WEBVTT - Ep89 "Why do you love some flavors and not others?" 0:00:05.120 --> 0:00:08.160 Why do you like the taste of something that your 0:00:08.200 --> 0:00:12.240 friend does not. Why new kids not like coffee but 0:00:12.520 --> 0:00:16.639 adults do. Can we consider smell and taste both part 0:00:16.640 --> 0:00:19.640 of something bigger? And what does any of this have 0:00:19.720 --> 0:00:23.920 to do with whether your culture eats spicy foods or 0:00:24.200 --> 0:00:31.440 whether women actually synchronize their menstruation or smelling someone's armpits. 0:00:32.320 --> 0:00:35.680 Welcome to Inner Cosmos with me David Eagleman. I'm a 0:00:35.720 --> 0:00:40.040 neuroscientist and author at Stanford and in these episodes, we 0:00:40.120 --> 0:00:44.320 sail deeply into our three pound universe to understand why 0:00:44.400 --> 0:00:56.279 and how our lives look the way they do. Over 0:00:56.320 --> 0:00:59.120 the last few months, I've received several requests to make 0:00:59.160 --> 0:01:02.920 an episode on the topic of smell and taste and 0:01:03.040 --> 0:01:06.960 flavor and why we like some more than others and 0:01:07.000 --> 0:01:10.360 why are some tastes acquired? So that's what we're gonna 0:01:10.360 --> 0:01:13.080 do today, and we're gonna start with taking a little 0:01:13.080 --> 0:01:17.880 time to appreciate how the stuff works. It hasn't escaped 0:01:17.920 --> 0:01:21.320 my notice that when I talk with people on airplanes, 0:01:21.640 --> 0:01:24.760 everyone seems to have a reasonable understanding of how their 0:01:25.120 --> 0:01:30.240 eyeballs work capturing light photons from the world. But if 0:01:30.319 --> 0:01:33.920 smell or taste comes up there generally seems to be 0:01:34.280 --> 0:01:38.240 less known about how that physically works. In other words, 0:01:38.600 --> 0:01:42.280 when there's an apple pie on the counter across the room, 0:01:43.120 --> 0:01:47.080 how precisely does your brain detect that? And let's say 0:01:47.080 --> 0:01:50.880 you taste some drink with molecules of this shape or 0:01:50.880 --> 0:01:54.080 that shape and you say, oh, that's blueberry flavored or 0:01:54.160 --> 0:01:58.120 oh that's lemon flavored. What is happening in your tongue 0:01:58.240 --> 0:02:01.120 and in your brain? So we're first going to understand 0:02:01.200 --> 0:02:05.080 that how these systems explore the world around us and 0:02:05.080 --> 0:02:09.239 how they work, and then we'll transition into cool questions 0:02:09.320 --> 0:02:11.720 like why did you not like coffee as a kid 0:02:11.760 --> 0:02:15.120 but you do now? Or why do you always choose 0:02:15.200 --> 0:02:18.120 this flavor of ice cream and your friend always chooses 0:02:18.200 --> 0:02:21.880 this other one? And I'll just say this episode is 0:02:22.200 --> 0:02:25.160 quite personal for me because when I was a kid, 0:02:25.200 --> 0:02:28.440 I fell off a roof and smashed my nose very badly, 0:02:28.480 --> 0:02:31.640 and as a result, I've always had a particularly terrible 0:02:31.720 --> 0:02:35.639 sense of smell. And watching people around me be able 0:02:35.680 --> 0:02:38.600 to identify things that a different level than I could 0:02:38.960 --> 0:02:42.040 has always made me very interested in this topic. So 0:02:42.200 --> 0:02:46.679 let's start by thinking about the senses generally, all your 0:02:46.720 --> 0:02:51.600 senses are just specialized detectors for picking up some sort 0:02:51.639 --> 0:02:57.000 of information stream from the world. Vision works by capturing 0:02:57.080 --> 0:03:01.119 and transforming the energy of photons which bound so objects. 0:03:01.800 --> 0:03:06.680 Hearing works by picking up these sensitive mechanical forces. On 0:03:06.720 --> 0:03:10.440 the ear drum. You have air compression waves that wiggle 0:03:10.480 --> 0:03:14.680 this membrane back and forth, and your brain analyzes that data. 0:03:14.960 --> 0:03:19.160 Touch is also a detector of mechanical forces. When you 0:03:19.560 --> 0:03:23.800 touch something, you physically distort these receptors in your skin. 0:03:24.200 --> 0:03:27.880 But taste and smell these are different from the others. 0:03:27.919 --> 0:03:32.519 They have a divergent strategy for picking up on information 0:03:32.680 --> 0:03:39.360 from the world. They work by being exquisitely sensitive molecule detectors. 0:03:40.040 --> 0:03:44.200 So let's start with taste. How does taste work. The 0:03:44.360 --> 0:03:48.040 sensitive taste is called gustation, and we see this in 0:03:48.080 --> 0:03:53.960 the Latin expression de gustibis known s disputandum, which means 0:03:54.280 --> 0:03:58.240 about taste. There is no disputing, or, as it's come 0:03:58.280 --> 0:04:01.200 down to us in English, there is there's no accounting 0:04:01.480 --> 0:04:05.320 for taste. Now, the ancient Romans were fond of this phrase, 0:04:05.720 --> 0:04:09.240 and as we'll see, there is something sufficiently profound about 0:04:09.240 --> 0:04:11.880 this observation that it has stuck with us for a 0:04:11.920 --> 0:04:17.360 couple thousand years. So how does taste Gustachian work biologically? 0:04:17.560 --> 0:04:22.920 Here's how. You have microscopic taste receptors all over your 0:04:22.960 --> 0:04:26.240 tongue and also, by the way, spread even more widely, 0:04:26.320 --> 0:04:28.680 even on your palate and the upper half of your 0:04:28.800 --> 0:04:33.960 esophagus and more. Now, these little taste receptors are embedded 0:04:34.000 --> 0:04:37.200 in the membrane of what we call taste cells, and 0:04:37.279 --> 0:04:41.599 taste cells clump together, about one hundred of them into 0:04:41.680 --> 0:04:47.000 taste buds, and taste buds cluster to form the bumps 0:04:47.080 --> 0:04:49.799 on your tongue that you can see with the naked eye. 0:04:49.960 --> 0:04:54.360 These are called pipille. Now, I know different listeners are 0:04:54.360 --> 0:04:57.880 doing different things while listening, but if you're able, either 0:04:57.960 --> 0:05:01.440 now or later, go to a mirror or bust out 0:05:01.440 --> 0:05:04.800 your front facing camera on your cell phone and take 0:05:04.839 --> 0:05:08.520 a look at your tongue. You'll notice the surface is 0:05:08.680 --> 0:05:12.680 not smooth. You have all these bumps, these papillae, and 0:05:12.720 --> 0:05:16.240 you'll notice that some of these are bigger bumps. These 0:05:16.240 --> 0:05:19.560 are called fungiform, meaning like a mushroom, and off to 0:05:19.600 --> 0:05:22.080 the sides you have some that look leaf like and 0:05:22.400 --> 0:05:25.119 others way in the back that look pimple. Like, what's 0:05:25.160 --> 0:05:27.880 cool is that you've had this tongue your whole life, 0:05:28.000 --> 0:05:31.000 and it's possible that you've never looked really carefully to 0:05:31.080 --> 0:05:34.240 see what you are made of. But this is all 0:05:34.320 --> 0:05:38.479 part of the joy of self discovery. Okay, So back 0:05:38.480 --> 0:05:41.120 to the structure here, so you can see these papilla 0:05:41.279 --> 0:05:43.920 on your tongue. But what you can't see because it's smaller, 0:05:44.480 --> 0:05:48.839 is that inside the taste buds, the taste cells arrange 0:05:48.880 --> 0:05:56.080 themselves so they're little receptors line a central pore. So architecturally, 0:05:56.200 --> 0:05:59.919 these guys are set up to catch chemicals. That's what 0:06:00.040 --> 0:06:03.680 they're built for. Even if you, the owner, are totally 0:06:03.760 --> 0:06:06.920 unaware of that. You're just walking around and you lick 0:06:06.960 --> 0:06:09.520 the ice cream and you say, oh yeah, that rocky 0:06:09.600 --> 0:06:12.920 Road tastes awesome, and you say, of course I can 0:06:12.920 --> 0:06:17.240 distinguish rocky Road from lemon from vanilla. But how does 0:06:17.279 --> 0:06:22.920 that work. The answer is, we have this miraculous microscopic 0:06:23.040 --> 0:06:25.920 engineering in ourselves. But the story gets even better because 0:06:25.960 --> 0:06:31.680 this whole system can detect and distinguish flavors with extraordinarily 0:06:31.760 --> 0:06:35.320 high specificity. So to understand how it does that, let's 0:06:35.360 --> 0:06:38.119 turn to the food that you put in your mouth. 0:06:38.360 --> 0:06:41.840 Everything that you eat or drink can be understood in 0:06:41.960 --> 0:06:44.520 terms of the molecules that they're made of, and we 0:06:44.600 --> 0:06:50.320 call these tastins Tasteans fall into five basic categories, and 0:06:50.360 --> 0:06:55.960 I know you're already well acquainted with four of them, sweet, salty, bitter, 0:06:56.200 --> 0:06:59.840 and sour. But what's the fifth taste category, which was 0:07:00.000 --> 0:07:03.640 added more recently. I'll give you a second. The fifth 0:07:03.680 --> 0:07:09.440 taste category is called umami. Umami this is a Japanese 0:07:09.440 --> 0:07:13.000 word meaning delicious taste, and we usually describe this flavor 0:07:13.080 --> 0:07:17.640 as savory. So for the sweet category, the standard thing 0:07:17.680 --> 0:07:22.640 we might think about is sucrose. For salty, a prototypical 0:07:22.720 --> 0:07:27.640 stimulus might be sodium chloride, which is table salt. For sour, 0:07:28.360 --> 0:07:32.520 the prototype is citric acid, like from a lemon. For bitter, 0:07:33.160 --> 0:07:37.480 it's quinine. And for umami, that fifth taste category, it's 0:07:37.920 --> 0:07:41.520 msgm MO, a sodium glutamate. Okay, So what happens when 0:07:41.520 --> 0:07:45.160 you put some taste int in your mouth? What happens 0:07:45.280 --> 0:07:49.920 is that the various chemicals, little molecules bind to the 0:07:50.040 --> 0:07:53.000 taste receptors. And even though I told you there are 0:07:53.040 --> 0:07:57.560 five categories, there are about fifty different receptors that we have, 0:07:58.080 --> 0:08:02.640 and there are all kinds of ways that these receptors work. Mechanically. 0:08:02.960 --> 0:08:07.520 Some cells activate when sodium physically flows through a channel 0:08:07.520 --> 0:08:11.040 in the membrane. Other cells activate because the key thing 0:08:11.240 --> 0:08:15.760 in sour compounds, which is hydrogen ions, block certain channels 0:08:15.800 --> 0:08:20.800 in the membrane. In other cases, taste ins bind to 0:08:20.880 --> 0:08:24.040 proteins in the cell membrane and change their shape, which 0:08:24.120 --> 0:08:27.480 leads to a cascade of changes inside the cell. So 0:08:27.520 --> 0:08:30.720 there's a huge variety of ways that these molecules that 0:08:30.720 --> 0:08:34.360 you've just put in your mouth physically get translated into 0:08:34.520 --> 0:08:38.720 signals that shoot off into your brain. So it seems 0:08:38.760 --> 0:08:41.800 like a strange zoo of things that can happen here. 0:08:42.160 --> 0:08:45.079 But as long as the signaling is consistent, that's all 0:08:45.120 --> 0:08:48.320 you need. So if you taste cinnamon, you get this 0:08:48.480 --> 0:08:51.240 very weird pattern of activation, and as long as you 0:08:51.280 --> 0:08:54.480 get that same pattern tomorrow, then you can identify that 0:08:54.640 --> 0:08:58.200 as cinnamon again. And this huge variety of these random 0:08:58.360 --> 0:09:01.760 tricks of mother nature gives us a lot of nuance 0:09:02.000 --> 0:09:05.600 and ability to discriminate when we sit down to enjoy 0:09:05.640 --> 0:09:09.960 a meal. So these signals shoot over to the brainstem, 0:09:10.000 --> 0:09:12.680 and then to the thalamus and finally to the core tex, 0:09:12.760 --> 0:09:16.880 a special area that we call the primary gustatory cortex. Okay, 0:09:16.960 --> 0:09:20.400 so how does taste actually get constructed? I told you 0:09:20.440 --> 0:09:24.280 that we have these taste receptors that respond preferentially to 0:09:24.640 --> 0:09:27.320 sweet or salty or something like that. But how do 0:09:27.360 --> 0:09:29.960 we get from something like that to something very specific 0:09:30.120 --> 0:09:35.120 like the taste of chocolate covered peanuts. With time and experience, 0:09:35.200 --> 0:09:40.199 we learn to recognize combinations of flavors, thousands and thousands 0:09:40.200 --> 0:09:42.440 of them. Now how do we get that kind of 0:09:42.480 --> 0:09:47.280 diversity from only fifty receptor types in five categories. Well, 0:09:47.320 --> 0:09:51.560 it's because each of these receptors will also activate in 0:09:51.640 --> 0:09:55.199 response to other types of taste ins if those are 0:09:55.360 --> 0:09:58.480 present and high enough concentrations. And it's not that each 0:09:58.640 --> 0:10:02.520 taste bud talks to one fiber going back into the brain, 0:10:02.800 --> 0:10:06.439 but instead multiple taste buds talk to a single fiber. 0:10:06.600 --> 0:10:09.160 So we can skip the details, but the bottom line 0:10:09.200 --> 0:10:12.040 is that anything you stick in your mouth triggers a 0:10:12.280 --> 0:10:17.040 pattern of taste receptor activation, which then stimulates a specific 0:10:17.320 --> 0:10:21.559 pattern of goostatory cells in the cortex. So this sense 0:10:21.600 --> 0:10:26.000 of taste uses populations of neurons to encode the sensation. 0:10:26.120 --> 0:10:30.760 This is what's called pattern encoding. It's not that chocolate 0:10:30.800 --> 0:10:36.439 covered peanuts activates this neuron. Instead, it activates thousands of neurons. 0:10:36.880 --> 0:10:40.000 This pattern of thousands of neurons maps onto lemon pie, 0:10:40.000 --> 0:10:43.000 and this other pattern over here maps on the bobagaoush, 0:10:43.080 --> 0:10:46.240 and this pattern maps on the anchovies and so on. 0:11:00.679 --> 0:11:03.960 Now what happens if there's damage to this gustatory system. 0:11:04.280 --> 0:11:08.800 It impairs your ability to taste. This is called dysgusia, 0:11:09.080 --> 0:11:12.160 or at the extreme agusia, and this can result from 0:11:12.200 --> 0:11:15.480 all kinds of different problems if it's at the level 0:11:15.600 --> 0:11:19.400 of the taste cells. Happily, these turnover quickly every couple 0:11:19.400 --> 0:11:23.080 of weeks, so damage to the taste cells themselves is 0:11:23.120 --> 0:11:27.040 often reversible. For example, there's a chef named Grant Ashats, 0:11:27.360 --> 0:11:30.600 and he made headlines after being diagnosed with tongue cancer 0:11:31.000 --> 0:11:33.200 and losing his sense of taste as a result of 0:11:33.320 --> 0:11:36.800 radiation treatment. And so he described how with time he 0:11:36.960 --> 0:11:41.199 regained taste sensation one category at a time. Now, people 0:11:41.240 --> 0:11:44.320 sometimes get dysgusia from COVID, and I'll come back to 0:11:44.360 --> 0:11:47.160 that a little later. But you can also get dysgusia 0:11:47.200 --> 0:11:50.960 from damage directly to the cortex, just like we see 0:11:51.000 --> 0:11:54.560 in all the other senses. If you damage the primary 0:11:54.640 --> 0:12:00.120 gustatory cortex, you don't understand basic taste anymore. If the 0:12:00.240 --> 0:12:04.880 damage happens in a higher level area, we move from 0:12:04.920 --> 0:12:09.960 the basic details to more abstract representation. So with damage 0:12:09.960 --> 0:12:15.120 to the secondary goustatory cortex, you can still identify basic tests, 0:12:15.160 --> 0:12:19.160 but you can't do more subtle recognition of food type 0:12:19.520 --> 0:12:23.080 and flavor intensity. Now what's fascinating is that if you 0:12:23.200 --> 0:12:28.080 lose other senses, that can also affect your sense of taste. 0:12:28.200 --> 0:12:32.880 For example, consider how your experience of each bite includes 0:12:33.320 --> 0:12:38.680 touch information in your mouth about texture and temperature, what 0:12:38.800 --> 0:12:43.560 is sometimes called the mouth feel of food. But by 0:12:43.760 --> 0:12:47.679 far the most influential sense on our perception of taste 0:12:48.400 --> 0:12:52.440 is smell. The nuance that we have and our perception 0:12:52.520 --> 0:12:56.840 of taste is cranked up way more from the interplay 0:12:56.960 --> 0:13:00.440 of taste and smell. Just think about what things like 0:13:00.480 --> 0:13:03.120 when you have a head cold. So now we turn 0:13:03.200 --> 0:13:06.320 to act too all about smell and then will come 0:13:06.480 --> 0:13:09.960 to the more general concept of flavor. So our other 0:13:10.160 --> 0:13:14.719 chemical sense smell, known as old faction. Lets us perceive 0:13:15.360 --> 0:13:19.680 airborne chemicals. Now, we humans don't rely on our sense 0:13:19.720 --> 0:13:22.360 of smell as much as we do on other sensory 0:13:22.559 --> 0:13:26.120 windows like vision and hearing. But other cousins of ours 0:13:26.160 --> 0:13:30.080 in the animal kingdom, like dogs and rodents, they capitalize 0:13:30.120 --> 0:13:33.840 on old faction to read the environment around them like 0:13:33.880 --> 0:13:37.920 a book. A dog can tell that a cat wandered 0:13:37.960 --> 0:13:42.120 onto the lawn hours ago, and rats can locate little, 0:13:42.160 --> 0:13:46.760 tiny morsels of food buried underneath layers of cage bedding. 0:13:47.280 --> 0:13:50.040 So here's the interesting bit. Although humans and dogs and 0:13:50.160 --> 0:13:53.680 rats we all rely on smell to different degrees, it 0:13:53.760 --> 0:13:57.599 all works in our brains the same way the floating 0:13:57.720 --> 0:14:00.920 chemical signals. These are called odorans, and you walk around 0:14:00.960 --> 0:14:03.959 all day vacuuming these in mostly through your nose a 0:14:03.960 --> 0:14:07.480 little through your mouth. Once these molecules have entered the 0:14:08.080 --> 0:14:12.240 giant vessel of your nasal cavity, they get sucked over 0:14:12.320 --> 0:14:14.640 to the main sensory organ, which is called the old 0:14:14.679 --> 0:14:18.280 factory epithelium, and that's at the back of that big 0:14:18.320 --> 0:14:23.120 space inside your nose. In humans, this structure is less 0:14:23.160 --> 0:14:25.560 than ten square centimeters, which is about the size of 0:14:25.600 --> 0:14:30.080 a half dollar coin. Compare that to dogs whose epithelium 0:14:30.480 --> 0:14:35.240 is seventeen times larger. Now, the old factory epithelium is 0:14:35.360 --> 0:14:38.120 covered with a layer of mucus, and that's how the 0:14:38.160 --> 0:14:43.160 molecules stick and come into contact with the little feelers 0:14:43.240 --> 0:14:47.960 of the receptor cells, called the dendrites. Okay, but how 0:14:47.960 --> 0:14:52.840 do we distinguish different smells like chlorine from lavender from 0:14:53.320 --> 0:14:56.040 wood burning at a campfire, given that these are all 0:14:56.160 --> 0:15:00.440 just molecules of different shapes. Well. A lot of the 0:15:00.480 --> 0:15:04.320 groundbreaking work in this field was done by Richard Axel 0:15:04.400 --> 0:15:08.160 and Linda Buck who discovered a huge number of old 0:15:08.280 --> 0:15:12.400 factory receptor genes in rats, about one thousand of them, 0:15:12.840 --> 0:15:14.400 and by the way, they won the two thousand and 0:15:14.440 --> 0:15:18.560 four Nobel Prize for that. Turns out, these old factory 0:15:18.600 --> 0:15:22.480 receptor genes are the largest gene family in the rodent genome. 0:15:22.760 --> 0:15:26.720 We humans also have that same gene family, but only 0:15:26.760 --> 0:15:29.840 about four hundred of these genes are still functional in US. 0:15:30.160 --> 0:15:34.600 So these receptors started becoming discovered in nineteen ninety one, 0:15:34.680 --> 0:15:37.720 not that long ago, and at first the guess was 0:15:37.720 --> 0:15:41.720 that maybe each receptor encodes an odor, But the way 0:15:41.720 --> 0:15:45.040 it turns out is that each odorant molecule has its 0:15:45.040 --> 0:15:49.160 particular shape, and it binds to several different receptor types, 0:15:49.560 --> 0:15:53.200 and a single receptor type responds to lots of different 0:15:53.240 --> 0:15:57.680 odorants that all happened to share some particular shape feature. So, 0:15:58.400 --> 0:16:02.560 just like the sense of taste, you have pattern encoding. 0:16:03.440 --> 0:16:08.200 This whole random looking population of neurons represents the scent 0:16:08.360 --> 0:16:12.400 of freshly cut grass, and this other group over here 0:16:12.800 --> 0:16:17.680 represents mint, and this other population represents hot chocolate and 0:16:17.680 --> 0:16:21.280 so on. Now, before I go on what's going on 0:16:21.400 --> 0:16:24.720 with COVID and smell. A lot of people end up 0:16:24.720 --> 0:16:27.400 with what's called a noosmia, which is a lack of 0:16:27.560 --> 0:16:32.080 smell and inability to smell. There's a growing literature on this, 0:16:32.200 --> 0:16:35.680 and the answer to why it happens isn't fully agreed on. 0:16:35.800 --> 0:16:39.200 But you can see how with the huge variety of 0:16:39.280 --> 0:16:44.360 molecular mechanisms underlying smell, it's not hard for a virus 0:16:44.400 --> 0:16:47.680 to throw a wrench in the machinery, in other words, 0:16:47.720 --> 0:16:51.800 to temporarily tweak things so that the signals the brain 0:16:51.840 --> 0:16:55.120 are used to have now been changed. Okay, so back 0:16:55.160 --> 0:16:58.800 to how smell works. From these receptors and the epithelium, 0:16:59.120 --> 0:17:02.560 the signals shoot to the olfactory bulb and from there 0:17:02.600 --> 0:17:06.960 to the primary olfactory cortex. From this part of the cortex, 0:17:07.520 --> 0:17:11.040 information zooms out to a network of other areas that 0:17:11.080 --> 0:17:16.640 are involved in higher order abstractions like familiarity and edibleness. 0:17:16.720 --> 0:17:21.600 And again we see that the primary sensory cortex represents 0:17:21.640 --> 0:17:26.320 the basic data and then higher level cortical processing becomes 0:17:26.400 --> 0:17:30.440 more abstract from there. So we have this sophisticated machinery 0:17:30.440 --> 0:17:33.719 to pick up on floating chemical signals in the world. 0:17:34.440 --> 0:17:37.440 So what do animals do with this? Well, obviously they 0:17:37.720 --> 0:17:42.320 identify things in the world like foods or toxins, but 0:17:42.400 --> 0:17:46.159 it goes beyond that. A lot of animals smell to 0:17:46.280 --> 0:17:51.400 understand not only what but where. They navigate space by 0:17:51.440 --> 0:17:54.640 smelling their way around. So think of a puppy finding 0:17:54.680 --> 0:18:00.639 its mother's nipple via smell, or lobsters locating their prey, 0:18:01.600 --> 0:18:04.800 or moths finding their lovers. All of these are done 0:18:04.840 --> 0:18:07.199 with smell, and this is also a large part of 0:18:07.240 --> 0:18:11.919 how pigeons find their way home or salmon return to 0:18:11.960 --> 0:18:17.280 their stream of origin. They create an olfactory map that 0:18:17.359 --> 0:18:21.600 links specific smells with locations during their travels. But it's 0:18:21.640 --> 0:18:25.719 not only about long distance stuff. Animals can navigate close 0:18:25.920 --> 0:18:29.040 space by smell. They can actually figure out which way 0:18:29.080 --> 0:18:32.879 to turn. Now, how could that work? Well, you probably 0:18:32.960 --> 0:18:37.560 know the brain can localize a sound by comparing the 0:18:37.640 --> 0:18:40.440 signal hitting the two ears, and the side where the 0:18:40.480 --> 0:18:43.359 signal arrives first tells you the side where the sound 0:18:43.520 --> 0:18:48.680 came from. Amazingly, the same strategy is used to find 0:18:48.680 --> 0:18:53.360 the source of a smell. You exploit the inter nostril 0:18:53.840 --> 0:18:58.280 time differences which nostril the odor got to first, And 0:18:58.320 --> 0:19:01.359 it's been shown this is how sharks aside which way 0:19:01.400 --> 0:19:04.679 to turn when they're following a plume of smell, and 0:19:04.720 --> 0:19:08.320 it's not as one might have intuited by the concentration difference. 0:19:08.800 --> 0:19:12.080 So this is a general strategy across senses. If you're 0:19:12.119 --> 0:19:17.200 trying to locate something, you can exploit the timing across 0:19:17.240 --> 0:19:20.720 two channels that are in slightly different places, like your 0:19:20.760 --> 0:19:25.679 two ears or your two nostrils. Okay, so where are 0:19:25.720 --> 0:19:29.320 we now. We've looked at how taste works by chemicals 0:19:29.359 --> 0:19:32.840 binding to receptors in and around the tongue, and we've 0:19:32.840 --> 0:19:37.720 seen how smell is about floating molecules binding to receptors 0:19:37.760 --> 0:19:40.280 at the back of the nasal cavity, and in both 0:19:40.320 --> 0:19:44.320 cases the brain is using pattern encoding. Think of this 0:19:44.440 --> 0:19:47.520 like the way that you strike multiple piano keys to 0:19:47.600 --> 0:19:52.600 make a cord. If you play these neurons, that's eucalyptus, 0:19:52.720 --> 0:19:57.679 This cord of neurons is peppermint, That chord is burnt toast, 0:19:57.880 --> 0:20:00.840 and so on. But the really interesting thing about taste 0:20:00.880 --> 0:20:03.880 and smell is how interactive they are, and a lot 0:20:03.920 --> 0:20:07.520 of times they really can't be separated. It's not always 0:20:07.600 --> 0:20:11.320 useful to think about taste and smell as independent senses, 0:20:11.640 --> 0:20:16.439 so some people talk about this as a composite flavor sense. 0:20:17.080 --> 0:20:21.880 As an example, certain odors like vanilla are consistently said 0:20:21.920 --> 0:20:26.000 to smell sweet, even though sweetness belongs to the domain 0:20:26.040 --> 0:20:29.199 of taste, and according to one study, sweet is the 0:20:29.240 --> 0:20:32.960 most common description of odor. Or we might say that 0:20:33.080 --> 0:20:37.240 something smells spicy or something smells sour, even though these 0:20:37.240 --> 0:20:41.879 are taste words, and food companies understand the interaction of 0:20:41.880 --> 0:20:44.800 smell and taste, so what they'll do is enhance the 0:20:44.920 --> 0:20:49.159 sweetness of a product by adding a sweet smelling odor. 0:20:49.520 --> 0:20:52.440 The same trick of adding a sweet odor can also 0:20:52.960 --> 0:20:57.200 reduce the perceived sourness of something that's acidic, so smell 0:20:57.240 --> 0:21:01.520 and taste are entangled. I mentioned earlier that foods lose 0:21:01.560 --> 0:21:04.760 their flavor when you have a cold because a plugged 0:21:04.840 --> 0:21:09.000 nose affects your sense of smell, and without smell, there's 0:21:09.080 --> 0:21:12.600 little flavor. So as a result of damaging my sense 0:21:12.640 --> 0:21:14.640 of smell when I was a kid, I've always had 0:21:15.080 --> 0:21:17.879 very little discrimination when it comes to food, which is 0:21:17.920 --> 0:21:21.200 not bad. I don't mind eating food that's boring or 0:21:21.240 --> 0:21:24.359 a little off. For food that's very spicy that my 0:21:24.480 --> 0:21:27.359 super taste or friends just can't handle, I'm fine with 0:21:27.400 --> 0:21:30.879 it all. None of the taste particularly stands out for 0:21:30.960 --> 0:21:34.320 me because I just don't experience that much smell. Okay, 0:21:34.359 --> 0:21:37.080 so we've been talking about the interaction between the senses, 0:21:37.080 --> 0:21:39.919 but I just want to return to smell in particular, 0:21:40.240 --> 0:21:43.840 because a discussion about noses would not be complete without 0:21:43.920 --> 0:21:48.360 talking about pheromones. What is a pheromone. It's a chemical 0:21:48.400 --> 0:21:53.480 that's broadcast by an animal to transmit information like identity 0:21:53.520 --> 0:21:57.320 and gender, and it can trigger behaviors in other members 0:21:57.320 --> 0:22:01.679 of the same species. So, for example, pheromones are given 0:22:01.760 --> 0:22:05.600 off by queen bees to halt the sexual development of 0:22:05.640 --> 0:22:08.800 the other females and trigger them to become workers. And 0:22:08.840 --> 0:22:12.320 what we generally see is that drifting molecules can carry 0:22:12.720 --> 0:22:17.040 a high density of information. In other cases, for example, 0:22:17.200 --> 0:22:22.879 pheromones carry information about a prospective mate, like their virility, 0:22:23.040 --> 0:22:26.760 or their genetics, or their age or their fitness. The 0:22:26.840 --> 0:22:30.399 effect of pheromones on sexual behavior has been studied a 0:22:30.400 --> 0:22:34.760 lot in the laboratory. So, for example, female mice are 0:22:34.800 --> 0:22:37.480 presented with a choice of males. It turns out that 0:22:37.520 --> 0:22:40.600 a female's choice of mate is not random, or it's 0:22:40.640 --> 0:22:44.800 not based on visible attributes. Instead, the choice results from 0:22:44.840 --> 0:22:48.920 the relationship between her genetics and that of her suitor. 0:22:49.520 --> 0:22:53.920 The trick is how she accesses that data. So mammals 0:22:54.000 --> 0:22:58.000 have a set of immune system genes that we summarize 0:22:58.000 --> 0:23:03.199 as the major histocompatibility complex or MHC, and following the 0:23:03.240 --> 0:23:08.119 strategy of keeping the gene pool well stirred, the female 0:23:08.160 --> 0:23:12.239 mouse will choose the mates whose MHC genes are the 0:23:12.280 --> 0:23:16.320 most different from hers. But how do the female mice, 0:23:16.359 --> 0:23:19.360 who are almost blind, figure out who is like them 0:23:19.400 --> 0:23:23.080 and who is unlike them? And the answer is inside 0:23:23.080 --> 0:23:27.520 their noses. There's a little organ called the vomeronasal organ 0:23:27.640 --> 0:23:32.359 and this detects the pheromones, which serve as little genetic 0:23:32.720 --> 0:23:38.880 calling cards. So these chemicals are carrying deep and important information. Now, 0:23:38.960 --> 0:23:43.600 the discovery of pheromones across mammals opens up the possibility 0:23:43.800 --> 0:23:49.920 that humans communicate unconsciously using olfaction in pheromones. And as 0:23:49.960 --> 0:23:54.080 it turns out, some receptors in your nose are identical 0:23:54.160 --> 0:23:58.560 to the receptors that mice use for pheromonal signaling. Now, 0:23:58.760 --> 0:24:01.760 it's not yet clear whether there our pheromonal systems are 0:24:01.800 --> 0:24:07.080 actually operational, but several groups have presented behavioral evidence that 0:24:07.119 --> 0:24:11.960 supports the possibility. So in one study, males wore t 0:24:12.160 --> 0:24:16.120 shirts for several days, allowing their sweat to soak into 0:24:16.160 --> 0:24:20.720 the cotton. Then females smelled the armpits of the shirts 0:24:21.240 --> 0:24:26.920 and selected the body odor that they preferred the most. Now, strikingly, 0:24:27.080 --> 0:24:29.520 and just as you might expect from the mice studies, 0:24:30.160 --> 0:24:35.480 the females favored the males with MHCs that were different 0:24:35.600 --> 0:24:39.840 from their own. So, although we're not consciously aware of 0:24:39.880 --> 0:25:02.080 our pheromonal signals, they might influence our attraction judgments. Beyond 0:25:02.240 --> 0:25:06.359 mate selection, pheromones also seem to offer some other kinds 0:25:06.400 --> 0:25:11.320 of data in humans. One study demonstrated that newborns prefer 0:25:11.600 --> 0:25:15.000 pads that have been brushed against their mother's breast over 0:25:15.200 --> 0:25:21.040 clean pads, presumably because of pheromones, and generally a mother 0:25:21.200 --> 0:25:24.439 and an infant can recognize one another based on smell. 0:25:24.920 --> 0:25:29.200 Humans can also recognize their parents and siblings based on scent, 0:25:29.680 --> 0:25:34.560 which is proposed to aid in incest avoidance. And beyond family, 0:25:34.640 --> 0:25:38.280 it's been shown that when a female sniffs the armpit 0:25:38.400 --> 0:25:42.000 sweat of another woman, the length of her menstrual cycle 0:25:42.160 --> 0:25:45.480 can change. By the way, side note, it's commonly believed 0:25:45.560 --> 0:25:49.879 that women who live together synchronize their menstrual cycles, but 0:25:49.920 --> 0:25:53.200 I just want to clarify that that claim is actually unsupported. 0:25:53.480 --> 0:25:55.679 There have been large scale studies on this and they 0:25:55.720 --> 0:26:00.480 demonstrate that synchronization doesn't happen, but you can get statistical 0:26:00.560 --> 0:26:04.679 fluctuations that give the illusion of synchrony. So although people 0:26:04.760 --> 0:26:08.000 thought this was true in the nineteen seventies, subsequent research 0:26:08.080 --> 0:26:11.840 has failed to replicate that finding. So pheromones convey some 0:26:12.160 --> 0:26:15.520 information in humans, but I'd say the amount they influence 0:26:15.600 --> 0:26:19.879 our behavior is still not totally clear. Human cognition is 0:26:20.000 --> 0:26:25.239 profoundly more complex than mouse cognition, and it's possible that 0:26:25.320 --> 0:26:30.280 pheromones have diminished to a pretty minor role, like legacy 0:26:30.400 --> 0:26:33.800 software that's left on a computer system that has been 0:26:33.880 --> 0:26:37.560 continuously updated. So I want to return now to the 0:26:37.640 --> 0:26:40.920 Latin phrase that I mentioned at the beginning, the gustabus 0:26:40.960 --> 0:26:45.840 known s disputantum or there's no accounting for taste? Why 0:26:45.840 --> 0:26:49.560 do some of us like some tastes and others don't? 0:26:49.640 --> 0:26:52.600 For example, I like Brussels sprouts and my wife doesn't. 0:26:52.800 --> 0:26:55.840 Why do people like different things? Whill As it turns out, 0:26:56.280 --> 0:27:00.560 there are lots of reasons for starters. There are genetic actors. 0:27:00.640 --> 0:27:05.399 Take the issue of taste sensitivity. You have genetic differences 0:27:05.440 --> 0:27:09.880 that determine how sensitive you are to certain tastes. As 0:27:09.880 --> 0:27:13.920 an example, there's a gene called TAS two R thirty eight, 0:27:14.119 --> 0:27:17.920 and whatever sequence you have in that gene that determines 0:27:18.040 --> 0:27:22.840 your sensitivity to bitter things like broccoli or coffee. If 0:27:22.880 --> 0:27:26.719 you have a heightened sensitivity to bitterness, you're probably not 0:27:26.760 --> 0:27:29.760 gonna like those flavors. So that's one example of many, 0:27:29.800 --> 0:27:33.679 But this goes further. Some people are known as super tasters, 0:27:33.960 --> 0:27:37.439 and they just have more taste buds, and as a result, 0:27:37.440 --> 0:27:41.800