Comment by Fraterkes
3 days ago
Dumb question, why do “sensitive” spots on the body need more nerves? Couldn’t you just have the normal touch-sensing nerves and map signals from specific spots on the body to stronger/pleasurable qualia in the brain?
3 days ago
Dumb question, why do “sensitive” spots on the body need more nerves? Couldn’t you just have the normal touch-sensing nerves and map signals from specific spots on the body to stronger/pleasurable qualia in the brain?
Not a dumb question. The shortest (and at a glance unsatisfactory) answer is because it works, and therefore it evolved that way.
Going in detail, first consider that for a feature to be evolutionarily selected for two things have to be true:
1. It must increase the fitness of the organism that carries it, i.e. the likelihood of its carrier having descendants as compared to non-carriers ( or be a side effect of another feature that improves fitness enough to be a net positive, etc etc )
2. It must be inheritable (and, in sexually reproduced organisms, mutually compatible during embryonic development).
One such a feature has reached dominance in a given population, as long as it continues to be important for fitness it cannot really be deprecated in favour of an alternative from scratch, even if that alternative is arguably better.
That's why, for instance, vertebrate ocular nerves connect to our retinas on the inside of our eyeball, resulting in us having a blind spot. Cephalopods, on the other hand, evolved their eyes independently the "reasonable" way, connecing their nerves from behind the eyeball. There's no way a vertebrate could mutate from scratch for its optical nerve to connect to the retina from behind without causing absolute mayhem in embryonic development. Our hacky solution for the blind spot? Let the brain hide it in software.
Going back to your question, some spots of the body being more sensitive than others became critical for evolutionary fitness long before nervous systems were complex enough to generate conscious qualia, let alone enough for them to be consistently involved in decision making. Furthermore, mapping of specific nerves to intensity of feeling on the CNS would imply complex hardcoding of something which is much easier to solve with "this place important, have more nerves", and maybe would even conflict with the fitness benefit of a CNS with enough neuroplasticity to learn anew during the development and lifetime of an organism.
So, in summary, the solution of having more nerves where it matters is simple, good enough, and has no reason to be rolled back in favour of a radically different alternative.
Nice post. Just this bit:
> Our hacky solution for the blind spot? Let the brain hide it in software.
I would say the solution is just having two eyes, since their respective blind spots don't overlap in the visual field.
I would also say that the brain doesn't hide the blind spots, but rather doesn't pay any attention to them in the first place. There's just a lack of information from them, and this deficit isn't normally noticeable because the other eye makes up for it. I think Dennett explains it that way somewhere, probably in Consciousness Explained
The blind spot still isn't noticeable if you close one eye, though. You have to look for it carefully with a specific setup that allows you to detect the discrepancy between what you see and what's actually there.
>1. It must increase the fitness of the organism that carries it, i.e. the likelihood of its carrier having descendants as compared to non-carriers
This isn't necessarily true. If you map out changes through the history of species, you'll find no significant changes but a lot of diversity for long periods, followed by big changes and low diversity for a short period. That's because during "abundant" times, the population will develop diversity as long as it doesn't significantly hinder reproductive rates. When an environnemental pressure comes up, the diversity dies down because the ones lucky enough to have adaptations that suddenly become useful and reproduce more.
So an animal might get a longer neck, but that doesn't significantly increase reproduction because food is aplenty. It's only when there's a drought that longer necks become an advantage and the trait is now selected for.
What you say is correct, and that is why I was referring specifically for what's necessary for "features" (the term I used for "phenotypes" to make it more HN) to become ubiquitous. It is a general rule of thumb in evolutionary biology that the more diversity is observed in a given population for a particular phenotype (e.g. hair colour, height, blood group, etc.) the less relevant it is for fitness within its observed range. When a phenotype is strongly selected for in a given population (e.g. bipedalism, opposable thumbs, the ability to speak) it soon becomes dominant and there is much less diversity.
As to your example about, for instance, neck length during abundant times, that follows the same rule: during abundant times neck length simply does not matter for fitness, therefore (all else being equal) there can be phenotypic diversity in the population.
One caveat though as to how a given phenotype may become ubiquitous without favourable selection is of course genetic drift[0], given a small enough population which is isolated for a long enough timeframe. Eventually that phenotype may become selectively "advantageous" inasmuch as it is no longer compatible with alternatives, and individuals from the isolated population who this phenotype can no longer have successful offspring with individuals of a different phenotype, resulting in speciation. That's what I meant with regards to a "make nerves on important places generate more pleasure/pain in brain" genotype being incompatible with a "have more nerves on important places" one. A hypothetical hybrid creature would be a mess.
[0] https://en.wikipedia.org/wiki/Genetic_drift
As a software dev, I think this is actually quite a satisfying and sensible answer. A simple reliable hardware solution in favour of a brittle “clever” software one
> mapping of specific nerves to intensity of feeling on the CNS would imply complex hardcoding of something which is much easier to solve with "this place important, have more nerves"
Not saying your answer is right or wrong, but I don't think this is a sufficient explanation. If the body can differentiate areas enough to produce more nerves in one area, then it could plausibly instead produce fewer nerves which inherently produce a stronger signal - just as we have nerves which respond differently to different stimuli (e.g. heat, light, etc). Also it could be neither and we kinda randomly ended up with what we have because no option was strongly disadvantageous at the time.
Having more independent samples helps filter out noise. If you had individual sensory neurons with outsized influence, then misfiring of such neurons would also have outsized influence.
This makes a lot of sense, thx!
Sounds plausible at least, but I think the question isn't necessarily making a valid assumption. Why do men have to have nipples? Why is our retina installed backwards? Why do sinuses drain upwards? It's just a path evolution took, it doesn't jump to some optimal design.
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Fingers, for instance, not only have higher sensitivity, but also much higher spatial resolution due to the more dense nerve network.
I can't tell why other areas may have needed higher spatial resolution; maybe it was evolutionary important in the past, and remains today. Or maybe just adding more nerves due to a random mutation correlated with better reproductive outcomes due to a stronger signal, or higher sensitivity, so more nerves are present for no other reason.
Wait, so what you’re saying is, we can use our genitals in a pinch that could be as good as fingers for finesse?
Yes. And the (thought) experiment of reading braille with your clitoris exists. Except youl'ld get horny really quickly.
Hence the phrase, "Can I use your dictaphone?"
Nerve density isn’t mainly about intensity, it’s about spatial resolution. More nerve endings per square centimeter means you can distinguish finer details of touch, texture, and pressure. The brain can’t invent spatial detail that isn’t in the incoming signal. Amplifying a sparse signal centrally would be like zooming into a low-res photo.
The brain does do some of what you’re describing though. The somatosensory cortex gives disproportionate space to certain body parts (the sensory homunculus). So there is central amplification, but it works on top of peripheral density, not instead of it. Without the dense nerve input, you’d basically have an on/off switch instead of nuanced sensation.
> Dumb question, why do “sensitive” spots on the body need more nerves? Couldn’t you just have the normal touch-sensing nerves and map signals from specific spots on the body to stronger/pleasurable qualia in the brain?
Think of a television. What gives you a better picture, quadrupling the number of pixels or making the existing pixels 4x as intense?
Perhaps encoding "software" is more expensive in terms of codons? So it's cheaper/more likely to "implement" physically.
I think you have it backwards. The brain doesn't "know" what's supposed to be sensitive or pleasurable. It boots up with no training data after all. It machine learns what's sensitive due to a combination of nerve density and other factors. We haven't figured out all the other factors yet. But that's why there's a correlation between nerve density and sensitivity: density means sensitivity.
This is just wrong. The brain does instinctively know that some things are good and bad.
Go take a picture in a dark room and edit the photo to try to make it like you've got the lights on in photo editing software. You get a noisy grainy mess and little to no detail.
It's not like evolution would leave a significant amount of signal/noise ratio on the table for all other nerves.
Presuming nerves are already optimized if you want more signal you have to add nerves.
Sensing nerves aren't especially energy hungry when considering their volume in the human body, so evolution doesn't have much reason to minimize them.