Why the brain’s connections to the body are crisscrossed
quantamagazine.org
quantamagazine.org
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_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.
If right side was connected right arm the nerves would be pressed “out”. When they cross there is less pressure in the spinal cord?
Its just a general extrapolation of the same principle. 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.h...
But we have two eyes, so if you directly connect the two inverted images observed by each retina, there would be a discontinuity in the middle of the joined image (peripheral light from the outer sides of each eye would be mapped to the middle of the joined retinal image). The original commenters point was that criss crossing the neural connections from the retina would resolve this discontinuity, allowing the brain to process a continuous image.
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?
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.
Worms don't have retinas or lenses, but they have hemispherically crossed nervous systems.
creatures who have photoreceptors only, and not eyes, have this same swapping.
we don't know why this happens. period.
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.
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.
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?
if you don't know, and you're talking to a layman audience, don't say you know. don't pretend you know, don't even present guesses, even when they are framed as guesses, because people will hear you saying you know.
"we think [educated guess] but we don't know" is the only acceptable phrasing, to me.
Remember this: science can never tell you when your hypothesis is correct. science can only ever tell you when your hypothesis is wrong.
I think the entire world has forgotten this.
Richard Feynman explains it very well in this lecture, particularly in the bit starting at 20:04.
> "we think [educated guess] but we don't know" is the only acceptable phrasing, to me.
I understand that it's irresponsible to communicate a greater degree of confidence than is supported by the evidence, but I'm saying that it can also be irresponsible to communicate a lesser degree of confidence than is supported by the evidence.
Consider, for example, the statement "vaccines don't cause autism". There is absolutely no evidence that vaccines cause autism (and if you want to argue that, go elsewhere--I'm not going to argue with assholes). Saying "we think vaccines don't cause autism, but we don't know" is irresponsible, because you have people saying, "Vaccines cause autism! I'm 100% sure!" A layman audience, hearing both people, hears that one isn't sure, and the other one is absolutely sure, and so they believe the absolutely sure one. The result is unvaccinated kids, the revitalization of nearly-extinct diseases, and widespread human death and suffering.
Pedantically using words that, by their dictionary definition, mean a certain thing, doesn't result in quality communication. If you say words that pedantically mean the truth, but fail to produce the true belief in the minds of your audience, you've failed to communicate. That's not entirely your fault--some of the responsibility for communication rests on the listener--but if you can improve your chances of communicating the truth by speaking with confidence that is supported by the evidence, it is your right and perhaps even obligation to do so.
> Remember this: science can never tell you when your hypothesis is correct. science can only ever tell you when your hypothesis is wrong.
This is trivially wrong:
Let's say your hypothesis is "vaccines cause autism". According to what you just said, we can't prove that hypothesis correct, we can only prove it wrong.
So let's say we've proven wrong the hypothesis, "vaccines cause autism". Now consider the hypothesis "vaccines don't cause autism". Haven't you just proven that hypothesis correct?
> I think the entire world has forgotten this.
Don't you mean "I think the entire world has forgotten this, but I don't know"? /s
Cut the melodrama. You're not so special that you have some unique or even rare knowledge that everyone else has forgotten. Lots of people know a hell of a lot more about science than either you or I do.
The Feynman video you linked doesn't load the video for me.
to elaborate on that point a tiny bit more: science cannot prove itself correct; science can only ever prove itself incorrect, because if your theory is incorrect, future experiments will eventually reveal that. if your theory is correct, you won't ever be proven correct, you will simply never be proven wrong.
this isn't about language and who is sure and who is not, this is about lying.
it's true that there is no evidence that vaccines cause autism. it's also true that scientists have looked and looked and looked for that link, and found nothing at all. with current technology and understanding, there is no link, and no one can prove a link between the two. no one can prove that link exists; not even those who are absolutely sure it exists.
see? no lies, no hyperbole, no guesswork, and the facts are clearly communicated.
I appreciate you trying to share your understanding though.
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.
PirB aka LILRB3 is also a protein involved in Alzheimers, toggling it affects neuroplasticity which includes vision.
why
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?
I had a sugar maple about 3 feet away from where I had to dig the foundation for the patio. An arborist said that it’s a bit close but the maple is strong and it should recover fine.
I cut out a bunch of roots and within a month the exact sections of the canopy corresponding to the roots I cut went brown.
It was incredible to me how the roots relate directly to the canopy. I always kinda thought it was one big circulatory system, where everything supports everything. I expected the whole tree to struggle a bit.
Next season the tree was fully recovered.
The neurological system seems to work the same way? It’s a directed graph where one root supports a very specific set of branches? I guess the circulatory system is like that too if you separate the two sets. Nature doesn’t really like cyclic graphs, does it?
neural convergence, and divergence, produce logic arrays that integrate many parameters to one integrated decision, vice versa
I had a course in computational neuroscience as part of my bachelor's and one of the things that we covered was that the timing of fires is important, in that depending on how soon before or after a neighbouring neuron fires, the connection may be weakened or strengthened. This is called Spike-timing-dependent plasticity:
> Under the STDP process, if an input spike to a neuron tends, on average, to occur immediately before that neuron's output spike, then that particular input is made somewhat stronger. If an input spike tends, on average, to occur immediately after an output spike, then that particular input is made somewhat weaker hence: "spike-timing-dependent plasticity"
From [1].
The implication of that, I believe, is that it prevents short cyclic graphs, for the sole reason of avoiding feedback loops that can cause the brain to go haywire (lol) due to the feedback loop. It sounds like an evolutionary adaptation to prevent short-circuiting.
From Hebbian Learning, we have that the cycles would become easier to trigger, meaning that it is a feedback loop that increases efficiency, however, without a mechanism to prevent this cyclical feedback loop, the brain could be filled with cycles that eventually turn to just rings, which is probably not a desirable property.
If anyone knows more about this please tell me. If it's a new idea, please remember to add my name :')
[1] https://en.wikipedia.org/wiki/Spike-timing-dependent_plastic...
neurons change functional, and structural state, depending on past events [hysterisis] and will shut down/modify state activities depending on feedback from post synapic neurons.
also neurons will get tired and handoff activity to similar neurons in a cohort.
https://www.frontiersin.org/articles/10.3389/fncom.2011.0002...
Root systems in general (for trees at least) mirror what's happening above ground. Pruning trees is a beneficial intervention, as it causes the corresponding roots to die and decompose. This not only makes precious biomatter available for recycling by the microorganisms in the soil, but also releases chemical signals that cause the tree itself and neighbouring plants to send out new growth.
I have some news for you: https://en.wikipedia.org/wiki/Circle_of_Willis
The issue described here is left-right symmetry, and it also applies to insects, so it is more general.
E.g.: someone else here was arguing that maybe the mirroring helps the brain keep processing inputs from the side that was hit. Evolution does not work this way! Flipping doesn’t “just” happen, that’s a huge morphological change. It had to have evolved incrementally, with each intermediate step having an immediate benefit.
The paper explains how and why this may have occurred.
There have been several papers published that outline how eyes could have evolved via a series of incremental steps.
I don't have references handy[1], but the process is thought to go something like this:
1. The earliest, primitive animals were tiny. Think small worms and the like.
2. Soft-bodied creatures are always at least somewhat translucent to light, at least to a certain depth: a few millimeters at least. Just shine a torch through the thin part of your hand between your thumb and forefinger! Similarly, this is why you know it is daylight or not even with your eyelids closed.
3. If a nerve in the brain of a primitive creature contains a chemical that is both neurologically active and light sensitive, then even without any "eye structure" at all, it can detect lightness and darkness. Melatonin is thought to be a candidate for this chemical. This can happen by accident but is immediately useful for seeking shelter, escaping a "sudden shadow" (predator), detecting day/night cycles, etc...
4. If those nerves migrate closer to the surface of the skin, then they receive more light, and the sensitivity becomes directional. The most obvious thing is for a spot to develop on top of the head, which is still seen in many creatures today!
5. Having multiple such spots over an area, sensitivity increases.
6. If that light-sensitive patch becomes concave[2], then this provides crude directional sensitivity. The more concave it becomes, the more accurate the direction sense, until it invaginates completely to form a cavity with a small hole as the pupil -- a pinhole camera. There are creatures living today with similar primitive eyes!
7. If the cavity becomes filled with a transparent variant of the skin, then it can be protected from filling up with dirt, etc...
8. If this transparent flesh has uneven index of refraction, it can focus light. This is a primitive lens.
9. Etc...
You get the idea. Essentially, every step can occur incrementally, providing benefits at every step, and these steps are littered throughout the tree of life, we just have to put the steps back in order to see the timeline.
[1] You can check the references in the Wiki article if you like: https://en.wikipedia.org/wiki/Evolution_of_the_eye
[2] Convex also works, and then you end up with insect eyes!
In some primitive ancestor, it might have 'just' happened randomly. Some mutations that are small in genetic code terms can have huge effects on body structure.
That said, wonderful and simple result.
[0] https://en.wikipedia.org/wiki/Contralateral_brain#Twist_theo...
"To make sense of the sensation [...] your brain would have to switch from one somatotopic map to another one with the opposite z-axis orientation".
This a textbook example of the "Cartesian theater" fallacy. It assumes a little person inside the brain who has to deal with an image projected upside-down. Of course that doesn't make sense.
So you’re right about the little man. I struggle to see why some arbitrary axis transform is “too hard”.
Obviously some parts of the skin are stretched, so a 2D map will cause a lot of deformation. Also some parts of the skin are more sensitive than other and will need more brain surface. But this is what is happening, there are a few maps in the brain https://en.wikipedia.org/wiki/Cortical_homunculus and they are quite deformed, and they even have a few cuts here and there.
Once you decide to cut the map in two parts, each part can be projected in both orientations without geometric problems.
> Odd things happen when we [project 3D space onto a 2D surface]. On a 2D map, an airplane taking the most direct path between two cities appears to travel in an arc, and satellites orbiting the globe appear to oscillate in a sinusoidal path
If you use the Gnomonic projection then all great circles become straight lines.[0] The only "catch" is that you need to cut the Earth into two hemispheres.
Imagine a sphere. You cut it in two halves, and you get something like this https://en.wikipedia.org/wiki/Nicolosi_globular_projection and use a scissor to cut it in the middle. Now imagine the initial sphere is made of rubber and it contracts to a body glove thigh around the person.
There are no discontinuity inside each half. Just a huge discontinuity between the two halves, but each one is processed by a different side of the brain anyway.
The projections in each half can be made in both orientations, the one that is like the skin and the mirror one. The non-mirrored is difficult to wire, but the mirrored one is easy to wire.
About the discontinuity, there is a huge discontinuity between the two sides of your body that are processed by the two sides of your brain, and you don't notice it.
Moreover, in each eye, there is a discontinuity because each half of each of your eyes is proceed in a different side of your brain https://www.quora.com/Which-side-of-the-brain-does-the-optic... With that hardware I expect to see a black vertical line in the middle of my eyes, but the transition is quite smooth.
I like the explanation in terms of predator avoidance behavior. Imagine a primitive fish with eyes that can detect motion. If it sees something moving, usually it wants to get away from that thing. If you see something moving in your left eye, the best way to swim away is to send a signal for a muscle contraction in your right side, which will cause you to curl and swim to the right. So the best wiring is a direct connection from left eye to right side, and right eye to left side. The brain is built up starting from that kind of connection.
...that's not what we have.
You're just making shit up. There's zero evidence for this.
You also don't seem to understand that inputs (senses) are also crossed, meaning that in your hypothetical inputs-directly-tied-to-outputs scenario, the movement produced would be in exactly the opposite direction to what you're describing, i.e. toward the predator.
And even if you found a way to resolve these problems, movement is far more complex than you're describing, and practically requires a more complex structure between sensation and reaction to coordinate muscular contractions which produce movement. The sort of reflexive muscular contraction you're describing would look more like a seizure than a contraction which produces useful movement.
I'm all for preferring the simplest explanation which explains our observations, but your explanation doesn't explain what we observe in any way.
...meaning you didn't understand it.
It may not be a good explanation, but the problem isn't that it's too abstract.
> If you see something moving in your left eye, the best way to swim away is to send a signal for a muscle contraction in your right side, which will cause you to curl and swim to the right. So the best wiring is a direct connection from left eye to right side, and right eye to left side. The brain is built up starting from that kind of connection.
If your fish only contracts the right side of it's body, that's not going to create effective motion, and while your fish is having the seizure you've described, it's going to get eaten.
Even if there were some sort of direct eye-to-muscle connection (which there isn't) the left eye would connect to the right side of the brain, which would then connect to the left side muscle, so you'd have left eye to left muscle (via the brain), not left eye to right muscle.
I'm amused at the audacity of the author for asking the webdevs to manually wire up a unique HTML span with a custom CSS transform solely to make a single word appear to be rendered as though seen in a mirror. :)
Very interesting idea described here. I think it would benefit from some animations.
I'm hoping that LLMs will be able to generate and animate SVGs to help with this.
Try it, YMMV.
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Normally when you look into your own eyes in a mirror they are each looking into themselves.
If you cross your eyes and get the distance just right, you can look into each eye from the other.
For me it causes a strange effect, or seems to. I would be interested to hear reports from others of their subjective experience of doing that?
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Edit to add: Reading aloud also has different effects than reading silently. I surmise that the extra feedback loop from voice to ear has something to do with it.
A common proofreading trick is to read your writing aloud. You catch errors that are [negative hallucination?] elided by, uh, non-external loops.
I find that it no longer works very well for me though: I often read aloud without experiencing or remembering anything I’ve read at all. It’s the same autopilot that takes over driving.
Cutting (or partially severing) the corpus callosum connecting the hemispheres of the brain has been shown to have some interesting results along those lines, though: https://en.wikipedia.org/wiki/Split-brain
For others interested, look up Roger Sperry's split-brain experiments[1], done at CalTech. He received the 1981 Nobel Prize for the work. I'm surprised the above article doesn't mention it.
We don't cut cut peoples' brains in half any more, so it was a unique moment in time when they had people available with this condition, and the results are quite illuminating.
Here's a video with some interviews with real patients: https://www.youtube.com/watch?v=aCv4K5aStdU
And a here's a timestamp where one of the experiments is performed: https://www.youtube.com/watch?v=aCv4K5aStdU&t=101s
Without injury the input should be shared just fine, but since the difference is so severe when the connections are damaged I guess (with no credentials or so to back it up) that there could be some hard-to-measure differences in thoughts formed depending on which eye is used.
Scientific American Frontiers : severed corpus callosum https://youtu.be/lfGwsAdS9Dc
left field of retina in both eyes communicates contralaterally with right hemispheric optic cortex, vice versa.
It seems un-testable in terms of eliminating any sort of placebo/expectation effect though.
I personally was born with a crossed eye. It's been corrected, but the reading acuity of my secondary eye is worse than my primary. Everything is in focus optically, but reading is more strenuous. It's almost as if there's some letter or syllable sized gaps my brain is interpolating around. Perhaps I only notice it when reading because of the density of high frequency content in all the letters.
Later it evolved to be the brain
After reading the piece, as he's proposition a potential theory based on mathematics and elegance of the solution, her reply still feels totally apt. That makes the title slightly misrepresent what he's trying to convey. We don't get the why, just a maybe.
Otherwise I'm not sure 3d mapping simplicity is enough of an answer when the brain can also adapt to way more complicated configurations when receiving partial damage for instance. It also feels like we have very few organs that developped along the most simple solution
Wouldn't a 3-D brain mapping also solve this issue? Do we just have this flipped symmetry because neural nets started out non-3-D in simple organisms, and we have all just inherited the 2-D structure as our brains have grown? Were there ever 1-D neural networks, and if so, how did they work?
as far as 1-d networks are concerned, electrical excitation is not an absolute property of neurons. unicellular organisms employ variations of electrical potential to initiate, coordinate, and buffer functions, and future changes of state.
For 3-D mapping onto 3-D I don't understand what you're saying. Except for vision, for which I understand there are optical properties that require image flipping and inversion.
sensory field is the snapshot state of each neuron in a 2-d array. cortical field is same but it is now destined for processing.
left and right fields differ by paralax and this difference is used, to construct a 3-d percept
field is the informatic state of all the neural elements, involved.
note: these are terms applied in the context of neuroscience.
Almost certainly not. Don't take this the wrong way, but you might want to get your brain checked out..
You only see quantum mechanical effects, like what you see in the double-slit experiment, in essentially undisturbed systems. That's why when you 'measure' which path the photons take, the interference pattern in the double-slit experiment fails to occur. Your body having lots and lots of atoms sloshing around at body temperature is analogues to taking measurements. It 'collapses' the wave function, if you pardon me using terminology from the Copenhagen interpretation.
See https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=91986... for some background. Or perhaps https://en.wikipedia.org/wiki/Quantum_decoherence first as an introduction.
As an aside: (thermal) noise demolishing quantum effects is a big part of why it's so hard to actually build a quantum computer.
What you described might be hallucinations, and so you might want to go to a neurologist or so.
its a tendency of biological systems to innovate one step at a time, as persistence is a demand, so the system is not overhauled large scale, that would be tantamount to major negative selection, thus non-persistent properties.
Look up Brown-Séquard syndrome for a House MD level quirk of the body.
Auditory signals don't cross. We should raise a human with eye flipping lenses
Ergo extreme impact trauma to the head results in the brain breaking the connections between the two hemispheres at the corpus callusom, resulting in immobolisation. However this could also be a temporary kill switch which immobolises the body until such time as the brain's neuroplasticity has restored the corpus callosum.
In such a condition the individual would likely be left for dead, be going hungry and quite likely thirsty which creates an increase in phagocytosis which in turn causes an increase in h2o2 and then catalase the enzyme from the liver breaks this down to water and oxygen, ergo the conditions to help the body live, if not in a slimmed down form (pun intended) still exists and lends credence to fasting.