I smell (with better sensitivity than) a rat (under certain conditions)

September 6, 2018 6:37 AM

Poor Human Olfaction is a Nineteenth Century Myth
"Strangely, the idea that humans have tiny olfactory bulbs and a poor sense of smell is derived in part from the religious politics of nineteenth century France."
via

Highlights:

• Paul Broca claimed that the smaller relative size of the olfactory bulb made room for the frontal lobe to be large enough to create free will, and the myth expanded from there.
• Freud was a hack.
• The myth grew in part because primate behaviors are not compelled by olfactory stimuli like other land mammals, but that is due to cognition, not reduced sensitivity.
• Though primate olfactory bulbs are small relative to brain size, in absolute terms they are bigger than many other mammals.
• Another myth, that the adult human brain does not grow new neurons, actually is true of the olfactory bulb.
• A recent study estimates humans can distinguish as many as one trillion distinct compounds by smell.
• Dogs are more sensitive to certain smells than humans - but also vice versa.
• Humans can follow scent trails, too.

posted by solotoro (45 comments total)

About to dig in, but if this is the first step towards some sort of Smell Dating service I will be very excited.

(You can’t just ask people if you can smell their neck, and yet it’s so important. Idk. Olfactory weirdos unite.)
posted by schadenfrau at 6:47 AM on September 6, 2018 [30 favorites]

Though primate olfactory bulbs are small relative to brain size, in absolute terms they are bigger than many other mammals.

This is literally what we mean when we say that humans have tiny olfactory bulbs: that humans devote a very small amount of brain space relative to the overall size of the brain, not that humans have absolutely smaller olfactory bulbs than, say, a mouse.

The idea that absolute size of brains or brain regions matters one whit compared to scaled relative sizes of these organs corrected for absolute body size is insane. For one thing, it's one of the big arguments that was used at about the same time to say that women were not as bright as men, because on average women have absolutely smaller brains than men do. (This is because women are, on average, smaller).

I need to read the full article, but this point is priming me to go in very skeptically.
posted by sciatrix at 6:48 AM on September 6, 2018 [18 favorites]

NOPE I was totally right, he's seriously making the argument that humans have absolutely more olfactory bulb volume than fucking mice as if it says something meaningful about human olfaction

oh my fucking god how did this get through peer review
posted by sciatrix at 6:49 AM on September 6, 2018 [19 favorites]

So does most brain function work on a 'relative size' basis? As a non-biologist, my first guess would have been that an olfactory bulb would have a certain level of complexity in order to distinguish smells, and wouldn't need to scale. Is it the case that humans would need a much larger olfactory bulb in order for the brain to (for want of a better term) 'pay attention' to smell?

Also, men have larger brains because of the need to contain a larger ego. I thought that was well-known.
posted by pipeski at 7:02 AM on September 6, 2018 [7 favorites]

God fucking dammit I love olfaction research but this smells fishy. I... can feel myself twitching and sketching out a rebuttal piece with proper citations.

I mean, I might be wrong, but. Hey, I might not be. And either way, I can see the holes....
posted by sciatrix at 7:07 AM on September 6, 2018

Patrick Süskind begs to disagree.
posted by adamvasco at 7:07 AM on September 6, 2018 [2 favorites]

I mean, I might be wrong, but. Hey, I might not be. And either way, I can see the holes....

So you’re saying it doesn’t pass the smell test?
posted by Celsius1414 at 7:09 AM on September 6, 2018 [28 favorites]

I dunno. I thought the arguments were pretty convincing. As he says, the olfactory system is generally the one glaring exception to allometric scaling laws for brain size, with the olfactory bulbs and odor-processing cortex generally scaling much more slowly with absolute brain size, hence the negative relationship between absolute brain size and relative olfactory system size. I see the argument as being that for the olfactory system specifically, neither absolute nor relative brain volume is particularly useful as a guide, and other measures (e.g., total neuron count) put humans more or less in line with most other mammals.

And anyway, the proper method to assess olfactory sensitivity is not anatomy or physiology of the olfactory system, but behavioral measures like detection and discrimination. If we trust the cited evidence, humans are also performing similarly to other mammals, with worse sensitivity for certain odorants but better sensitivity for others.
posted by biogeo at 7:21 AM on September 6, 2018 [13 favorites]

Yeah, I'm currently scanning the literature in irritation, but I do have to point out that olfaction is also unique among sensory modalities in that it is extremely specific to particular odorants in a way that just isn't the case for other modalities like vision, audition, or more exotic senses like magnetoception or electroception. One of the things that is annoying me about this argument is the concept that humans can't be, for example, more sensitive to some olfactory cues while being generally poor at distinguishing between many other compounds. That seems totally wrong-headed to me.

This is actually one of the reasons that his casual dismissal of the reduction in human olfactory receptor genes along the primate evolutionary tree is so frustrating. Olfaction is unlike any other sense in that the variety and expression density of those proteins is the specific driver of olfactory flexibility and sensitivity, and the pieces I've read so far as I dig through don't really seem to differentiate between those two aspects of sensation. And that just blows my mind, especially when we know from vast swathes of interspecies work on other modalities that these are two different aspects of sensation and perception that really, really do not need to go together in intraspecies organization.
posted by sciatrix at 7:28 AM on September 6, 2018 [2 favorites]

Next on science hot takes: empirical determination of who is, in fact, a good boy.
posted by Halloween Jack at 7:29 AM on September 6, 2018 [7 favorites]

Huh. I am actually coming around on a lot of this, and I'm historically pretty skeptical about human olfaction work, largely because of huge variation in human olfactory sensitivity (and particularly the intersex variation thereof). (Although... I need to see how much variation research has actually been published on these things. Variation on olfactory sensitivity between male and female humans, incidentally, hasn't gotten discussed here a lot but is also a pretty robust and well-replicated result, if I remember right. Women are considerably more sensitive on average, and we're not really sure why except that olfactory sensitivity also tends to increase with pregnancy.) The historical aspects of the work are not particularly helpful to me; I'm much less irritated having gotten down to the meat of the actual argument, and the relative brain size thing is much less egregious than I thought initially. (Score one for framing, I suppose.)

(You can also improve your sampling of smells by taking several short, light breaths and wiggling your nose, incidentally--there are behavioral techniques that can help humans who are actually focusing on their olfactory processing.)

One thing that is continuing to seriously bother me is the lack of focus on variety of specific oderants in this analysis. We do know from olfactory evolution that you can create surprisingly strong behavioral changes by simply changing sensitivity or reward responses to particular oderants--for example, one of these is driving a lot of sympatric evolution in the famous Rhagoletis pomonella study, and changes in reward response to cuticular hydrocarbons drive a surprising number of interspecies interactions in Drosophila species more generally. You don't necessarily have to change a given receptor gene: you just need to either change its expression levels or else change what happens when the receptor protein binds a ligand downstream.

So in light of this, that loss of functional odorant receptor genes in human evolutionary history is really big and weird and needs to be dug into. And because sensitivity is likely to be so dependent on the specific compounds we're measuring, we also need to be testing a large number of potential odorants against a lot of species before we can really make broad determinations about how much relative attention different species pay to specific sensory modalities.

That's going to be largely a function of human failure to pay attention to chemosensory signaling, of course, but it's definitely bothering me that the genetic work is so blithely dismissed in this context.
posted by sciatrix at 7:55 AM on September 6, 2018 [8 favorites]

I think it was Richard Feynman who had a party trick where he'd say that he could lay out some books, leave the room, have someone handle one of the books, and then come back in and identify which book was handled by smelling it.

I could have some of those details wrong, though.
posted by clawsoon at 7:57 AM on September 6, 2018

Sorry for any poor framing! I'm not an expert or even an enthusiastic amateur in this field, I just posted an article I found interesting that challenged a bit of "everybody-knows" wisdom I'd never thought to probe any more deeply. And I'm super excited to read your takes, both hot and less so!
posted by solotoro at 7:57 AM on September 6, 2018 [1 favorite]

(Also I had a friend when I was a kid who was incredibly no fun to play hide-and-seek with, because he found us all every time pretty much right away, and he swore he was tracking us by scent. I never even CONSIDERED believing him then, but now - well, now I still think he was probably just cheating tbh, but still....)
posted by solotoro at 8:00 AM on September 6, 2018 [2 favorites]

"Paul Broca claimed that the smaller relative size of the olfactory bulb made room for the frontal lobe to be large enough to create free will, and the myth expanded from there."

I love this one since even free will is kind of antiquated concept these days. Though, in some ways, they are at least acknowledging the illusion of free will is in the brain rather than something weird like a soul or ghost or whatever.

Is this factoid of humans having small bulbs and poor senses of smell common? While I've heard often of other animals with better senses of smell, I don't recall ever being told that humans are just bad at it.
posted by GoblinHoney at 8:04 AM on September 6, 2018

Paging Patrick Süskind . . .
posted by aspersioncast at 8:04 AM on September 6, 2018

I mean, people think I’m crazy when I tell them I smelled it when my cat’s cancer came back, but...little dude smelled different.

Also, like half the reason I know I am very much a lesbian on a primal level is how ...wrong dudes smell to me. Not necessarily “bad,” just...wrong. And while I’m super sensitive to how people (and I guess animals?) smell, I’m fucking blank on a lot of other smells. Like they don’t even register.

We don’t really have a great vocabulary for smells, huh?
posted by schadenfrau at 8:10 AM on September 6, 2018 [7 favorites]

Sorry for any poor framing!

Oh, let me be clear: I'm irritated with McGann, not you. You buried that particular lede in the good faith idea that it wouldn't seem out of place at all in the middle of interesting takeaways inside a cut; McGann by contrast led with that idea knowing full well how neurobiologists would react to it. I'm not criticizing you at all!
posted by sciatrix at 8:10 AM on September 6, 2018 [2 favorites]

Also, like half the reason I know I am very much a lesbian on a primal level is how ...wrong dudes smell to me. Not necessarily “bad,” just...wrong.

I occasionally complain that men smell strongly to me of chlorine, but it's probably more accurate to say that I find the smell of Dude approximately as pleasant as a strong scent of chlorine. I occasionally feel bad about this--I know y'all can't help it!--but it's the truth.
posted by sciatrix at 8:13 AM on September 6, 2018 [1 favorite]

I think it was Richard Feynman who had a party trick where he'd say that he could lay out some books, leave the room, have someone handle one of the books, and then come back in and identify which book was handled by smelling it.

If it was really Richard Feynman you're talking about, he'd probably just cheat by making a show of smelling it while discreetly checking which book was warmer than the others.
posted by a snickering nuthatch at 8:16 AM on September 6, 2018 [2 favorites]

I always describe dude smell as kinda gamey, but I have no idea if that’s true because I’ve never been, like, hunting. Or sometimes like how I imagine a t-rex would smell? I recognize this also makes no sense.

While we’re checking on shared smell experiences: does anyone else suspect they can tell when people consume a lot of dairy products? (I can also smell it on myself for a day or two after ice cream. I do not love it.)

Follow up question: is this possibly related to why white people sometimes smell like wet dog?

I HAVE SO MANY SMELL QUESTIONS.
posted by schadenfrau at 8:20 AM on September 6, 2018 [4 favorites]

The wet dog smell is from not taking your laundry out of the washer fast enough. Also some people own dogs.
posted by runcibleshaw at 8:22 AM on September 6, 2018 [2 favorites]

Feynman was the first thing I thought of as well here is an excerpt from Surely You're Joking, Mr Feynman called Testing Bloodhounds.

He says his first attempt was with a coke bottle handled a bit by his wife, and could tell by the warmth as well as the smell, so he repeated it with her just opening and closing a book and returning it.
posted by borkencode at 8:25 AM on September 6, 2018 [4 favorites]

The wet dog smell is from not taking your laundry out of the washer fast enough. Also some people own dogs.

Wait is this a known thing, or just a weird hypothesis? Because I have definitely smelled it in circumstances where laundry was um not really an issue. (I’m also white, just...sensitive to how people smell.)
posted by schadenfrau at 8:28 AM on September 6, 2018

Wait is this a known thing, or just a weird hypothesis? Because I have definitely smelled it in circumstances where laundry was um not really an issue. (I’m also white, just...sensitive to how people smell.)

This is just from my own observation. My clothes smell musty like wet dog if I let them sit in the washer for too long before I dry them. It stays even if you dry the clothes and use extra dryer sheets. The smell then gets on my skin if I wear those clothes all day. I would be surprised if white people somehow genetically exude wet dog smell from their actual bodies, but who knows?
posted by runcibleshaw at 8:36 AM on September 6, 2018

Between this and the 98.6 F is a lie post yesterday, I'm feeling a little old. "Everything you were taught is now not true thanks to new generations of science!"
posted by Fig at 8:40 AM on September 6, 2018 [3 favorites]

"I always describe dude smell as kinda gamey"

What? I don't make a habit of smelling dudes in particular, but even just in my tiny office right now I know of at least 4 different dude smells. Also, gamey to me is largely an adjective for taste, that kind of coppery taste of some meat. Not even going to touch the race smell thing, already been exposed to way too many similar comments about races over the years, and as far as I can tell them comment never has anything meaningful to say about the race they're targeting, but do say lots about the speaker.
posted by GoblinHoney at 8:53 AM on September 6, 2018

Also, gamey to me is largely an adjective for taste, that kind of coppery taste of some meat.

Yes. It’s not so much “smells like” as “elicits a similar response from my brain.” I did lament specifically the lack of vocabulary to describe smells.

Not even going to touch the race smell thing

Sure. Given my experiences with smell — in particular that men have a distinct smell (or base scent/note, if you will), as do particular diets — it’s really not crazy to me that people with genetic ancestry from a particular locality would share a scent...profile? Like people with irish ancestry are a lot more likely to have red hair, celiac’s, or any other number of genetic quirks. Whether that scent profile is generalizable to “white people” is another question, but it’s how it’s always been presented to me (if it matters, by POC — prior to that I thought it was just a diet thing, and maybe it is).
posted by schadenfrau at 9:08 AM on September 6, 2018

(Which is what I posited in my comment that I’m starting to suspect you didn’t actually read.)
posted by schadenfrau at 9:10 AM on September 6, 2018

I didn't realize that this myth existed! I just figured we were good at smelling but it was a half-vestigial sense. What good is it really to go around smelling such urban smellscape non-delights as motor vehicle emissions, waste, and industry all day?

(Apparently air pollution has been shown to make it harder to smell the flowers. No wonder nobody stops to smell 'em!)
posted by aniola at 9:47 AM on September 6, 2018

schadenfrau: does anyone else suspect they can tell when people consume a lot of dairy products?

Or, as South Park put it in their cover of Elvis Presley's "In the Ghetto", "Why do poor people always smell like sour milk?"

I've never noticed the smell myself, but that's probably because I consume a lot of dairy products. Sweet, sweet milk...
posted by clawsoon at 10:45 AM on September 6, 2018

Yep, borkencode, that's what I was thinking of!

I had forgotten the part about him crawling around on the ground to see if he could track like a bloodhound.
posted by clawsoon at 10:49 AM on September 6, 2018

I can't go super far into the neuroanatomy and size questions as its not my area, but I did specialize in flavor chemistry for my doctorate in agricultural and environmental chemistry and have studied sensory perception and analysis, sensometrics, and the like, as well as been on the public-science interface a lot regarding the senses and food.

I like this paper, and this area, because it cuts to questions about the philosophy of science and how assumed values impact what science we think is worth doing. McGann's thesis here, basically, is that the human sense of smell is not less-developed than in other mammals. But, even if humans have a lower sensitivity than mice or dogs (which, clearly, is not clear-cut), does it then follow that, within the human sensory realm, smell is less acute than the other senses? And, even if it were (I don't think it is), does it follow that the sense of smell is mostly unimportant for humans?

I actually don't think it's necessary to demonstrate that the human sense of smell is strong amongst the human senses (although I believe multiple streams of evidence show that it is) or even that the human sense of smell is quantitatively similar in sensitivity to mice or dogs (which McGann is doing a good job of, for me, here) to accept the more fundamental premise that the sense of smell is important to for humans. This may sound like an academic or pedantic argument, but look at McGann's quotes from Negus, Freud, and others (drawing on Broca's osmatique/nonosmatique dichotomy)— smell “probably has not contributed greatly to the evolution of the human brain and will, therefore, not be considered further" and furthermore, humans who enjoy smell are psychosexually stunted.

To quote McGann, "Freud and Broca thus provided a pseudoscientific gloss on the idea that smell operates in opposition to a disembodied rationality that makes humans civilized and distinct from other mammals"; I would add that this attitude isn't a purely 19th-century one, but (courtesy of Gordon Shepherd's Neurogastronomy, which I highly recommend to anyone in this thread who wants a good popular in-dpeth read on more of this topic) goes back at least as far as 350 BC: "The legacy of thinking of smell in this way began with Aristotle. In discussing the senses in De Anima (On the Soul), he observed that “our sense of smell is inferior to that of all other living creatures, and also inferior".

McGann doesn't really mention this, but it is not widely known outside people in the smell-related specialties that a hugely important use of smell for humans is when we're eating. Smell is probably the most important part of flavor perception. We tend to think of smell or olfaction only happening when we sniff in through out nostrils, but it can also occur as "retronasal olfaction" where odorous molecules pass from our mouths, up the back of the throat and into the nasal cavity (the same route as when milk comes out your nose when you laugh). The brain localizes this perception as happening in the mouth, but it actually occurs in the same place as sniffing. Retronasal olfaction (flavor-smell) is enabled by the anatomy of our skulls rather than necessarily our olfactory bulb, and there is some hypotheses (not tested as far as I know) that humans experience much more retronasal olfaction than other species with longer muzzles that are more famously orthnasally (sniffing-smell) sensitive.

So, when thinking about smell's role in the lives of humans, consider not just sniffing out our environment, but also the multiplicity of flavors we experience when we eat—pickled, pungent, herbal, brown, floral, spiced, green, fresh, boiled, garlicky, fruity, citrus just a tiny handful among them. Those "tastes"/flavors wouldn't exist without smell!

Smell, both in the sniffing out of food sense and in the pleasure-when-we-eat sense, is a hugely important component in getting us to seek out and enjoy a biodiverse diet, which is what most early human forager-hunters would need to do to get a lot of useful molecules and nutrition into their diet.

Gordon Shepherd, who is a neurobiologist, and others like Harry Klee a tomato breeder have laid out some really interesting evidence of this evolutionary role of smell/flavor, both specifically in terms of retronasal olfaction and looking at smell molecules as cues for nutrition and health (for example, the class of molecules called carotenoids includes vitamin A and is smell-less, but this group of molecules breaks down in plants into smaller molecules with odors. We're WAY more sensitive to the smell of the breakdown products of vitamin A carotenoids than the less-useful, non-vitamin A carotenoids.

Bear in mind also that the superfamily of genes coding for olfactory receptors is the largest in our genome, makes up at least 2% of our genome, which is a staggering amount of genetic information to conserve over our evolution if not under some kind of selection pressure. I make this statement not to get into an argument about genome or gene family size etc with experts in those areas, but to illustrate how smell has been overlooked.

One last piece is that the sense of smell is highly combinatorial— each receptor type (whether we have 390 functional ones or if the 465 pseduogenes are actually in use) is able to bind with a variety of different smell molecules, and each smell molecule can bind with varying affinities to multiple types of receptors, leading to individual smells being sent and stored in the brain more as maps or images than the firing of the receptor for, e.g., "chocolate smell". This means that for a limited number of receptors we can detect a much larger number of odor molecules. We also perceive mixtures of odor molecules in funny ways, with addition, blending, suppression, and synergy going on such that the smell of a mixture can be perceived very differently than would be expected just from adding up the perception of its components. Incidentally, this where the "trillions of odors" idea comes from—not necessarily trillions of molecules, but since mixtures can be so different and distinguishable from each other while being perceived as fairly unitary things themselves, that there are potentially trillions of mixtures we can smell that will have different smells from each other.

here's some further reading:

Neurogastronomy the book

The Human Sense of Smell: Are We Better Than We Think?

An Odor is Not Worth a Thousand Words: From Multidimensional Odors to Unidimensional Odor Objects

Humans can Discriminate more than one Trillion Olfactory Stimuli

Plant Volatile Compounds: Sensory Cues for Health and Nutritional Value?

[on preview]

that turned out longer than I thought! not to hijack the thread (apologies solotoro); I'm writing a book about flavor and smell and this all just kind of came out.
posted by zingiberene at 11:00 AM on September 6, 2018 [42 favorites]

I appreciated it, for one.
posted by sciatrix at 11:05 AM on September 6, 2018 [3 favorites]

flagged as motherfucking fantastic for two.
posted by schadenfrau at 11:12 AM on September 6, 2018 [2 favorites]

well shucks, you've given me the confidence to post my book to MeFi Projects when it comes closer to its release date (Fall 2021, so far and yet so close)
posted by zingiberene at 11:18 AM on September 6, 2018 [11 favorites]

there's also some interesting hypothesizing in Neurogastronomy about how humans use much more cognition in olfaction in terms of memory, naming, and emotion, therefore MORE of our brain is devoted to smell than other animals, but I'm a little rustier on that.

Also, we tend to think we're bad at smelling when really we're bad at NAMING smells, which is more a factor of practice and culture than innate ability (see link #3, "An Odor is Not Worth a Thousand Words")
posted by zingiberene at 11:22 AM on September 6, 2018 [7 favorites]

cogito ergo no dealto
posted by condour75 at 11:23 AM on September 6, 2018 [3 favorites]

Permalink for the "Via" in the OP, since the account's timeline will eventually scroll it away: https://mastodon.social/@whitequark/100677636145999980 (OP: Message the mods if you'd like the link updated)

Is this the first time a Metafilter post was sourced through a Mastodon toot?
posted by ardgedee at 11:45 AM on September 6, 2018 [2 favorites]

He noted that the non-osmatics could be subdivided into two categories, aquatic animals like cetaceans (e.g. whales and dolphins) who lacked basic olfactory structures

Huh. So whales don't have olfactory structures? Do they smell? Wouldn't smelling be useful, even in the water?
posted by clawsoon at 1:00 PM on September 6, 2018

the ordering of our common experimental subjects in order of increasing numbers of olfactory bulb neurons would be: human male, mouse, hamster, guinea pig, human female, macaque monkey, rat.

Is that why y'all are picking up so many different dude smells?
posted by clawsoon at 1:09 PM on September 6, 2018

So the wet dog / clothes left damp for too long thing... for some reason that smell for me is 'a really strong smell of peas'. I wonder if it's something like the way almonds and cherries smell similar for some people (probably because they both contain benzaldehyde).

I also used to wonder why line-dried washing smells a bit like a photocopier. It's because both smell of ozone.
posted by pipeski at 4:00 PM on September 6, 2018 [1 favorite]

About to dig in, but if this is the first step towards some sort of Smell Dating service I will be very excited.

(You can’t just ask people if you can smell their neck, and yet it’s so important. Idk. Olfactory weirdos unite.)

I don't have a strong sense of smell (though strangely, it is getting much more sensitive as I get older), but I would 100% sign up for this dating service, or even just for basic socialization. People tend to smell either very good or very bad to me, and it seems like important basic information. (At the risk of really sounding like an olfactory weirdo, this is one of the benefits for me of cultures where people hug and cheek-kiss as greetings versus standing at arms-length.)
posted by Dip Flash at 6:53 PM on September 6, 2018

(You can’t just ask people if you can smell their neck, and yet it’s so important. Idk. Olfactory weirdos unite.)

I want to be part of the Smell Dating service. Can't tell you how many times I have gotten very excited about someone who seemed smart and like a decent person only to discover that once I got close enough for a sniff, they did not smell right.

Men often don't know what I'm talking about. "Just tell him to change his cologne!" a friend told me when I described a failed romance with a perfectly nice guy who had an oddly citrusy smell. Women are more likely to nod vigorously when I bring it up. But the fact that not everyone can smell this, or at least is not aware of smelling it, makes me wonder what other people are talking about when they talk about sexual chemistry.
posted by bunderful at 7:16 PM on September 6, 2018 [1 favorite]

"We even appear to unconsciously smell our hands after shaking hands with strangers." This rabbit hole is deep and twisty indeed!
posted by drdanger at 2:01 PM on September 7, 2018