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.

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Olfaction is both ancient and emotionally charged.

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 in Frontiers 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).

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Odor perception and recognition is a complex process.

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 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.

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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 retched 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.