[HN Gopher] Why the brain's connections to the body are crisscro... ___________________________________________________________________ Why the brain's connections to the body are crisscrossed Author : rolph Score : 163 points Date : 2023-04-22 15:17 UTC (7 hours ago) (HTM) web link (www.quantamagazine.org) (TXT) w3m dump (www.quantamagazine.org) | killdevil wrote: | I have always assumed that bilaterally symmetric animals have | neural cross-wiring so that the two hemispheres of the brain are | forced to cooperate more than they otherwise would. Simple | animals are prevented from, say, hoarding resources selfishly and | maladaptively on the right. | Waterluvian wrote: | Tangentially related to this is when I installed a patio in my | yard two summers ago. Okay, let me explain: | | 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? | ciconia wrote: | > 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. | | 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. | rolph wrote: | i think of it like a plinko ball setup, in analog. | | neural convergence, and divergence, produce logic arrays that | integrate many parameters to one integrated decision, vice | versa | Waterluvian wrote: | If your model includes a tiny Bob Barker commentating on the | transmission of neural signals, I'm on board. | PartiallyTyped wrote: | > Nature doesn't really like cyclic graphs, does it? | | 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... | rolph wrote: | just keep inmind, this, and other phenomenon dont happen in | all neurons, or in any particular neurons 100% of the time. | | 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. | IIAOPSW wrote: | Its very much not a directed graph. We do have brain loops. | That's where brain waves come from, circular paths of neurons | activating themselves in a circular firing squad. | gus_massa wrote: | I still don't buy it. Once you cut a person vertically in two | half with a guillotine, you get two topological disks of skin. | Each disk can be mapped into the 2D surface of the brain. It | doesn't matter if the parts of the skin are in the same plane or | rotated 90deg. | | 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. | [deleted] | N659 wrote: | There's a theory that says an ancestor of all vertebrates just | flipped its head around and it stayed that way. (Source: | https://arxiv.org/abs/1003.1872). Can someone who knows more | about animal physiology explain this paper? Thanks. | Shorel wrote: | But that issue is related to the fact that insects have their | nerves in the inferior part of their bodies, and the digestive | tract on top, while we have our nerves in the back (protected | by bones), and the digestive tract on the front, which would be | the lower part if we were still walking with four limbs. | | The issue described here is left-right symmetry, and it also | applies to insects, so it is more general. | N659 wrote: | So far I have read that these are related theories and the | "somatic twist" theory is an expansion on the earlier theory | of inversion. And there seems to be another related but | separate theory called the axial twist theory (explained in | the linked paper). | jiggawatts wrote: | This should be the top comment here: a reference to an actually | _scientific_ analysis that uses an argument that is plausible | from an evolutionary perspective. | | 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. | m3kw9 wrote: | I think it evolved to be crisscrossed, likely non crisscrossed | ones were just eliminated, I'm guessing for redundant reasons. | Having it cross covers a bigger area and may serve to cover | issues if part of the other side get severed. | renewiltord wrote: | Fascinating. The magazine doesn't appear to have a way to pay to | subscribe. | | 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. | crazygringo wrote: | > _Since Quanta is a nonprofit foundation-funded publication, | all of its resources go toward producing responsible, freely | accessible journalism that is meticulously researched, | reported, edited, copy-edited and fact-checked. And our | editorial independence ensures the impartiality of our science | coverage -- our articles do not reflect or represent the views | of the Simons Foundation._ | [deleted] | nnlsoccer wrote: | Odd that in the entire explanation they completely ignore | embryology and the pathway dependence that came with evolutionary | progression between species | oncotic wrote: | 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_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. | thevagrant wrote: | 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? | punnerud wrote: | 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 wrote: | 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. | bookofjoe wrote: | https://news.ycombinator.com/item?id=35647341 | canjobear wrote: | I don't find the TFA explanation convincing; it's too abstract. | | 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. | ProjectArcturis wrote: | But sensory input is also reversed. If you're touched on the | right side of your body, it projects to the left side of your | brain, which then projects back to the right side of your body | again. If the goal were to minimize wire length, everything | would stay on the right side. | rhn_mk1 wrote: | As much as I like Quanta Magazine, they dropped the ball on this | one. I didn't understand anything of the summary of the paper. I | hope they attach some images to the next piece about topology... | amelius wrote: | Also, it didn't really answer the "why" question. | rhn_mk1 wrote: | I presume it's somewhere in the incomprehensible explanation | :) | dandanua wrote: | It's just for power balance, I think. Otherwise the left and | right parts would be too independent. And this causes conflicts | of interests. | anonymouskimmer wrote: | > While there are lots of solutions to this wiring problem, the | most elegant is to have two bilaterally symmetrical systems of | wiring between the brain and the body, with the connections from | each side of the body crossing the midline. | | 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? | rolph wrote: | the problem is mirror image one, when mapping sensory field | stimlus, to cortical field state. | | 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. | anonymouskimmer wrote: | > the problem is mirror image one, when mapping sensory field | stimlus, to cortical field 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. | rolph wrote: | light bounces off objects and travels through the lens of | the eye. when light exits the lens it is inverted, so | things to your right are projected on the left side of the | retina, the left retinae then communicate contralaterally | to the right optical cortex. | | 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 | anonymouskimmer wrote: | I thought by "cortical" you were referring to the | cerebral cortex. | rolph wrote: | yes cortical in this case is applied to the cortical | structure, of the optic lobes of the cerebrum, summarized | as optical cortex | | field is the informatic state of all the neural elements, | involved. | | note: these are terms applied in the context of | neuroscience. | svnt wrote: | I also didn't follow the leap requiring animal experience to | represent 3D phenomena in 2D mapping in brain space -- it feels | like an oversimplification in order to force the peg to fit the | hole. | anonymouskimmer wrote: | I think it has to do with the brain being built up in layers. | Even though our brains are 3-D in shape, the layers are | effectively thick 2-D surfaces. This is just a guess though. | rolph wrote: | you have the proper concept | amelius wrote: | If this is true, then perhaps we should wire motherboards in a | way such that the connections are crisscrossed too. | rolph wrote: | this may be effective, when the time comes to allow a | synthetic, to have a mobile body, that must model its sensory | input to a common format to parse against a stored data array, | in order to decide what scheme to employ when negotiating a | physical, 3+d world using 2-d arrays of neuromime excitation. | xwdv wrote: | Will you get different thoughts and ideas if you read a book with | one eye closed vs the other? | kirse wrote: | That is such a wild idea. I wonder if you dance with one eye | covered or learn a language solely thru input from right vs. | left eye (and ears) would it matter? Or what about musicians | like Jimi Hendrix who played their instrument backwards? | | It seems un-testable in terms of eliminating any sort of | placebo/expectation effect though. | sgtnoodle wrote: | Various corrective surgeries for vision over the years have | intentionally mismatched focal distance between eyes. | Presumably there are at least tens of thousands of people | that can only read up close with one eye, and at a distance | with the other. | | 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. | eimrine wrote: | Too slow connection between the secondary eye and letter | recognition center? | gibolt wrote: | This is a mind blowing question. If so, does that mean your | dominant eye is the default path? | thulle wrote: | I read some brain encyclopedia ages ago, so details are a bit | foggy, but when the connections between the two hemispheres of | the brain gets injured/damaged it can have some really strange | effects. In the book they retold tests where they did things | like showing a picture of a shovel to only one eye and ask the | person to think of a garden tool, and they'd say shovel. Then | they'd ask the person to explain why they're thinking of a | shovel, and they'd tell some elaborate story like having met | their neighbour a few days ago who said that they need to buy a | shovel, or something similar. This due to the connection | between the part(s) of the brain that constructs the story | having severed connections to the parts the receives and | processes the visual input of that eye. There was a few hundred | pages of tests like these with wildly different effects | depending on where the connections had been damaged. | | 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. | callesgg wrote: | An intressting thing with that, is that the side of the brain | that controls speech might be completly accurate. It could | have said showel cause that side of the brain thoght of its | neighbors shovel. The only thing that it would be missing | would be that it was influenced to think of that shovel event | by input given to the other brain half. | shagie wrote: | A severed corpus callosum - sometimes done to mitigate the | effects of sever epilepsy (the seizure on one side of the | brain can't travel to the other side). | | Scientific American Frontiers : severed corpus callosum | https://youtu.be/lfGwsAdS9Dc | loa_in_ wrote: | So it seems the answer would be yes, but we can't say how or | to what extent. | carapace wrote: | I do. | | Try it, YMMV. | | - - - - | | 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? | | - - - - | | 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. | svnt wrote: | Your auditory memory is a different space than other forms of | memory, so when you read aloud to yourself it permits you to | buffer additional information and get to it later, in | addition to mapping it through different perceptual space as | you've noted. So at least the perceptual circuit and the time | constraints are changed by doing so. | | 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. | loa_in_ wrote: | Both may be true! | skirmish wrote: | A life hack I found useful: if I need to remember a few | numbers for a few minutes before I can write them down, I | keep quietly repeating them in my mind. It works much | better for me than trying to memorize them since I think | it uses auditory memory; after a few repetitions the | sequence of words becomes automatic and easy to repeat | without thinking. | lookdangerous wrote: | I have noticed my right eye twitch after working on the | computer too long. I felt like my left brain had been dominant | for too long. | loa_in_ wrote: | Be wary, in my personal case it was neck tightness | restricting blood flow to the face and jaw. Normally you | should be able to make a square by touching fingers to your | opposing shoulders and touch wrists to your nose with no pain | or tightness, I could not. | rolph wrote: | the contralateral "wiring" is also conserved in the retinal | processing. | | left field of retina in both eyes communicates contralaterally | with right hemispheric optic cortex, vice versa. | gus_massa wrote: | I agree, eyes are even more weird. There is a nice image here | showing how they are conected https://www.quora.com/Which- | side-of-the-brain-does-the-optic... | rolph wrote: | you may be interested: | | https://en.wikipedia.org/wiki/Evolution_of_the_eye | | https://academic.oup.com/jhered/article/96/3/171/2187545 | JonathonW wrote: | Not in a healthy human. | | 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 | euroderf wrote: | I recall reading that cannabis use was supposed to inhibit | this communication between the hemispheres via the corpus | callosum. | modzu wrote: | yes, if you severed the corpus collosum | eimrine wrote: | No if chiasm is still there. | [deleted] | cratermoon wrote: | "Some of the most famous examples of confabulation come "split- | brain" patients, whose left and right brain hemispheres have | been surgically disconnected for medical treatment. | Neuroscientists have devised clever experiments in which | information is provided to the right hemisphere (for instance, | pictures of naked people), causing a change in behavior | (embarrassed giggling). Split-brain individuals are then asked | to explain their behavior verbally, which relies on the left | hemisphere. Realizing that their body is laughing, but unaware | of the nude images, the left hemisphere will confabulate an | excuse for the body's behavior ("I keep laughing because you | ask such funny questions, Doc!")." | https://www.edge.org/response-detail/11513 | turtledragonfly wrote: | It is a fascinating series of experiments. | | 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 | | [1] http://scihi.org/roger-wolcott-sperry-split-brain/ | d--b wrote: | This doesn't explain why the left brain controls the right side | of the body and vice versa, does it? | rolph wrote: | this would be dependency of new structure, on founding | structure, and methods. a plan is developed early,and departure | from that initial architechture, is a scorched earth style | revision, versus patching old proven fixes,on new hardware. | | 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. | Acumen321 wrote: | A theory I heard that I like, is that if you are getting attacked | from the right, and suffer damage to the right side of your head, | you probably want your right arm to work more than your left for | the highest chance of fighting off the attack. | Tagbert wrote: | If your brain is hurt to that degree on either side, you are | unlikely to be able to continue to fight. Evolution doesn't | usually work to offset unusual events like this. It is more | about optimizing operations on a daily basis. | everyone wrote: | I once read that an ancestor of ours evolved to have its head | rotated 90' relative to its body (like a flatfish or something), | but then later when it evolved a normally aligned head again, | instead of twisting back 90' the way it had come it just twisted | another 90'degrees | makeitdouble wrote: | > The nervous system is cross-wired [...] I asked my doctor last | week why this should be, all I got was a shrug. So I asked | Catherine Carr, a neuroscientist at the University of Maryland, | College Park. "No good answer," she replied. | | 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 | spicyramen_ wrote: | [dead] | dghughes wrote: | The recurrent laryngeal nerve too I'm not sure if it's all or | most or some animals. It loops around the heart like a noob PC | builder's first build and poor cable management. Giraffes have | the best most extreme example of this. | khana wrote: | [dead] | kibwen wrote: | _> Letters on your T-shirt appear reversed for the same reason | that the name "Quanta" would appear flipped, as "Quanta" if you | wrote it with your finger on a frosty window and then went | outside to look at it._ | | 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. :) | pbhjpbhj wrote: | It's the inline style is in a hidden div at the end of the | story and within #postbody, I suspect the author did it -- it's | part of the story content at least -- and so didn't need the | webdevs to do anything different. It's a nice touch. | venk12 wrote: | I understand that 2d brain to 3d brain connection requires an | intersection (criss-cross) I drew that on a piece of paper. But I | didn't really get why that has to be the case with 3D brain to 3D | brain mapping. I am able to connect them without a need for an | intersection. Are there easier examples to understand the | intuition behind this hypothesis? | squillion wrote: | After an explanation that without L/R crossing the body map in | the brain would be flipped upside down, comes this revealing | passage: | | "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. | squillion wrote: | The pictures with the finger at the end are even more | confusing. They conflate the mapping from body parts to | cerebral cortex (the cortical homunculus) with the mapping from | the homunculus to some other part of the brain were the 3D | environment is allegedly mapped. They don't make sense to me. | refactor_master wrote: | Slightly related: there's a YouTube video [1] of a guy teaching | himself how to ride a bicycle "in reverse", i.e. you turn left, | and it goes right. It takes some practice but in the end it | just "clicks". So now he's balancing _and_ moving completely | opposite to perception. | | So you're right about the little man. I struggle to see why | some arbitrary axis transform is "too hard". | | [1] https://youtu.be/MFzDaBzBlL0 ___________________________________________________________________ (page generated 2023-04-22 23:00 UTC)