Olfaction
is a fascinating sense. Although our nasal passages constantly detect aromatic
molecules, most of us give little thought to how smell works. Odors also hold a
surprising ability to evoke powerful emotions; here we discuss why that it is.
We are all well aware that we can
detect odors via the holes in our noses. We also know that aromas are capable
of evoking powerful memories and, conversely, deep revulsion.
We
take these feelings for granted, but how does smell work? And why does
olfaction bring intense memories and emotions to the forefront of our
consciousness?
The
detection of an odor - pineapple, for instance - brings to mind a pineapple, of
course.
In
some of us, that same smell might also remind us of a pineapple-flavored
medicine that we detested as a child, sparking feelings of dread and nausea.
An
aroma has the ability to cause an upswell in old emotions - all from a
minuscule concentration of airborne molecules.
Olfaction
plugs deep into our brains. A single odor can be attached to physical
realities, memories, and dreams, all in one fell swoop. In this article, we
will have a brief sniff around olfaction and try to understand why it is so
intimately tied to emotion and memory.
The evolution
of odor detection
In
humans, smell (detecting airborne chemicals) and taste (detecting water-soluble
chemicals) are separate, although they overlap significantly. For water-based
creatures, because they only have access to water-soluble molecules, the two
are combined; this is referred to as chemosensing.
Chemosensing
is the oldest sensory system and marked the first way in which two organisms
could contact each other. Even the simplest organisms - slime molds and
bacteria, for instance - are sensitive to external chemicals.
Olfaction
predates the rise of the mammals by millions of years and is widely considered
to be the oldest vertebrate sense. For humans and other primates, vision has
taken over as the most vital sense; but, for many species, olfaction is still
their most valued tool for detecting potential partners, predators, and lunch.
Although the specific mechanisms controlling olfaction
across species show a great deal of variety, its basic structure has been
maintained across 500 million years - an impressive feat that demonstrates a
winning design. Similarities between species include the structure of odor receptor
proteins, the organization of the olfactory central nervous system, and odor-guided memory and
behavior.
Are human noses
any good?
Anyone who has paid attention to a dog will know that our
sense of smell is less developed than many other mammals. However, even with
our reduced overall sensitivity, we can actually detect millions of airborne odorants in very small amounts.
Many animals have more sensitive nostrils than us, but, if
necessary, humans can track down the source of an odor fairly accurately. For
instance, a study in 2007 asked humans to follow a chocolate odor for 10
meters. Around two thirds of participants successfully pinned
down the source.
The researchers also found that the distance between our
nostrils gives us a slight stereo-odor advantage when tracking smells. The
minute variation between the two signals helps us hone in on the target. As an
aside, some scientists believe that hammerhead sharks may use this stereo-olfaction method
to the ultimate degree, through the placement of their nostrils at either end
of their majestic hammer-like heads.
The
authors of the chocolate odor study also found that repetition improved the hit
rate; they concluded:
"These findings reveal fundamental mechanisms of
scent-tracking and suggest that the poor reputation of human olfaction may
reflect, in part, behavioral demands rather than ultimate abilities."
So,
we may be better at tracking smells than we think.
In fact, our ability to distinguish between smells appears
to be better than our conscious brain realizes. A study published inFrontiers in Behavioral
Neuroscience demonstrated
this point in 2014.
Pool
et al. used Pavlovian conditioning to plumb the depths of human olfaction
detection. They used two odors that are chemically different but smell so
similar that they are impossible to tell apart. They paired one of the smells
with a pleasant taste.
Although
the participants reported that they were not able to distinguish between the
odors at all, their physiological responses told a different story. The smell
that had previously been paired with a pleasant taste resulted in faster
reactions, more inhalation of air, and higher skin conductance (a measure of
emotional response).
In
other words, the participant's olfactory receptors could detect the most subtle
differences between odor chemistry, but their conscious brain was left in the
dark.
How do you
smell?
The first phase of olfaction is detection. The nasal
cavity is covered by an olfactory epithelium, which, in a human, contains an
estimated 20 million olfactory sensory neuron cells (for comparison's sake, a
bloodhound has around 220 million).
These cells produce receptor proteins
that lie in wait for an aroma to pass by; humans have around 450 different
types of olfactory receptor.
Olfactory receptors utilize the largest gene family in
vertebrates - 900 genes in
humans, making up 3-5 percent of the total gene content.
Once
an odor arrives in the nasal cavity, it binds to a receptor of the appropriate
shape. This binding triggers a nerve impulse that is transmitted to the
olfactory bulbs, a region where olfactory neurons converge. The olfactory bulbs
are two pea-sized blobs that sit just below the frontal lobe.
At
this stage, a major difference between olfaction and the other senses becomes
clearer. For the other sensory modalities - sight, hearing, touch - once a
stimulus has been detected by their organ of choice - eyes, ears, skin - it
travels to the thalamus, where the signals are processed. Next, the information
is shipped out to other parts of the brain for further scrutiny.
Olfaction
is an odd case. The olfactory bulb is directly plumbed into the brain via the
first cranial nerve. It does not need to travel a circuitous route via the
thalamus before it is processed.
Odor perception and recognition is a complex process.
Odor and emotion entwined
In a
very real way, olfaction can be considered an environmental probe. The nervous
system comes into direct contact with the air. It is the only biological
structure where the brain sends its neurons directly into the outside world.
Another
feature that helps olfaction stand out from the sensory crowd is the region in
which it is processed. The aromatic signals plucked from the environment are
sent straight to the brain's limbic system, a group of brain structures heavily
involved in emotion, motivation, learning, and memory.
The
limbic system includes the amygdala, hippocampus, and orbitofrontal cortex, and
the majority of our emotional lives are played out within its walls. It
operates by influencing endocrine systems (hormone secretion) and the autonomic
nervous system- a system in charge of automatic physiological activities, such
as heart rate, digestion, urination, and sexual arousal.
Olfaction's
direct connection with these centers in the brain are the reason why odor,
memory, and emotion seem so thoroughly entwined.
The
tight relationship with the endocrine system also explains how a smell can
generate a whole-body response - whether pleasure or disgust.
Because the limbic system is also
important in the formation of memories, it is no wonder that smells seem to
stick fast in our brains better than other senses.
For
instance, if someone gives you a phone number to remember, you may well manage
to retain it, but it will be long gone a decade later.
However,
if you wretched on pineapple-flavored medicine 20 years ago, the smell will
still induce the same revulsion and bring a snapshot of childhood emotions
rushing to the fore.
As
humans navigating a modern world where little emphasis is put on
"survival," it is easy to wonder why olfaction should generate such
strong emotions in us. After all, it only signifies a cocktail of chemicals
drifting through the air. However, as we evolved, and our ancestors evolved
before that, smells were vital for survival. Emotions, too, used to serve a
deeper purpose than remembering that time you took pineapple-flavored medicine
and nearly vomited.
As a
creature in the wild, pairing experiences with odors was vital. The
unmistakable whiff of a bear, the distant aroma of a fruit tree, the sweat of a
friend, the lip-curling honk from a rancid foodstuff. Emotions are designed to
evoke a response.
The
smell of a bear brings instant fear and a readiness to fight or fly - not only
do you recognize the odor, but the tight relationship between the limbic and
endocrine system prepares you physiologically for danger. Similarly, spoiled
food has the potential to make an animal sick, a swift gut reaction to that
aroma can be the difference between life and death.
For
these reasons, olfaction needed to be an instant and deep trigger. Today, that
need is less pronounced for humans, but it is still present. So, although our
firm, emotionally ingrained response to smell might seem like overkill in the
modern world, it wasn't always so.
Odors have a direct link to parts of the brain involved in memory and emotion.
Because the limbic system is also
important in the formation of memories, it is no wonder that smells seem to
stick fast in our brains better than other senses.
For
instance, if someone gives you a phone number to remember, you may well manage
to retain it, but it will be long gone a decade later.
However,
if you wretched on pineapple-flavored medicine 20 years ago, the smell will
still induce the same revulsion and bring a snapshot of childhood emotions
rushing to the fore.
As
humans navigating a modern world where little emphasis is put on
"survival," it is easy to wonder why olfaction should generate such
strong emotions in us. After all, it only signifies a cocktail of chemicals
drifting through the air. However, as we evolved, and our ancestors evolved
before that, smells were vital for survival. Emotions, too, used to serve a
deeper purpose than remembering that time you took pineapple-flavored medicine
and nearly vomited.
As a
creature in the wild, pairing experiences with odors was vital. The
unmistakable whiff of a bear, the distant aroma of a fruit tree, the sweat of a
friend, the lip-curling honk from a rancid foodstuff. Emotions are designed to
evoke a response.
The
smell of a bear brings instant fear and a readiness to fight or fly - not only
do you recognize the odor, but the tight relationship between the limbic and
endocrine system prepares you physiologically for danger. Similarly, spoiled
food has the potential to make an animal sick, a swift gut reaction to that
aroma can be the difference between life and death.
For
these reasons, olfaction needed to be an instant and deep trigger. Today, that
need is less pronounced for humans, but it is still present. So, although our
firm, emotionally ingrained response to smell might seem like overkill in the
modern world, it wasn't always so.
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