January 27, 2025 • 35 mins
Why do you like the taste of things that your friend doesn't? Why do kids not like coffee but adults do? What does any of this have to do with smelling people’s armpits, whether women really synchronize their menstruation, whether your culture eats a lot of spicy foods, and how animals sense the world? Join Eagleman this week to understand why there's no accounting for taste.
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Speaker 1 (00:05):
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.
(00:32):
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
(00:56):
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
(01:17):
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,
(01:38):
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
(02:01):
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
(02:22):
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
(02:46):
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
(03:10):
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
(03:32):
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
(03:54):
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?
(04:17):
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
(04:41):
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
(05:04):
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
(05:25):
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
(05:48):
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
(06:09):
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
(06:31):
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,
(06:56):
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
(07:17):
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
(07:37):
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,
(07:58):
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
(08:20):
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.
(08:42):
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
(09:05):
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
(09:27):
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
(09:51):
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
(10:12):
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.
(10:36):
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.
(11:00):
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
(11:23):
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
(11:44):
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
(12:04):
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.
(12:28):
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
(12:48):
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
(13:10):
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
(13:33):
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
(13:57):
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
(14:18):
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
(14:43):
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
(15:04):
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
(15:26):
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,
(15:49):
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
(16:12):
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.
(16:35):
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,
(16:59):
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
(17:21):
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
(17:42):
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
(18:04):
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
(18:25):
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
(18:48):
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.
(19:08):
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
(19:29):
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,
(19:52):
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.
(20:17):
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
(20:37):
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
(21:01):
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
(21:24):
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
(21:48):
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
(22:08):
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
(22:30):
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
(22:54):
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,
(23:16):
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,
(23:38):
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
(24:01):
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,
(24:27):
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
(25:02):
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.
(25:24):
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
(25:45):
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
(26:08):
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
(26:30):
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.
(26:52):
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,
(27:14):
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,
(27:37):
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.
(27:57):
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
(28:21):
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
(28:42):
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
(29:03):
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
(29:27):
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.
(29:50):
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
(30:12):
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,
(30:38):
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
(31:00):
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
(31:23):
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,
(31:45):
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
(32:07):
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,
(32:27):
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
(32:48):
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,
(33:08):
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
(33:33):
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
(33:54):
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
(34:15):
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
(34:38):
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
(34:59):
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
(35:20):
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.
(00:32):
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
(00:56):
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
(01:17):
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,
(01:38):
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
(02:01):
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
(02:22):
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
(02:46):
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
(03:10):
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
(03:32):
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
(03:54):
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?
(04:17):
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
(04:41):
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
(05:04):
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
(05:25):
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
(05:48):
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
(06:09):
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
(06:31):
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,
(06:56):
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
(07:17):
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
(07:37):
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,
(07:58):
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
(08:20):
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.
(08:42):
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
(09:05):
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
(09:27):
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
(09:51):
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
(10:12):
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.
(10:36):
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.
(11:00):
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
(11:23):
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
(11:44):
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
(12:04):
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.
(12:28):
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
(12:48):
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
(13:10):
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
(13:33):
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
(13:57):
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
(14:18):
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
(14:43):
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
(15:04):
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
(15:26):
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,
(15:49):
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
(16:12):
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.
(16:35):
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,
(16:59):
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
(17:21):
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
(17:42):
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
(18:04):
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
(18:25):
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
(18:48):
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.
(19:08):
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
(19:29):
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,
(19:52):
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.
(20:17):
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
(20:37):
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
(21:01):
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
(21:24):
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
(21:48):
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
(22:08):
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
(22:30):
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
(22:54):
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,
(23:16):
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,
(23:38):
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
(24:01):
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,
(24:27):
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
(25:02):
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.
(25:24):
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
(25:45):
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
(26:08):
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
(26:30):
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.
(26:52):
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,
(27:14):
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,
(27:37):
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.
(27:57):
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
(28:21):
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
(28:42):
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
(29:03):
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
(29:27):
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.
(29:50):
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
(30:12):
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,
(30:38):
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
(31:00):
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
(31:23):
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,
(31:45):
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
(32:07):
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,
(32:27):
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
(32:48):
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,
(33:08):
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
(33:33):
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
(33:54):
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
(34:15):
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
(34:38):
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
(34:59):
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
(35:20):
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,
Host
David Eagleman
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