When the olfactory neurons detect a specific substance (a molecule of cheese, a ketone from something sweet, something emanating from the mouth of someone with questionable dental hygiene) they send electrical signals to the olfactory bulb, which relays this information to areas such as the olfactory nucleus and piriform cortex, meaning you experience a smell.
Smell is very often associated with memory. The olfactory system is located right next to the hippocampus and other primary components of the memory system, so close in fact that early anatomical studies thought that’s what the memory system was for. But they’re not just two separate areas that happen to be side by side, like an enthusiastic vegan living next to a butcher. The olfactory bulb is part of the limbic system, just like the memory-processing regions, and has active links to the hippocampus and the amygdala. As a result, certain smells are particularly strongly associated with vivid and emotional memories, like how a smell of roast dinner can suddenly remind you of Sundays at your grandparents’ house.
You’ve probably experienced this yourself on many occasions, how a certain smell or odor can trigger powerful memories of childhood and/or bring about emotional moods associated with smells. If you spent a lot of happy time as a child at your grandfather’s house and he smoked a pipe, you will likely have a sort of melancholy fondness for the smell of pipe smoke. Smell being part of the limbic system means it has a more direct route to triggering emotions than other senses, which would explain why smell can often elicit a more powerful response than most other senses. Seeing a fresh loaf of bread is a fairly innocuous experience, smelling one can be very pleasurable and oddly reassuring, as it’s stimulating and coupled with the enjoyable memories of things associated with the smell of baking, which invariably ends up with something pleasant to eat. Smell can have the opposite effect too, of course; seeing rotten meat isn’t very nice, but smelling it is what’ll make you throw up.
The potency of smell and its tendency to trigger memories and emotions hasn’t gone unnoticed. Many try to exploit this for profit: real estate agents, supermarkets, candle-makers and more all try to use smell to control people’s moods and make them more prone to handing over money. The effectiveness of this approach is known but probably limited by the way in which people vary considerably—someone who’s had food poisoning from vanilla ice-cream won’t find that odor reassuring or relaxing.
Another interesting misconception about smell: for a long time, it was widely believed that smell can’t be “fooled.” However, several studies have shown this to be not true. People experience illusions of smell all the time, such as thinking a sample smell is pleasant or unpleasant depending on how it’s labeled (for instance, “Christmas tree” or “toilet cleaner”—and for the record this isn’t a joke example; it’s a real one from a 2001 experiment by researchers Herz and von Clef).
The reason it was believed there were no olfactory illusions seems to be because the brain only gets “limited” information from smell. Tests have shown that, with practice, people can “track” things via their scent, but it’s generally restricted to basic detection. You smell something, you know something is nearby that’s giving off that smell, and that’s about it; it’s either “there” or “not there.” So if the brain scrambles the smell signals, so that you end up smelling something that’s different from what’s actually producing the odor, how would you even know? Smell may be powerful, but it has a limited range of applications for the busy human.
Olfactory hallucinations,? smelling things that aren’t there, also exist, and can be worryingly common. People often report the phantom smell of burning—toast, rubber, hair or just a general “scorched” smell. It’s common enough for there to be numerous websites dedicated to it. It’s often linked to neurological phenomena, such as epilepsy, tumors or strokes, things that could end up causing unexpected activity in the olfactory bulb or elsewhere in the smell-processing system, and be interpreted as a burning sensation. That’s another useful distinction: illusions occur when the sensory system gets something wrong, has been fooled. Hallucinations are more typically an actual malfunction, where something’s actually awry in the brain’s workings.
Smell doesn’t always operate alone. It’s often classed as a “chemical” sense, because it detects and is triggered by specific chemicals. The chemical sense is taste. Taste and smell are often used in conjunction; most of what we eat has a distinct smell. There’s also a similar mechanism as receptors in the tongue and other areas of the mouth respond to specific chemicals, usually molecules soluble in water (well, saliva). These receptors are gathered in taste buds, which cover the tongue. It’s generally accepted that there are five types of taste bud: salt, sweet, bitter, sour and umami. The last responds to monosodium glutamate, essentially the “meat” taste. There are actually several more “types” of taste, such as astringency (for instance from cranberries), pungency (ginger) and metallic (what you get from . . . metal).
Smell is underrated, but taste, by contrast, is a bit rubbish. It is the weakest of our main senses; many studies show taste perception to be largely influenced by other factors. For example, you may be familiar with the practice of wine tasting, where a connoisseur will take a sip of wine and declare that it is a fifty-four-year-old Shiraz from the vineyards of southwest France, with hints of oak, nutmeg, orange and pork (just guessing here) and that the grapes were crushed by a twenty-eight-year-old named Jacques with a verruca on his left heel.
All very impressive and refined, but many studies have revealed that such a precise palate is more to do with the mind than the tongue. Professional wine tasters are typically very inconsistent with their judgements; one professional taster might declare that a certain wine is the greatest ever, while another with identical experience declares it’s basically pond water.3 Surely a good wine will be recognized by everyone? Such is the unreliability of taste that no, it won’t. Wine tasters have also been given several samples of wine to taste and been unable to determine which is a celebrated vintage and which is mass-produced cheap slop. Even worse are tests that show wine tasters, given samples of red wine to evaluate, are apparently unable to recognize that they’re drinking white wine with food dye in it. So clearly, our sense of taste is no good when it comes to accuracy or precision.