For those who study the brain, our sense of smell has always been a bit of a wild child. While other senses are rule-abiding and organized, smell’s mysterious route through the brain has resisted full scientific comprehension. In a testament to how much we still have to learn, a recent article in Science tells us we can actually detect about 1 trillion different odors—blasting away old estimates of about 10,000.
It’s becoming more and more apparent that smell is enormously powerful. And while science is still figuring out exactly how and why, we do know some pieces of the puzzle. Here are the bare bones of how a smell makes its way to your brain and the delightful ruckus it stirs up along the way.
Step 1: A rose in the nose
There’s a rose in front of you and you breathe in. Along with the air, you suck up a few molecules that contain the rose’s scent. When those molecules hit your olfactory epithelium (a tiny, square-inch patch high up in your nasal cavity), they’re in business. That’s where receptor neurons turn the rose-scent molecules into electrical signals—meaning the message is now free to travel through your brain.
(Warning: Don’t get too attached to your receptor neurons. Unlike almost any other neuron we know of, these neurons are constantly regenerating. This shuffling-around of neurons may help explain why our sense of smell changes over time.)
At this point, your brain still doesn’t know what it’s smelling. For now, it’s just a scrambled message—like a connect-the-dots drawing before you’ve connected any dots.
Step 2: Time to get organized
The electrical-signal version of that rose smell is now traveling rapidly up your receptor neuron highway. Its first pit stop is your olfactory bulb, a tiny piece of brain right behind your eyes. This is where all those scrambled signals finally swirl into a pattern that your brain can recognize: rose! Precisely how this happens is still being debated. But according to a popular theory, each smell lights up a specific pattern of receptor neurons. So your brain can assume, for example, that if receptor neurons A, B, and D are lit up, this unique pattern must mean “rose.”
Step 3: Interpretation
Knowing it’s a rose is all well and good, but your brain doesn’t stop there. The olfactory bulb is a part of the limbic system (sometimes called the “emotional brain”), which has frequent-flier privileges to the feeling and memory areas: the amygdala and hippocampus. So now the rose becomes more than just a rose. It’s your dear grandmother’s garden where she taught you how to prune. It’s the next-day bouquet after a first date. It’s the corsage from your seventh-grade Valentine’s dance. And you feel emotions accordingly: nostalgia or nervous hope or cringe-worthy pre-teen awkwardness.
Usually, our brains latch on to the first memory we associate with a smell, which helps explain why the same scent can have different effects on people. We see this a lot with perfume: A perfume that transports you to a childhood spent snuggling against your mother’s neck might remind someone else of a crazy ex-girlfriend. Same smell, different emotional reactions. But those associations can change with repeated exposure, so don’t write off a great perfume just because your ex liked it, too.
Step 4: Amygdala and beyond
Let’s spend another minute in the amygdala, because it’s sort of like the busy CEO of smells. It ignores all the unimportant smell information you get every time you take a breath, hones in on what’s relevant, makes decisions, and tells the rest of your brain how to act on them. It tells you when the milk is spoiled (dump it), when there may be a fire (run!), or even whether you’re attracted to that guy at the bar (say hi).
As proof of its filtering power, scientists ran a (slightly gross) experiment where people were told to smell sweat generated by exercise, then sweat generated by emotional distress. The amygdala knew what it was looking for and lit up only for sweat generated by negative emotions.
Other studies show that people with bigger amygdalas tend to have greater emotional intelligence and a bigger network of friends. Not to make any great scientific leaps here, but it’s not hard to imagine that we perfume lovers are probably drawn in by the way our amygdalas interpret different fragrances. So the more into scent you are, the bigger your amygdala? Maybe. Let’s just assume that it also means we’re all smart and popular.