How Smell, Taste & Pheromones Shape Behavior | Huberman Lab Essentials
Date: 2025-05-01 | Duration: 00:34:28
Transcript
0:00 Welcome to Huberman Lab Essentials, where we revisit past episodes for the most potent and actionable science-based tools for mental health, physical health, and performance. I’m Andrew Huberman, and I’m a professor of neurobiology and ophthalmology at Stanford School of Medicine. This podcast is separate from my teaching and research roles at Stanford. Today we’re going to talk about chemical sensing. We’re going to talk about the sense of smell, our ability to detect odors in our environment. We’re also going to talk
0:30 about taste, our ability to detect chemicals and make sense of chemicals that are put in our mouth and into our digestive tract. And we are going to talk about chemicals that are made by other human beings that powerfully modulate the way that we feel, our hormones, and our health. Now, that last category is sometimes called pheromones. However, whether or not pheromones exist in humans is rather controversial. There actually hasn’t been a clear example of a true human
1:00 pheromonal effect. But what is absolutely clear, what is undeniable, is that there are chemicals that human beings make and release in things like tears, onto our skin, in sweat, and even breath that powerfully modulate or control the biology of other individuals. There are things floating around in the environment which we call volatile chemicals. So when you actually smell something—let’s say you smell
1:30 a wonderfully smelling rose or cake—yes, you are inhaling the particles into your nose. Literally, little particles of those chemicals are going up into your nose and being detected by your brain. Other ways of getting chemicals into our system is by putting them in our mouth, by literally taking foods and chewing them or sucking on them and breaking them down into their component parts. And that’s one way that
2:00 we sense chemicals with our tongue. So these chemicals we sometimes bring into our body, into our biology through deliberate action. We select a food, we chew that food, and we do it intentionally. Sometimes they’re coming into our body through non-deliberate action. We enter an environment and there’s smoke, and we smell the smoke, and as a consequence, we take action. Sometimes, however, other people are actively making chemicals with their
2:30 body. Typically this would be with their breath, with their tears, or possibly—I want to underscore possibly—by making what are called pheromones: molecules that they release into the environment, typically through the breath, that enter our system through our nose or our eyes or our mouth that fundamentally change our biology. I’ll just give an example, which is a very salient and interesting one that was published about 10 years ago in the journal Science, showing that
3:00 humans—men in particular in this study—have a strong biological response and hormonal response to the tears of women. What they did is they had women—and in this case, it was only women for whatever reason—cry, and they collected their tears. Then those tears were smelled by male subjects, or male subjects got what was essentially the control, which was the
3:30 saline. Men that smelled these tears that were evoked by sadness had a reduction in their testosterone levels that was significant. They also had a reduction in brain areas that were associated with sexual arousal. They actually recruited subjects that had a high propensity for crying at sad movies, which was not all women. What they were really trying to do is just get tears that were authentically cried in response to sadness as opposed to
4:00 putting some irritant in the eye and collecting tears that were evoked by something else, like just having the eyes irritated. Nonetheless, what this study illustrates is that there are chemicals in tears that are evoking or changing the biology of other individuals. Now, I didn’t select this study as an example because I want to focus on the effects of tears on hormones per se, although I do find the results really interesting. I chose it because I wanted to just
4:30 emphasize or underscore the fact that chemicals that are made by other individuals are powerfully modulating our internal state. And that’s something that most of us don’t appreciate. I think most of us can appreciate the fact that if we smell something putrid, we tend to retract, or if we smell something delicious, we tend to lean into it. But there are all these ways in which chemicals are affecting our biology, and interpersonal communication using
5:00 chemicals is not something that we hear that often about, but it’s super interesting. So let’s talk about smell and what smell is and how it works. I’m going to make this very basic, but I am going to touch on some of the core elements of the neurobiology. So here’s how smell works. Smell starts with sniffing. Now, that may come as no surprise, but no volatile chemicals can enter our nose unless we inhale them. If our nose is occluded or if we’re actively exhaling, it’s much more difficult for
5:30 smells to enter our nose, which is why people cover their nose when something smells bad. Now, the way that these volatile odors come into the nose is interesting. The nose has a mucosal lining—mucus that is designed to trap things, to actually bring things in and get stuck there at the base of your brain. So you could actually imagine this, or if you wanted, you could touch the roof of your mouth, but right about 2 cm up is your olfactory bulb. The
6:00 olfactory bulb is a collection of neurons, and those neurons actually extend out of your skull into your nose, into the mucosal lining. So what this means in a literal sense is that you have neurons that extend their little dendrites and axon-like things, or little processes as we call them, out into the mucus, and they respond to different odorant compounds. Now, the olfactory neurons also send a branch
6:30 deeper into the brain, and they split off into three different paths. So one path is for what we call innate odor responses. So you have some hardwired aspects to the way that you smell the world that were there from the day you were born and that will be there until the day you die. These are the pathways and the neurons that respond to things like smoke, which, as you can imagine,
7:00 there’s a highly adaptive function to being able to detect burning things because burning things generally means lack of safety or impending threat of some kind. It calls for action, and indeed these neurons project to a central area of the brain called the amygdala, which is often discussed in terms of fear, but it’s really fear and threat detection. You also have neurons in your nose that respond to odorants or combinations of odorants that evoke a sense of desire
7:30 and what we call appetitive behaviors—approach behaviors that make you want to move toward something. So when you smell a delicious cookie or some dish that’s really savory that you really like, that’s because of these innate pathways that require no learning whatsoever. Now, some of the pathways from the nose, these olfactory neurons into the brain, are involved in learned associations with odors. Many
8:00 people have this experience that they can remember the smell of their grandmother’s home or the smell of particular items baking or on the stove in a particular environment. Typically these memories tend to be of a nurturing sort of feeling—safe and protected. But one of the reasons why olfaction is so closely tied to memory is because olfaction is the most ancient sense that we have. So we have a
8:30 pathway for innate responses and a pathway for learned responses. And then we have this other pathway. In humans, it’s a little bit controversial as to whether or not it sits truly separate from the standard olfactory system or whether or not it’s its own system embedded in there, but they call it the accessory olfactory pathway. The accessory olfactory pathway is what in other animals is responsible for true pheromone effects. For example, in rodents and in some
9:00 primates, including mandrills. If you’ve ever seen a mandrill, they have these big beak noses. You may have seen them at the zoo. Look them up if you haven’t seen them already: M-A-N-D-R-I-L-L-S. Mandrills. There are strong pheromone effects. Some of those include things like if you take a pregnant female rodent or mandrill, you take away the father that created those fetuses and you introduce the scent of the urine or the fur of a novel male, she will spontaneously abort or miscarry those fetuses. It’s a very powerful effect. Another example of a pheromone effect is called the Vandenberg effect, named after the person who discovered this effect, where you take a female of a given species that has not entered puberty, you expose her to the scent or the urine from a
10:00 sexually competent, meaning post-pubertal, male, and she spontaneously goes into puberty earlier. So something about the scent triggers something through this accessory olfactory system. This is a true pheromonal effect and creates ovulation and menstruation—or in rodents, it’s an estrus cycle, not a menstrual cycle. So this is not to say that the exact same things happen in humans. In humans,
10:30 as I mentioned earlier, there is chemical sensing between individuals that may be independent of the nose, but those are basically the three paths by which smells and odors impact us. So, I want to talk about the act of smelling. And if you are not somebody who’s very interested in smell, but you are somebody who’s interested in making your brain work better, learning faster, remembering more things, this next little segment is for you. Because it turns out that how you smell—meaning
11:00 the act of smelling, not how good or bad you smell, but the act of smelling, sniffing, and inhalation—powerfully impacts how your brain functions and what you can learn and what you can’t learn. Noam Sobel’s group, originally at UC Berkeley and then at the Weizmann Institute, has published a number of papers that I’d like to discuss today. One of them, “Human non-olfactory cognition phase-locked with inhalation,” was published in Nature Human Behaviour, an excellent
11:30 journal. What this paper shows is that as we inhale, the level of alertness goes up in the brain. And this makes sense because, as the most primitive and primordial sense by which we interact with our environment and bring chemicals into our system and detect our environment, inhaling is a cue for the rest of the brain to essentially pay attention to what’s happening, not just to the odors.
12:00 And as the name of this paper suggests, human non-olfactory cognition is phase-locked with inhalation. What that means is that the act of inhaling itself wakes up the brain. It’s not about what you’re perceiving or what you’re smelling. And indeed, sniffing as an action, inhaling as an action, has a powerful effect on your ability to be alert, your ability to attend, to focus, and your ability to remember information. When we exhale,
12:30 the brain goes through a subtle but nonetheless significant dip in level of arousal and ability to learn. How should you use this knowledge? Well, you could imagine, and I think this would be beneficial for most people, to focus on nasal breathing while doing any kind of focused work that doesn’t require that you speak or eat or ingest something. There is a separate paper published in the Journal of Neuroscience that showed
13:00 that indeed if human subjects are restricted to breathing through their nose, they learn better than if they have the option of breathing through their mouth or a combination of their nose and mouth. Now, there are other ways to wake up your brain more as well. For instance, the use of