossed neurology predates the existence of vision in our evolutionary heritage. The article mentions worms, for example: how does your hypothesis explain why their nerves crossed hemispherically given they don't have lenses and retinas?