Comment by xg15 | Hacker News Reader

Comment by xg15 | Hacker News Reader

Parent Full thread

> Is the neocortex not an outer layer? That is not present in reptiles? So how is the stratified model incorrect as a high-level structural categorization?

Not sure about reptiles, but the author write this about mammals:

> Neurobiologists do not debate whether any cortical regions are evolutionarily newer in some mammals than others. To be clear, even the prefrontal cortex, a region associated with reason and action planning, is not a uniquely human structure. Although there is debate concerning the relative size of the prefrontal cortex in humans compared with nonhuman animals (Passingham & Smaers, 2014; Sherwood, Bauernfeind, Bianchi, Raghanti, & Hof, 2012; Teffer & Semendeferi, 2012), all mammals have a prefrontal cortex.

I also read some interesting papers a few years ago about corvids - particular New Caledonian Crows: Those animals do not have a neocortex and hence were thought incapable of many higher-level cognitive tasks, such as planning, tool use, etc. Turned out the crows were in fact capable of them. One hypothesis I read suggested that, as crow brains are structured differently, another structure may have taken the role that the neocortex has in humans.

So even if it's an anatomical distinction, it's an unreliable indicator of mental capacity.

OK, but again -- none of that refutes the triune brain hypothesis at all.

Literally nobody is claiming that only humans have neocortexes.

Nor is anyone claiming it's an indicator of mental capacity. The recent discoveries about crows were interesting, but that doesn't have anything to do with the fact that the neocortex in mammals plays a particular, functional, well-recognized role.

The refutation is of the idea that it’s a strictly chronological layering, with the old layers inside and intact.

The correct view is that while the neocortex is indeed mostly new, the “older” more central structures were modified throughout evolution.

No one thinks the older structures are static. No one is arguing that. It's a simplified model about the origin.

This argument is akin to saying we're not "newer" apes ala

> The refutation is of the idea that it’s a strictly chronological ordering of species, with the old species still inside and intact. The correct view is that while homosapiens are indeed mostly "newer", the “older” apes were also modified throughout evolution

Obviously.

I mean… the quote from TFA that they are arguing against is

> As Paul MacLean (1964), originator of the triune-brain theory, stated,

>> man, it appears, has inherited essentially three brains. Frugal Nature in developing her paragon threw nothing away. The oldest of his brains is basically reptilian; the second has been inherited from lower mammals; and the third and newest brain is a late mammalian development which reaches a pinnacle in man and gives him his unique power of symbolic language.

And they quote other textbooks that are making claims along these lines too; this is right at the beginning of TFA. So I think you are wrong that “no one thinks that”.

Of course they don’t think the old brains are 100% static but there are claims that they are largely conserved.

> there are claims that they are largely conserved.

That they're not static doesn't necessarily mean that they aren't largely conserved.

MacLean doesn't say the brain is an onion with a tiny reptile inside!

It’s an incorrect and misleading model about the origin. Bat wings and bird wings share many traits but the common answer didn’t fly it’s simply convergent evolution. The brain structures of that same common ancestor is only vaguely related to modern structures in mammals and reptiles.

Modern reptiles, birds, and humans have experienced the same amount of evolution. There’s similar traits, but we don’t call whale flippers hands because that’s what the structures was in some of their ancestors or even what it is in related species. Similarly we don’t call human hands flippers because that’s what the structure started out as even further back and it’s still preforming that function in modern fish.

The human visual cortex is larger than most creatures entire brain, it’s a wildly different structure than you find on a frog which plays a wider role in cognition than just decoding vision.

But you could say that whale flippers are 'basically' their hands.

Every scientific model and analogy is flawed in some way, but many are still useful.

How exactly is this theory useful? Yes, frogs and humans process visual information because they both have eyes. That’s about all those structures have in common.

For nearly all of human history everyone thought that those frog vs. human eyes were completely separate and designed by some magical being living up in the cloud.

By noticing that while they have significant differences, those structures also have significant similarities, we were able to correctly answer one of the greatest questions of all time: where did humans come from?

Of all models, this one is particularly useful.

To be useful models must accurately represent some aspect of reality, Reptiles don’t represent the brain of our common assessor particularly well. So this model is just wildly wrong without any predictive power.

Compare the giant spheres used by snakes, bird, frog etc to process visual information and try and map that to a rodent, bat, or human. Hint you’re not going to be able to locate an obvious sphere because the fundamental morphology is just different. You can say we both have areas of the brain devoted to processing vision but so do fruit flies.

Eyes are a good example of conservation and path dependence, development is initiated by a fairly highly conserved gene:

https://en.wikipedia.org/wiki/PAX6#Species_distribution

Being reasonably conserved has minimal predictive power. Yes, knocking it or a closely related gene out out harms eye formation in a wide range of species but it says little about what specific structure ends up being formed.

Putting this particular theory aside the generalization that evolution is a nested hierarchy of structures is pretty obvious on the macroscopic level. Traits like breathing oxygen, having symmetrical bodies, backbones, limbs. These are all examples of structures building on top of one another sequentially and conserved across species. Sure each of these subcomponents continues to specialize but the core functionality remains largely the same.

Going back to your analogy saying visual cortex of frog is wildly different from a human visual cortex. That's similar to saying a frog backbone is wildly different from a human backbone. Sure that's true but there's also a shared common core functionality largely conserved across time

Again the issue is the theory is misleading. Those traits are conserved through evolution largely because they’re useful, but if you look at say owl skulls you find wild asymmetries because that’s more useful even our lungs are very different from each other. Hagfish lost backbones and snakes lost limbs because nothing is sacred to evolution.

So yes frogs and humans both have a visual cortex, because we both have eyes. It really doesn’t explain anything beyond that point. The human visual cortex doesn’t even map to the same structures as it expanded into nearby ones.

>It really doesn’t explain anything beyond that point.

I think taking a macroscopic view like this points at interesting types of analysis. Evolution is analogous to a hill climbing algorithm navigating towards local optima in a probability field. Macroscopic evolutionary trends give a low resolution peek into the underlying probability distributions.

That’s not what this theory is suggesting.

Frogs, birds, etc have very distinct spherical visual cortexes. Humans and squirrels don’t have that kind of spherical structure. So, other than naming where vision is processed the same thing the overlap isn’t particularly strong even at the most basic structural level.

It’s perfectly reasonable to extrapolate from studied organisms to species that share recent common ancestors. But reptiles don’t represent the probability distribution, they represent different results from that distribution under different evolutionary pressures. Extrapolation backwards and our common ancestor was very different from all decedents.

> No one thinks the older structures are static.

It would seem enough people did to warrant this person to write about the topic.

From the wikipedia:

> The triune brain consists of the reptilian complex (basal ganglia), the paleomammalian complex (limbic system), and the neomammalian complex (neocortex), viewed each as independently conscious, and as structures sequentially added to the forebrain in the course of evolution. According to the model, the basal ganglia are in charge of our primal instincts, the limbic system is in charge of our emotions and the neocortex is responsible for objective or rational thoughts.

We disproved:

- the sequentially added nature

- what these zones are responsible for

So in the end we're only left with a semi-arbitrary split of the brain in 3 zones and none of the hypothised implications. That to me refutes the hypothesis as a whole, and we're keeping the zone naming by sheer inertia.

Reply on news.ycombinator.com