The classic “homunculus” has gone through a radical revision | Hacker News Reader

The classic “homunculus” has gone through a radical revision

scientificamerican.com

72 pointstaubek30 comments

> this neural representation of the body is sliced into three sections, one for the feet, another for the hands and a third for the mouth

This smells like Inverse Kinematics to me.

If correct, that'd mean we don't generally move joints manually. Rather, at the planning level the brain just says "put the hand at relative position X" and the motor cortex works backwards how to make it happen.

It could be much weirder than that; the way movement works could be the brain very strongly predicting that a limb will be at a certain position and the rest of the body scrambling to prove that prediction true. See https://slatestarcodex.com/2017/09/06/predictive-processing-...

Wow, very interesting, thanks for sharing!

Yes. Body movement is very strong inverse kinematics. This is why personal trainers exist: "lift thing" is intuitive, "lift thing with specific muscles along a specific path" is not.

I'm also wondering where the inertial measurement unit (IMU) is in there. People often forget about the inner ear a sensory organ. Moving body parts is one thing, but moving the body from here to there requires tight coupling of this this additional feedback. We clearly have a 3-axis gyro in each ear ;-)

Apart from the inner ear, there are also other feedback paths worth considering: muscular sensing, skin stretch sensing, even acoustic transmission along ligaments.

Yeah, proprioception in general is amazing. I always find it fun to do stuff like close my eyes, then quickly spread my arms apart, and just as quickly bring them back together and try to touch specific finger tips together; seems to be generally easiest with index and middle fingers for me. I also enjoy being able to extend that proprioception to tools - being able to feel the engagement of a screwdriver with a screw head, etc.

I took a few classes taught by Graziano, one of the motor researchers cited in this article. I remember he was convinced that the motor cortex does much higher level planning and orchestration than other people believed, and that it stored entire movement templates across different areas as described here. One of the main ones was picking up something and bringing it to your mouth to eat.

I honestly would not be surprised to learn that a substantial amount of the specific content of spoken (or indeed keyboarded) utterances originates in movements of mouth and hands planned by the motor cortex. The reason being that sometimes the speed of production precludes the involvement of ‘higher’ cortical functions, which proceed at a slower pace.

"It has been eye-opening to realize how powerfully my thinking is shaped by prior assumptions. My trust in textbook teachings blinded me to data that were right there in front of us. As time went on, our confidence in our data grew to the point that we eventually trusted it more than the prevailing neuroscience doctrine."

Science advances one gravestone at a time...

Not in this case, it seems. At the end, the author mentions that the original image was never supposed to be taken very seriously. It was a coarse (my interpretation) model for teaching medical students.

And that should’ve been obvious to any sensible person, let alone a scientist or medical professional.

Idk. The brain was (and really still is) very badly understood. But even back then, they already knew that nerves connected roughly in that area. Then a theory that posits a minimal topological mapping between the two makes sense. I can imagine the motor cortex evolving like that, or growing with the fetal development. So in the early days, this was a sensible model. It overstayed its welcome, as so many theories do. Many scientists still hold on to coarse brain regionality.

... no pic of revision in article

See the referenced Nature article for a series of brain maps.

That's largely what the authors are presenting as an alternative. There is no straightforward homunculus, though Figure 4 shows an approximate body-regions mapping.

<https://www.nature.com/articles/s41586-023-05964-2>

Fig. 4: <https://www.nature.com/articles/s41586-023-05964-2/figures/4>

The homunculus never made sense, but did anyone think it did?

"As a student and later as a practitioner in the field, I had accepted the homunculus as unquestioned fact. As a professor, I would dutifully teach students that the distorted figure represented a key aspect of brain organization."

- Nico U.F. Dosenbach

Again . . . did anyone think it made sense? At least Dosoanbach (did I spell that correctly?) gets 15 minutes of fame, but did it mean anything?

It kinda made sense, even the modern diagram isn't so far off from the original.

It was a crude model built with the tools available at the time, I've seen it referenced in research articles and this professor says it was taught in universities so presumably it was.

He was also published in nature along with ~30 other people so I don't think it's about the 15 minutes of fame.

>The homunculus never made sense, but did anyone think it did?

Most in the field? And most still do. So there's that.

> The homunculus never made sense, but did anyone think it did?

Why doesn't it make sense? Seriously, it was so widely accepted by literally MILLIONS of people because it DOES make sense: clearly my hands are MUCH more dexterous and sensitive than my elbows, my mouth and lips more sensitive than my forehead and having more dedicated brain connections and brain matter explains this nicely.

I first saw this image years ago in Psych 101, and many times since, and not ONCE did it occur to me that this "doesn't make sense".

In what way did it not make sense? It's not a mechanistic explanation of motor control, but it is a description of specific scientific observations, like what part of the body you can evoke movement in by stimulating with an electrode in the brain. I'm not sure what "doesn't make sense" about that, aside from it not really explaining a specific theory of how movements are planned or controlled.

I studied neuroscience and remembered the first time that I had seen a "homunculus" diagram and 3D chap. Not something you can unsee.

The one that I first saw in grad school was the illustration by Slagter.

https://anatomytool.org/content/slagter-drawing-cortical-mot...

I remember thinking at the time that nature had tucked the genitals inside wall of the gyrus in order to give it as much protection from blunt force trauma as possible.

The homunculus chap pictured in that article is the SFW/SFK variety. A more NSFW version can be seen here https://movementum.co.uk/journal/2020/1/23/homunculus-man

Last night I after seeing this in HN, I did some googling and found this interesting article

https://www.scientificamerican.com/article/where-are-genital...

TL;DR:

"Penfield wrote that the lack of responses from other patients may have been because of 'a false sense of modesty.'"

"But, somewhat amazingly, scientists are still without a firm answer as to how to draw a homunculus with the correct placement of male and female genitalia."

"The key to finally pinning down the location of the genitals, as with everything in science, will be large sample sizes and repeated experiments with consistent methods. In the past 40 years, 17 studies using several different imaging and stimulation techniques have mapped the genitals of 264 people—an average of just 15 people per study. Small wonder that they disagree."

> The somewhat frightening creature is prominent in every neuroscience textbook and even has its own Dungeons & Dragons character, with bat wings for good measure.

I hope Dr. Dosenbach realizes that the D&D homunculus is based on the older (and probably still more common) sense of that word and not the neuroscience diagram, and this is just a dumb joke rather than a serious statement...

This seems to be the motor version. The article is a huge, flaccid missed opportunity to show the sensory version.

What do you imagine that would look like?

A giant penis with a series of small dots to signify non-penis parts.

Well, that would not be very accurate since less than half of humanity is a penis bearer

Based on this, it's also inaccurate for the lesser half that does have one: https://commons.wikimedia.org/wiki/File:Sensory_and_motor_ho...

(Unrelated, but the phrasing "bearer" makes me think of "ring-bearer" in Lord of the Rings, and that makes for a very different film…)

Especially the throwing into the volcano part.

The other half has a clitoris, which, as far as I know, is similar to a penis from a sensory perspective.

I think the hands would still necessarily be large.

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