Why do you like the taste of something that your friend does not. Why new kids not like coffee but adults do. Can we consider smell and taste both part of something bigger? And what does any of this have to do with whether your culture eats spicy foods or whether women actually synchronize their menstruation or smelling someone's armpits. Welcome to Inner Cosmos with me David Eagleman. I'm a neuroscientist and author at Stanford and in these episodes, we sail deeply into our three pound universe to understand why and how our lives look the way they do. Over the last few months, I've received several requests to make an episode on the topic of smell and taste and flavor and why we like some more than others and why are some tastes acquired? So that's what we're gonna do today, and we're gonna start with taking a little time to appreciate how the stuff works. It hasn't escaped my notice that when I talk with people on airplanes, everyone seems to have a reasonable understanding of how their eyeballs work capturing light photons from the world. But if smell or taste comes up there generally seems to be less known about how that physically works. In other words, when there's an apple pie on the counter across the room, how precisely does your brain detect that? And let's say you taste some drink with molecules of this shape or that shape and you say, oh, that's blueberry flavored or oh that's lemon flavored. What is happening in your tongue and in your brain? So we're first going to understand that how these systems explore the world around us and how they work, and then we'll transition into cool questions like why did you not like coffee as a kid but you do now? Or why do you always choose this flavor of ice cream and your friend always chooses this other one? And I'll just say this episode is quite personal for me because when I was a kid, I fell off a roof and smashed my nose very badly, and as a result, I've always had a particularly terrible sense of smell. And watching people around me be able to identify things that a different level than I could has always made me very interested in this topic. So let's start by thinking about the senses generally, all your senses are just specialized detectors for picking up some sort of information stream from the world. Vision works by capturing and transforming the energy of photons which bound so objects. Hearing works by picking up these sensitive mechanical forces. On the ear drum. You have air compression waves that wiggle this membrane back and forth, and your brain analyzes that data. Touch is also a detector of mechanical forces. When you touch something, you physically distort these receptors in your skin. But taste and smell these are different from the others. They have a divergent strategy for picking up on information from the world. They work by being exquisitely sensitive molecule detectors. So let's start with taste. How does taste work. The sensitive taste is called gustation, and we see this in the Latin expression de gustibis known s disputandum, which means about taste. There is no disputing, or, as it's come down to us in English, there is there's no accounting for taste. Now, the ancient Romans were fond of this phrase, and as we'll see, there is something sufficiently profound about this observation that it has stuck with us for a couple thousand years. So how does taste Gustachian work biologically? Here's how. You have microscopic taste receptors all over your tongue and also, by the way, spread even more widely, even on your palate and the upper half of your esophagus and more. Now, these little taste receptors are embedded in the membrane of what we call taste cells, and taste cells clump together, about one hundred of them into taste buds, and taste buds cluster to form the bumps on your tongue that you can see with the naked eye. These are called pipille. Now, I know different listeners are doing different things while listening, but if you're able, either now or later, go to a mirror or bust out your front facing camera on your cell phone and take a look at your tongue. You'll notice the surface is not smooth. You have all these bumps, these papillae, and you'll notice that some of these are bigger bumps. These are called fungiform, meaning like a mushroom, and off to the sides you have some that look leaf like and others way in the back that look pimple. Like, what's cool is that you've had this tongue your whole life, and it's possible that you've never looked really carefully to see what you are made of. But this is all part of the joy of self discovery. Okay, So back to the structure here, so you can see these papilla on your tongue. But what you can't see because it's smaller, is that inside the taste buds, the taste cells arrange themselves so they're little receptors line a central pore. So architecturally, these guys are set up to catch chemicals. That's what they're built for. Even if you, the owner, are totally unaware of that. You're just walking around and you lick the ice cream and you say, oh yeah, that rocky Road tastes awesome, and you say, of course I can distinguish rocky Road from lemon from vanilla. But how does that work. The answer is, we have this miraculous microscopic engineering in ourselves. But the story gets even better because this whole system can detect and distinguish flavors with extraordinarily high specificity. So to understand how it does that, let's turn to the food that you put in your mouth. Everything that you eat or drink can be understood in terms of the molecules that they're made of, and we call these tastins Tasteans fall into five basic categories, and I know you're already well acquainted with four of them, sweet, salty, bitter, and sour. But what's the fifth taste category, which was added more recently. I'll give you a second. The fifth taste category is called umami. Umami this is a Japanese word meaning delicious taste, and we usually describe this flavor as savory. So for the sweet category, the standard thing we might think about is sucrose. For salty, a prototypical stimulus might be sodium chloride, which is table salt. For sour, the prototype is citric acid, like from a lemon. For bitter, it's quinine. And for umami, that fifth taste category, it's msgm MO, a sodium glutamate. Okay, So what happens when you put some taste int in your mouth? What happens is that the various chemicals, little molecules bind to the taste receptors. And even though I told you there are five categories, there are about fifty different receptors that we have, and there are all kinds of ways that these receptors work. Mechanically. Some cells activate when sodium physically flows through a channel in the membrane. Other cells activate because the key thing in sour compounds, which is hydrogen ions, block certain channels in the membrane. In other cases, taste ins bind to proteins in the cell membrane and change their shape, which leads to a cascade of changes inside the cell. So there's a huge variety of ways that these molecules that you've just put in your mouth physically get translated into signals that shoot off into your brain. So it seems like a strange zoo of things that can happen here. But as long as the signaling is consistent, that's all you need. So if you taste cinnamon, you get this very weird pattern of activation, and as long as you get that same pattern tomorrow, then you can identify that as cinnamon again. And this huge variety of these random tricks of mother nature gives us a lot of nuance and ability to discriminate when we sit down to enjoy a meal. So these signals shoot over to the brainstem, and then to the thalamus and finally to the core tex, a special area that we call the primary gustatory cortex. Okay, so how does taste actually get constructed? I told you that we have these taste receptors that respond preferentially to sweet or salty or something like that. But how do we get from something like that to something very specific like the taste of chocolate covered peanuts. With time and experience, we learn to recognize combinations of flavors, thousands and thousands of them. Now how do we get that kind of diversity from only fifty receptor types in five categories. Well, it's because each of these receptors will also activate in response to other types of taste ins if those are present and high enough concentrations. And it's not that each taste bud talks to one fiber going back into the brain, but instead multiple taste buds talk to a single fiber. So we can skip the details, but the bottom line is that anything you stick in your mouth triggers a pattern of taste receptor activation, which then stimulates a specific pattern of goostatory cells in the cortex. So this sense of taste uses populations of neurons to encode the sensation. This is what's called pattern encoding. It's not that chocolate covered peanuts activates this neuron. Instead, it activates thousands of neurons. This pattern of thousands of neurons maps onto lemon pie, and this other pattern over here maps on the bobagaoush, and this pattern maps on the anchovies and so on. Now what happens if there's damage to this gustatory system. It impairs your ability to taste. This is called dysgusia, or at the extreme agusia, and this can result from all kinds of different problems if it's at the level of the taste cells. Happily, these turnover quickly every couple of weeks, so damage to the taste cells themselves is often reversible. For example, there's a chef named Grant Ashats, and he made headlines after being diagnosed with tongue cancer and losing his sense of taste as a result of radiation treatment. And so he described how with time he regained taste sensation one category at a time. Now, people sometimes get dysgusia from COVID, and I'll come back to that a little later. But you can also get dysgusia from damage directly to the cortex, just like we see in all the other senses. If you damage the primary gustatory cortex, you don't understand basic taste anymore. If the damage happens in a higher level area, we move from the basic details to more abstract representation. So with damage to the secondary goustatory cortex, you can still identify basic tests, but you can't do more subtle recognition of food type and flavor intensity. Now what's fascinating is that if you lose other senses, that can also affect your sense of taste. For example, consider how your experience of each bite includes touch information in your mouth about texture and temperature, what is sometimes called the mouth feel of food. But by far the most influential sense on our perception of taste is smell. The nuance that we have and our perception of taste is cranked up way more from the interplay of taste and smell. Just think about what things like when you have a head cold. So now we turn to act too all about smell and then will come to the more general concept of flavor. So our other chemical sense smell, known as old faction. Lets us perceive airborne chemicals. Now, we humans don't rely on our sense of smell as much as we do on other sensory windows like vision and hearing. But other cousins of ours in the animal kingdom, like dogs and rodents, they capitalize on old faction to read the environment around them like a book. A dog can tell that a cat wandered onto the lawn hours ago, and rats can locate little, tiny morsels of food buried underneath layers of cage bedding. So here's the interesting bit. Although humans and dogs and rats we all rely on smell to different degrees, it all works in our brains the same way the floating chemical signals. These are called odorans, and you walk around all day vacuuming these in mostly through your nose a little through your mouth. Once these molecules have entered the giant vessel of your nasal cavity, they get sucked over to the main sensory organ, which is called the old factory epithelium, and that's at the back of that big space inside your nose. In humans, this structure is less than ten square centimeters, which is about the size of a half dollar coin. Compare that to dogs whose epithelium is seventeen times larger. Now, the old factory epithelium is covered with a layer of mucus, and that's how the molecules stick and come into contact with the little feelers of the receptor cells, called the dendrites. Okay, but how do we distinguish different smells like chlorine from lavender from wood burning at a campfire, given that these are all just molecules of different shapes. Well. A lot of the groundbreaking work in this field was done by Richard Axel and Linda Buck who discovered a huge number of old factory receptor genes in rats, about one thousand of them, and by the way, they won the two thousand and four Nobel Prize for that. Turns out, these old factory receptor genes are the largest gene family in the rodent genome. We humans also have that same gene family, but only about four hundred of these genes are still functional in US. So these receptors started becoming discovered in nineteen ninety one, not that long ago, and at first the guess was that maybe each receptor encodes an odor, But the way it turns out is that each odorant molecule has its particular shape, and it binds to several different receptor types, and a single receptor type responds to lots of different odorants that all happened to share some particular shape feature. So, just like the sense of taste, you have pattern encoding. This whole random looking population of neurons represents the scent of freshly cut grass, and this other group over here represents mint, and this other population represents hot chocolate and so on. Now, before I go on what's going on with COVID and smell. A lot of people end up with what's called a noosmia, which is a lack of smell and inability to smell. There's a growing literature on this, and the answer to why it happens isn't fully agreed on. But you can see how with the huge variety of molecular mechanisms underlying smell, it's not hard for a virus to throw a wrench in the machinery, in other words, to temporarily tweak things so that the signals the brain are used to have now been changed. Okay, so back to how smell works. From these receptors and the epithelium, the signals shoot to the olfactory bulb and from there to the primary olfactory cortex. From this part of the cortex, information zooms out to a network of other areas that are involved in higher order abstractions like familiarity and edibleness. And again we see that the primary sensory cortex represents the basic data and then higher level cortical processing becomes more abstract from there. So we have this sophisticated machinery to pick up on floating chemical signals in the world. So what do animals do with this? Well, obviously they identify things in the world like foods or toxins, but it goes beyond that. A lot of animals smell to understand not only what but where. They navigate space by smelling their way around. So think of a puppy finding its mother's nipple via smell, or lobsters locating their prey, or moths finding their lovers. All of these are done with smell, and this is also a large part of how pigeons find their way home or salmon return to their stream of origin. They create an olfactory map that links specific smells with locations during their travels. But it's not only about long distance stuff. Animals can navigate close space by smell. They can actually figure out which way to turn. Now, how could that work? Well, you probably know the brain can localize a sound by comparing the signal hitting the two ears, and the side where the signal arrives first tells you the side where the sound came from. Amazingly, the same strategy is used to find the source of a smell. You exploit the inter nostril time differences which nostril the odor got to first, And it's been shown this is how sharks aside which way to turn when they're following a plume of smell, and it's not as one might have intuited by the concentration difference. So this is a general strategy across senses. If you're trying to locate something, you can exploit the timing across two channels that are in slightly different places, like your two ears or your two nostrils. Okay, so where are we now. We've looked at how taste works by chemicals binding to receptors in and around the tongue, and we've seen how smell is about floating molecules binding to receptors at the back of the nasal cavity, and in both cases the brain is using pattern encoding. Think of this like the way that you strike multiple piano keys to make a cord. If you play these neurons, that's eucalyptus, This cord of neurons is peppermint, That chord is burnt toast, and so on. But the really interesting thing about taste and smell is how interactive they are, and a lot of times they really can't be separated. It's not always useful to think about taste and smell as independent senses, so some people talk about this as a composite flavor sense. As an example, certain odors like vanilla are consistently said to smell sweet, even though sweetness belongs to the domain of taste, and according to one study, sweet is the most common description of odor. Or we might say that something smells spicy or something smells sour, even though these are taste words, and food companies understand the interaction of smell and taste, so what they'll do is enhance the sweetness of a product by adding a sweet smelling odor. The same trick of adding a sweet odor can also reduce the perceived sourness of something that's acidic, so smell and taste are entangled. I mentioned earlier that foods lose their flavor when you have a cold because a plugged nose affects your sense of smell, and without smell, there's little flavor. So as a result of damaging my sense of smell when I was a kid, I've always had very little discrimination when it comes to food, which is not bad. I don't mind eating food that's boring or a little off. For food that's very spicy that my super taste or friends just can't handle, I'm fine with it all. None of the taste particularly stands out for me because I just don't experience that much smell. Okay, so we've been talking about the interaction between the senses, but I just want to return to smell in particular, because a discussion about noses would not be complete without talking about pheromones. What is a pheromone. It's a chemical that's broadcast by an animal to transmit information like identity and gender, and it can trigger behaviors in other members of the same species. So, for example, pheromones are given off by queen bees to halt the sexual development of the other females and trigger them to become workers. And what we generally see is that drifting molecules can carry a high density of information. In other cases, for example, pheromones carry information about a prospective mate, like their virility, or their genetics, or their age or their fitness. The effect of pheromones on sexual behavior has been studied a lot in the laboratory. So, for example, female mice are presented with a choice of males. It turns out that a female's choice of mate is not random, or it's not based on visible attributes. Instead, the choice results from the relationship between her genetics and that of her suitor. The trick is how she accesses that data. So mammals have a set of immune system genes that we summarize as the major histocompatibility complex or MHC, and following the strategy of keeping the gene pool well stirred, the female mouse will choose the mates whose MHC genes are the most different from hers. But how do the female mice, who are almost blind, figure out who is like them and who is unlike them? And the answer is inside their noses. There's a little organ called the vomeronasal organ and this detects the pheromones, which serve as little genetic calling cards. So these chemicals are carrying deep and important information. Now, the discovery of pheromones across mammals opens up the possibility that humans communicate unconsciously using olfaction in pheromones. And as it turns out, some receptors in your nose are identical to the receptors that mice use for pheromonal signaling. Now, it's not yet clear whether there our pheromonal systems are actually operational, but several groups have presented behavioral evidence that supports the possibility. So in one study, males wore t shirts for several days, allowing their sweat to soak into the cotton. Then females smelled the armpits of the shirts and selected the body odor that they preferred the most. Now, strikingly, and just as you might expect from the mice studies, the females favored the males with MHCs that were different from their own. So, although we're not consciously aware of our pheromonal signals, they might influence our attraction judgments. Beyond mate selection, pheromones also seem to offer some other kinds of data in humans. One study demonstrated that newborns prefer pads that have been brushed against their mother's breast over clean pads, presumably because of pheromones, and generally a mother and an infant can recognize one another based on smell. Humans can also recognize their parents and siblings based on scent, which is proposed to aid in incest avoidance. And beyond family, it's been shown that when a female sniffs the armpit sweat of another woman, the length of her menstrual cycle can change. By the way, side note, it's commonly believed that women who live together synchronize their menstrual cycles, but I just want to clarify that that claim is actually unsupported. There have been large scale studies on this and they demonstrate that synchronization doesn't happen, but you can get statistical fluctuations that give the illusion of synchrony. So although people thought this was true in the nineteen seventies, subsequent research has failed to replicate that finding. So pheromones convey some information in humans, but I'd say the amount they influence our behavior is still not totally clear. Human cognition is profoundly more complex than mouse cognition, and it's possible that pheromones have diminished to a pretty minor role, like legacy software that's left on a computer system that has been continuously updated. So I want to return now to the Latin phrase that I mentioned at the beginning, the gustabus known s disputantum or there's no accounting for taste? Why do some of us like some tastes and others don't? For example, I like Brussels sprouts and my wife doesn't. Why do people like different things? Whill As it turns out, there are lots of reasons for starters. There are genetic actors. Take the issue of taste sensitivity. You have genetic differences that determine how sensitive you are to certain tastes. As an example, there's a gene called TAS two R thirty eight, and whatever sequence you have in that gene that determines your sensitivity to bitter things like broccoli or coffee. If you have a heightened sensitivity to bitterness, you're probably not gonna like those flavors. So that's one example of many, But this goes further. Some people are known as super tasters, and they just have more taste buds, and as a result, they're more sensitive to lots of flavors, especially bitterness and sweetness, so they find certain foods just too intense, while people at the other end of the spectrum with fewer taste buds, sometimes called non tasters, prefer more robust flavors because they need that to get the same punch. But that's not all. Your particular tastes are also about your early experiences. Your taste preferences begin to develop even before birth from your mother's diet, which passes into the amniotic fluid and influences your preferences. The general story is that repeated exposure to particular tastes leads to a preference for those flavors. In other words, the foods that you're exposed to early on shape your lifelong tastes. If you're raised in a culture where spicy foods are common, you're more likely to develop a preference for spicy flavors. What one culture considers a delicacy, another might find totally unappealing. But you learn these cultural preferences from your family traditions and your social groups. By the way, on the flip side of preferences, you can also develop an aversion to some foods given some negative experience that you had, like getting sick after eating it once. This is called a conditioned response. And on this topic of individual preferences, I'll also just mention that these can change even by the hour. You might crave particular flavors at particular times because your body is signaling a need for specific nutrients in the same way that you want water when you're thirsty. If you're craving salty foods, you might be responding to a need for sodium. So there are a lot of factors that combine to create your highly individual taste preferences. Your biological makeup, your upbringing, your experience. These all play their roles in shaping what you find delicious or distasteful. Now there's a related issue which I've always found fascinating, which is that my kids don't like coffee. Every once in a while, they'll say, hey, can I try a sip of that? And I give them a sip, and they contort their faces and they're truly incredulous that I and other adults would swill this black liquid. It's totally aversive to them. And I remember feeling the same way when I was a kid, But now I love my daily addiction to coffee, and I'm sure my kids will someday as well. So this always led me to be fascinated by the concept of the acquired taste. So how do we understand this? Well, given what I said a moment ago, there are obviously social and cultural factors that play in here. Children observe adults consuming things and they come to associate that with maturity or desirable social behaviors. Coffee, in particular, commonly symbolizes adulthood and provides a sense of sophistication, making it appealing to children as they get older and more generally, the drive to fit in with social groups can also influence us to develop a taste for certain foods and drinks, And there's also the psychological association. Kids come to associate certain flavors with positive experiences, like a fun morning routine with the family, and that leads to a preference for those smells or tastes. And acquired tastes are about even more than that, because there's also physical stuff that happens. Children's taste buds are more sensitive and they tend to prefer sweet flavors. This is thought to be a biological safeguard to ensure that they consume calorie rich foods for growth, and bitter flavors like coffee are thought to be even more unpleasant because bitterness can signal toxins in nature. But as children grow, their taste buds change, their desire for sweetness goes down. They become less sensitive to bitterness, and that makes certain foods and drinks more palatable over time, like coffee. There's also the issue of repeated exposure. So a kid might initially dislike a bitter or strong flavor like coffee, but with repeated tasting, they get more accustomed to it. This is called taste acculturation or flavor learning, where exposure to certain flavors gradually diminishes their negative reactions. And we also acquire a taste for coffee because we come to predict what it will do to us, how it will make us feel. And finally, when we're talking about acquire tastes, we can't ignore cognitive development. As children mature, they're just more open to trying new foods and flavors, and they're growing cognitive abilities help them to better appreciate complex flavors, recognizing the nuances and foods that they previously just didn't want to try. And so when we think about acquired tastes, there are so many things underlying this, social issues, developmental issues, and physical issues, all the way down to the level of the taste buds. So I hope you enjoyed this deep dive into the incredible world of taste and smell and how they work biologically. Now, I told you we have about fifty different types of taste receptors and about four hundred types of olfactory receptors, and that high dimensional space is what defines the boundaries of what you taste and smell. In other words, everything that you could possibly experience in your entire life lives inside this space. But other animals have more receptors for smell and for taste, many more, as carved by their evolutionary needs. So what would it be like to experience something well outside the dimensions of human flavor. We are, of course, very special species, a runaway species in terms of so many of our talents, but we lag behind most of the animal kingdom on this one metric of chemical sensing. For much of the rest of our cousins, smell into some of you, taste is their main window for picking up information from the world. So the next time you're out walking with your dog, try to think about the world as your cane perceives it. You are surrounded by a universe of smells. Just try to imagine seeing those as colorful plumes rising up all around you. And so when the ancient Romans said there was no accounting for taste, they were just talking about personal preference. But they couldn't have imagined just how vast the worlds of taste and smell are when we look beyond humans into the much vaster realm of all the animals and their noses tailored by evolution. So the next time that you enjoy a delicious meal, or smell a great floral bouquet, or walk alongside your dog, just remember that you're only scratching the surface of a much larger sensory cosmos. Go to Eagleman dot com slash podcast for more information and to find further reading. Send me an email at podcast at eagleman dot com with questions or discussion and check out Subscribe to Inner Cosmos on YouTube for videos of each episode and to leave comments. Until next time. I'm David Eagleman and this is inner cosmos,

Why do you like the taste of something that your friend does not. Why new kids not like coffee but adults do. Can we consider smell and taste both part of something bigger? And what does any of this have to do with whether your culture eats spicy foods or whether women actually synchronize their menstruation or smelling someone's armpits. Welcome to Inner Cosmos with me David Eagleman. I'm a neuroscientist and author at Stanford and in these episodes, we sail deeply into our three pound universe to understand why and how our lives look the way they do. Over the last few months, I've received several requests to make an episode on the topic of smell and taste and flavor and why we like some more than others and why are some tastes acquired? So that's what we're gonna do today, and we're gonna start with taking a little time to appreciate how the stuff works. It hasn't escaped my notice that when I talk with people on airplanes, everyone seems to have a reasonable understanding of how their eyeballs work capturing light photons from the world. But if smell or taste comes up there generally seems to be less known about how that physically works. In other words, when there's an apple pie on the counter across the room, how precisely does your brain detect that? And let's say you taste some drink with molecules of this shape or that shape and you say, oh, that's blueberry flavored or oh that's lemon flavored. What is happening in your tongue and in your brain? So we're first going to understand that how these systems explore the world around us and how they work, and then we'll transition into cool questions like why did you not like coffee as a kid but you do now? Or why do you always choose this flavor of ice cream and your friend always chooses this other one? And I'll just say this episode is quite personal for me because when I was a kid, I fell off a roof and smashed my nose very badly, and as a result, I've always had a particularly terrible sense of smell. And watching people around me be able to identify things that a different level than I could has always made me very interested in this topic. So let's start by thinking about the senses generally, all your senses are just specialized detectors for picking up some sort of information stream from the world. Vision works by capturing and transforming the energy of photons which bound so objects. Hearing works by picking up these sensitive mechanical forces. On the ear drum. You have air compression waves that wiggle this membrane back and forth, and your brain analyzes that data. Touch is also a detector of mechanical forces. When you touch something, you physically distort these receptors in your skin. But taste and smell these are different from the others. They have a divergent strategy for picking up on information from the world. They work by being exquisitely sensitive molecule detectors. So let's start with taste. How does taste work. The sensitive taste is called gustation, and we see this in the Latin expression de gustibis known s disputandum, which means about taste. There is no disputing, or, as it's come down to us in English, there is there's no accounting for taste. Now, the ancient Romans were fond of this phrase, and as we'll see, there is something sufficiently profound about this observation that it has stuck with us for a couple thousand years. So how does taste Gustachian work biologically? Here's how. You have microscopic taste receptors all over your tongue and also, by the way, spread even more widely, even on your palate and the upper half of your esophagus and more. Now, these little taste receptors are embedded in the membrane of what we call taste cells, and taste cells clump together, about one hundred of them into taste buds, and taste buds cluster to form the bumps on your tongue that you can see with the naked eye. These are called pipille. Now, I know different listeners are doing different things while listening, but if you're able, either now or later, go to a mirror or bust out your front facing camera on your cell phone and take a look at your tongue. You'll notice the surface is not smooth. You have all these bumps, these papillae, and you'll notice that some of these are bigger bumps. These are called fungiform, meaning like a mushroom, and off to the sides you have some that look leaf like and others way in the back that look pimple. Like, what's cool is that you've had this tongue your whole life, and it's possible that you've never looked really carefully to see what you are made of. But this is all part of the joy of self discovery. Okay, So back to the structure here, so you can see these papilla on your tongue. But what you can't see because it's smaller, is that inside the taste buds, the taste cells arrange themselves so they're little receptors line a central pore. So architecturally, these guys are set up to catch chemicals. That's what they're built for. Even if you, the owner, are totally unaware of that. You're just walking around and you lick the ice cream and you say, oh yeah, that rocky Road tastes awesome, and you say, of course I can distinguish rocky Road from lemon from vanilla. But how does that work. The answer is, we have this miraculous microscopic engineering in ourselves. But the story gets even better because this whole system can detect and distinguish flavors with extraordinarily high specificity. So to understand how it does that, let's turn to the food that you put in your mouth. Everything that you eat or drink can be understood in terms of the molecules that they're made of, and we call these tastins Tasteans fall into five basic categories, and I know you're already well acquainted with four of them, sweet, salty, bitter, and sour. But what's the fifth taste category, which was added more recently. I'll give you a second. The fifth taste category is called umami. Umami this is a Japanese word meaning delicious taste, and we usually describe this flavor as savory. So for the sweet category, the standard thing we might think about is sucrose. For salty, a prototypical stimulus might be sodium chloride, which is table salt. For sour, the prototype is citric acid, like from a lemon. For bitter, it's quinine. And for umami, that fifth taste category, it's msgm MO, a sodium glutamate. Okay, So what happens when you put some taste int in your mouth? What happens is that the various chemicals, little molecules bind to the taste receptors. And even though I told you there are five categories, there are about fifty different receptors that we have, and there are all kinds of ways that these receptors work. Mechanically. Some cells activate when sodium physically flows through a channel in the membrane. Other cells activate because the key thing in sour compounds, which is hydrogen ions, block certain channels in the membrane. In other cases, taste ins bind to proteins in the cell membrane and change their shape, which leads to a cascade of changes inside the cell. So there's a huge variety of ways that these molecules that you've just put in your mouth physically get translated into signals that shoot off into your brain. So it seems like a strange zoo of things that can happen here. But as long as the signaling is consistent, that's all you need. So if you taste cinnamon, you get this very weird pattern of activation, and as long as you get that same pattern tomorrow, then you can identify that as cinnamon again. And this huge variety of these random tricks of mother nature gives us a lot of nuance and ability to discriminate when we sit down to enjoy a meal. So these signals shoot over to the brainstem, and then to the thalamus and finally to the core tex, a special area that we call the primary gustatory cortex. Okay, so how does taste actually get constructed? I told you that we have these taste receptors that respond preferentially to sweet or salty or something like that. But how do we get from something like that to something very specific like the taste of chocolate covered peanuts. With time and experience, we learn to recognize combinations of flavors, thousands and thousands of them. Now how do we get that kind of diversity from only fifty receptor types in five categories. Well, it's because each of these receptors will also activate in response to other types of taste ins if those are present and high enough concentrations. And it's not that each taste bud talks to one fiber going back into the brain, but instead multiple taste buds talk to a single fiber. So we can skip the details, but the bottom line is that anything you stick in your mouth triggers a pattern of taste receptor activation, which then stimulates a specific pattern of goostatory cells in the cortex. So this sense of taste uses populations of neurons to encode the sensation. This is what's called pattern encoding. It's not that chocolate covered peanuts activates this neuron. Instead, it activates thousands of neurons. This pattern of thousands of neurons maps onto lemon pie, and this other pattern over here maps on the bobagaoush, and this pattern maps on the anchovies and so on. Now what happens if there's damage to this gustatory system. It impairs your ability to taste. This is called dysgusia, or at the extreme agusia, and this can result from all kinds of different problems if it's at the level of the taste cells. Happily, these turnover quickly every couple of weeks, so damage to the taste cells themselves is often reversible. For example, there's a chef named Grant Ashats, and he made headlines after being diagnosed with tongue cancer and losing his sense of taste as a result of radiation treatment. And so he described how with time he regained taste sensation one category at a time. Now, people sometimes get dysgusia from COVID, and I'll come back to that a little later. But you can also get dysgusia from damage directly to the cortex, just like we see in all the other senses. If you damage the primary gustatory cortex, you don't understand basic taste anymore. If the damage happens in a higher level area, we move from the basic details to more abstract representation. So with damage to the secondary goustatory cortex, you can still identify basic tests, but you can't do more subtle recognition of food type and flavor intensity. Now what's fascinating is that if you lose other senses, that can also affect your sense of taste. For example, consider how your experience of each bite includes touch information in your mouth about texture and temperature, what is sometimes called the mouth feel of food. But by far the most influential sense on our perception of taste is smell. The nuance that we have and our perception of taste is cranked up way more from the interplay of taste and smell. Just think about what things like when you have a head cold. So now we turn to act too all about smell and then will come to the more general concept of flavor. So our other chemical sense smell, known as old faction. Lets us perceive airborne chemicals. Now, we humans don't rely on our sense of smell as much as we do on other sensory windows like vision and hearing. But other cousins of ours in the animal kingdom, like dogs and rodents, they capitalize on old faction to read the environment around them like a book. A dog can tell that a cat wandered onto the lawn hours ago, and rats can locate little, tiny morsels of food buried underneath layers of cage bedding. So here's the interesting bit. Although humans and dogs and rats we all rely on smell to different degrees, it all works in our brains the same way the floating chemical signals. These are called odorans, and you walk around all day vacuuming these in mostly through your nose a little through your mouth. Once these molecules have entered the giant vessel of your nasal cavity, they get sucked over to the main sensory organ, which is called the old factory epithelium, and that's at the back of that big space inside your nose. In humans, this structure is less than ten square centimeters, which is about the size of a half dollar coin. Compare that to dogs whose epithelium is seventeen times larger. Now, the old factory epithelium is covered with a layer of mucus, and that's how the molecules stick and come into contact with the little feelers of the receptor cells, called the dendrites. Okay, but how do we distinguish different smells like chlorine from lavender from wood burning at a campfire, given that these are all just molecules of different shapes. Well. A lot of the groundbreaking work in this field was done by Richard Axel and Linda Buck who discovered a huge number of old factory receptor genes in rats, about one thousand of them, and by the way, they won the two thousand and four Nobel Prize for that. Turns out, these old factory receptor genes are the largest gene family in the rodent genome. We humans also have that same gene family, but only about four hundred of these genes are still functional in US. So these receptors started becoming discovered in nineteen ninety one, not that long ago, and at first the guess was that maybe each receptor encodes an odor, But the way it turns out is that each odorant molecule has its particular shape, and it binds to several different receptor types, and a single receptor type responds to lots of different odorants that all happened to share some particular shape feature. So, just like the sense of taste, you have pattern encoding. This whole random looking population of neurons represents the scent of freshly cut grass, and this other group over here represents mint, and this other population represents hot chocolate and so on. Now, before I go on what's going on with COVID and smell. A lot of people end up with what's called a noosmia, which is a lack of smell and inability to smell. There's a growing literature on this, and the answer to why it happens isn't fully agreed on. But you can see how with the huge variety of molecular mechanisms underlying smell, it's not hard for a virus to throw a wrench in the machinery, in other words, to temporarily tweak things so that the signals the brain are used to have now been changed. Okay, so back to how smell works. From these receptors and the epithelium, the signals shoot to the olfactory bulb and from there to the primary olfactory cortex. From this part of the cortex, information zooms out to a network of other areas that are involved in higher order abstractions like familiarity and edibleness. And again we see that the primary sensory cortex represents the basic data and then higher level cortical processing becomes more abstract from there. So we have this sophisticated machinery to pick up on floating chemical signals in the world. So what do animals do with this? Well, obviously they identify things in the world like foods or toxins, but it goes beyond that. A lot of animals smell to understand not only what but where. They navigate space by smelling their way around. So think of a puppy finding its mother's nipple via smell, or lobsters locating their prey, or moths finding their lovers. All of these are done with smell, and this is also a large part of how pigeons find their way home or salmon return to their stream of origin. They create an olfactory map that links specific smells with locations during their travels. But it's not only about long distance stuff. Animals can navigate close space by smell. They can actually figure out which way to turn. Now, how could that work? Well, you probably know the brain can localize a sound by comparing the signal hitting the two ears, and the side where the signal arrives first tells you the side where the sound came from. Amazingly, the same strategy is used to find the source of a smell. You exploit the inter nostril time differences which nostril the odor got to first, And it's been shown this is how sharks aside which way to turn when they're following a plume of smell, and it's not as one might have intuited by the concentration difference. So this is a general strategy across senses. If you're trying to locate something, you can exploit the timing across two channels that are in slightly different places, like your two ears or your two nostrils. Okay, so where are we now. We've looked at how taste works by chemicals binding to receptors in and around the tongue, and we've seen how smell is about floating molecules binding to receptors at the back of the nasal cavity, and in both cases the brain is using pattern encoding. Think of this like the way that you strike multiple piano keys to make a cord. If you play these neurons, that's eucalyptus, This cord of neurons is peppermint, That chord is burnt toast, and so on. But the really interesting thing about taste and smell is how interactive they are, and a lot of times they really can't be separated. It's not always useful to think about taste and smell as independent senses, so some people talk about this as a composite flavor sense. As an example, certain odors like vanilla are consistently said to smell sweet, even though sweetness belongs to the domain of taste, and according to one study, sweet is the most common description of odor. Or we might say that something smells spicy or something smells sour, even though these are taste words, and food companies understand the interaction of smell and taste, so what they'll do is enhance the sweetness of a product by adding a sweet smelling odor. The same trick of adding a sweet odor can also reduce the perceived sourness of something that's acidic, so smell and taste are entangled. I mentioned earlier that foods lose their flavor when you have a cold because a plugged nose affects your sense of smell, and without smell, there's little flavor. So as a result of damaging my sense of smell when I was a kid, I've always had very little discrimination when it comes to food, which is not bad. I don't mind eating food that's boring or a little off. For food that's very spicy that my super taste or friends just can't handle, I'm fine with it all. None of the taste particularly stands out for me because I just don't experience that much smell. Okay, so we've been talking about the interaction between the senses, but I just want to return to smell in particular, because a discussion about noses would not be complete without talking about pheromones. What is a pheromone. It's a chemical that's broadcast by an animal to transmit information like identity and gender, and it can trigger behaviors in other members of the same species. So, for example, pheromones are given off by queen bees to halt the sexual development of the other females and trigger them to become workers. And what we generally see is that drifting molecules can carry a high density of information. In other cases, for example, pheromones carry information about a prospective mate, like their virility, or their genetics, or their age or their fitness. The effect of pheromones on sexual behavior has been studied a lot in the laboratory. So, for example, female mice are presented with a choice of males. It turns out that a female's choice of mate is not random, or it's not based on visible attributes. Instead, the choice results from the relationship between her genetics and that of her suitor. The trick is how she accesses that data. So mammals have a set of immune system genes that we summarize as the major histocompatibility complex or MHC, and following the strategy of keeping the gene pool well stirred, the female mouse will choose the mates whose MHC genes are the most different from hers. But how do the female mice, who are almost blind, figure out who is like them and who is unlike them? And the answer is inside their noses. There's a little organ called the vomeronasal organ and this detects the pheromones, which serve as little genetic calling cards. So these chemicals are carrying deep and important information. Now, the discovery of pheromones across mammals opens up the possibility that humans communicate unconsciously using olfaction in pheromones. And as it turns out, some receptors in your nose are identical to the receptors that mice use for pheromonal signaling. Now, it's not yet clear whether there our pheromonal systems are actually operational, but several groups have presented behavioral evidence that supports the possibility. So in one study, males wore t shirts for several days, allowing their sweat to soak into the cotton. Then females smelled the armpits of the shirts and selected the body odor that they preferred the most. Now, strikingly, and just as you might expect from the mice studies, the females favored the males with MHCs that were different from their own. So, although we're not consciously aware of our pheromonal signals, they might influence our attraction judgments. Beyond mate selection, pheromones also seem to offer some other kinds of data in humans. One study demonstrated that newborns prefer pads that have been brushed against their mother's breast over clean pads, presumably because of pheromones, and generally a mother and an infant can recognize one another based on smell. Humans can also recognize their parents and siblings based on scent, which is proposed to aid in incest avoidance. And beyond family, it's been shown that when a female sniffs the armpit sweat of another woman, the length of her menstrual cycle can change. By the way, side note, it's commonly believed that women who live together synchronize their menstrual cycles, but I just want to clarify that that claim is actually unsupported. There have been large scale studies on this and they demonstrate that synchronization doesn't happen, but you can get statistical fluctuations that give the illusion of synchrony. So although people thought this was true in the nineteen seventies, subsequent research has failed to replicate that finding. So pheromones convey some information in humans, but I'd say the amount they influence our behavior is still not totally clear. Human cognition is profoundly more complex than mouse cognition, and it's possible that pheromones have diminished to a pretty minor role, like legacy software that's left on a computer system that has been continuously updated. So I want to return now to the Latin phrase that I mentioned at the beginning, the gustabus known s disputantum or there's no accounting for taste? Why do some of us like some tastes and others don't? For example, I like Brussels sprouts and my wife doesn't. Why do people like different things? Whill As it turns out, there are lots of reasons for starters. There are genetic actors. Take the issue of taste sensitivity. You have genetic differences that determine how sensitive you are to certain tastes. As an example, there's a gene called TAS two R thirty eight, and whatever sequence you have in that gene that determines your sensitivity to bitter things like broccoli or coffee. If you have a heightened sensitivity to bitterness, you're probably not gonna like those flavors. So that's one example of many, But this goes further. Some people are known as super tasters, and they just have more taste buds, and as a result, they're more sensitive to lots of flavors, especially bitterness and sweetness, so they find certain foods just too intense, while people at the other end of the spectrum with fewer taste buds, sometimes called non tasters, prefer more robust flavors because they need that to get the same punch. But that's not all. Your particular tastes are also about your early experiences. Your taste preferences begin to develop even before birth from your mother's diet, which passes into the amniotic fluid and influences your preferences. The general story is that repeated exposure to particular tastes leads to a preference for those flavors. In other words, the foods that you're exposed to early on shape your lifelong tastes. If you're raised in a culture where spicy foods are common, you're more likely to develop a preference for spicy flavors. What one culture considers a delicacy, another might find totally unappealing. But you learn these cultural preferences from your family traditions and your social groups. By the way, on the flip side of preferences, you can also develop an aversion to some foods given some negative experience that you had, like getting sick after eating it once. This is called a conditioned response. And on this topic of individual preferences, I'll also just mention that these can change even by the hour. You might crave particular flavors at particular times because your body is signaling a need for specific nutrients in the same way that you want water when you're thirsty. If you're craving salty foods, you might be responding to a need for sodium. So there are a lot of factors that combine to create your highly individual taste preferences. Your biological makeup, your upbringing, your experience. These all play their roles in shaping what you find delicious or distasteful. Now there's a related issue which I've always found fascinating, which is that my kids don't like coffee. Every once in a while, they'll say, hey, can I try a sip of that? And I give them a sip, and they contort their faces and they're truly incredulous that I and other adults would swill this black liquid. It's totally aversive to them. And I remember feeling the same way when I was a kid, But now I love my daily addiction to coffee, and I'm sure my kids will someday as well. So this always led me to be fascinated by the concept of the acquired taste. So how do we understand this? Well, given what I said a moment ago, there are obviously social and cultural factors that play in here. Children observe adults consuming things and they come to associate that with maturity or desirable social behaviors. Coffee, in particular, commonly symbolizes adulthood and provides a sense of sophistication, making it appealing to children as they get older and more generally, the drive to fit in with social groups can also influence us to develop a taste for certain foods and drinks, And there's also the psychological association. Kids come to associate certain flavors with positive experiences, like a fun morning routine with the family, and that leads to a preference for those smells or tastes. And acquired tastes are about even more than that, because there's also physical stuff that happens. Children's taste buds are more sensitive and they tend to prefer sweet flavors. This is thought to be a biological safeguard to ensure that they consume calorie rich foods for growth, and bitter flavors like coffee are thought to be even more unpleasant because bitterness can signal toxins in nature. But as children grow, their taste buds change, their desire for sweetness goes down. They become less sensitive to bitterness, and that makes certain foods and drinks more palatable over time, like coffee. There's also the issue of repeated exposure. So a kid might initially dislike a bitter or strong flavor like coffee, but with repeated tasting, they get more accustomed to it. This is called taste acculturation or flavor learning, where exposure to certain flavors gradually diminishes their negative reactions. And we also acquire a taste for coffee because we come to predict what it will do to us, how it will make us feel. And finally, when we're talking about acquire tastes, we can't ignore cognitive development. As children mature, they're just more open to trying new foods and flavors, and they're growing cognitive abilities help them to better appreciate complex flavors, recognizing the nuances and foods that they previously just didn't want to try. And so when we think about acquired tastes, there are so many things underlying this, social issues, developmental issues, and physical issues, all the way down to the level of the taste buds. So I hope you enjoyed this deep dive into the incredible world of taste and smell and how they work biologically. Now, I told you we have about fifty different types of taste receptors and about four hundred types of olfactory receptors, and that high dimensional space is what defines the boundaries of what you taste and smell. In other words, everything that you could possibly experience in your entire life lives inside this space. But other animals have more receptors for smell and for taste, many more, as carved by their evolutionary needs. So what would it be like to experience something well outside the dimensions of human flavor. We are, of course, very special species, a runaway species in terms of so many of our talents, but we lag behind most of the animal kingdom on this one metric of chemical sensing. For much of the rest of our cousins, smell into some of you, taste is their main window for picking up information from the world. So the next time you're out walking with your dog, try to think about the world as your cane perceives it. You are surrounded by a universe of smells. Just try to imagine seeing those as colorful plumes rising up all around you. And so when the ancient Romans said there was no accounting for taste, they were just talking about personal preference. But they couldn't have imagined just how vast the worlds of taste and smell are when we look beyond humans into the much vaster realm of all the animals and their noses tailored by evolution. So the next time that you enjoy a delicious meal, or smell a great floral bouquet, or walk alongside your dog, just remember that you're only scratching the surface of a much larger sensory cosmos. Go to Eagleman dot com slash podcast for more information and to find further reading. Send me an email at podcast at eagleman dot com with questions or discussion and check out Subscribe to Inner Cosmos on YouTube for videos of each episode and to leave comments. Until next time. I'm David Eagleman and this is inner cosmos,