Why Scent Brings Back Memories

Smells are detected and processed in the brain through the olfactory bulb, a bulge of nerve tissue fed by loads of olfactory receptors on the insides of the nose and airways. (Extremely oddly, sperm cells also have olfactory receptors, apparently to help them smell their way to an egg.) We've only recently discovered that those receptors themselves seem to transmit data about how pleasant a scent is. Yes, your nose is judgy.

One of the big reasons that smell is closely tied to human emotion appears to show up in brain structure; the primary olfactory cortex is connected to a bunch of other brain regions, including the amygdala, hippocampus, and orbitofrontal cortex, that are related to how we manage and experience emotions. It's this closeness that, neurobiologist Rachel Herz argues, explains we feel emotional responses to smells first and understand why later:

"Due to the intimate neural circuitry between the olfactory cortex and the amygdala, emotional responses triggered by odors can occur instantly upon odor exposure and thus seem immediate and without conscious/cognitive mediation."

Scents have one of the most direct lines to emotional processing. Visual, auditory, and tactile stimuli first travel through the brain stem and thalamus, but the olfactory system doesn't — which may be why smell has such an intense emotional connection for us. Perhaps the information is less diluted. We're not sure, though.

There's a lot of scientific argument about whether we learn to dislike certain scents or if a kind of suspicion of them is innate in our brains from birth. It's evolutionarily useful to be wary of various smells: rotting flesh, rancid food, and stagnant water are "unsafe," and it would make sense if children seemed to have dislikes of those smells even if they'd never smelled them before. That doesn't seem to be the case, though. It seems that a lot of our education about smells and how we view them is based on associative learning; how we "rate" scents is based on the contexts in which we're exposed to them and what happens to us as a result. We learn, in other words, that certain smells are pleasant and others are unsafe, which means that our "smell libraries" may be intensely personal.

One of the most lauded properties of smell is its capacity to bring back memories (you can always tell an undergraduate writing assignment because somebody has a flashback after smelling something). How this actually works, though, is a deeply odd bit of science.

That it exists isn't denied by scientists. One survey of 1000 Chicago residents about the smells that reminded them of their childhood found that baking and domestic smells popped up a lot, while those who were older tended to have more nature-oriented smell memories, reflecting the city's growing urbanization. But how it's embedded in the brain, and how the recall of a memory through scent actually works, is viewed as part of our emotional processing system. Again, it's an evolutionary thing that may have helped us survive in the past, according to memory specialists Anne-Marie Mouly and Regina Sullivan:

"In animals, memory for environmental odors plays a vital role because it regulates many behaviors that are crucial for survival. Highly emotional or at least particularly ethologically relevant olfactory learning occur during an animal’s major life events."

Perhaps because of the strength of the olfactory system's connection to our emotional brain centers, the memories brought up by smell can go all the way back to infancy. Our peak time for memories induced by smell, according to a Swedish study, is age five; for visual and verbal inductions, it's our teens and 20s. Autobiographical memories brought back by scent are also more intense on the neural level than ones brought back by words; a study in 2014 found that, when memories were recalled by smells, brain activity in the limbic system (which is in charge of hedonic pleasure and pain) and visual vividness was stronger than when they were brought back by words. The memories aren't more accurate, and our brains respond more intensely if the smell is pretty unpleasant, but it's a remarkable response.

So smell can have a strong role in memory and emotion, but can it also change the way we act? Current science says the answer's yes. Herz, whose work we encountered earlier, has suggested that scent can play a role in helping us do better on tests, for instance; if we smell a particular scent while studying, and then take the scent into an exam, it seems that we may encounter a memory boost. But, she noted to Scientific American, it's difficult to generalize about how people will respond to certain scents (with more productivity, for example, or with relaxation), because so much of how we feel about scent appears to be intertwined with our past experiences with it.

If you do hit on a smell that many people respond positively to, she said, the behavioral responses can be worthwhile:

"People exposed to the smells of baking cookies or roasting coffee were more inclined to help a stranger than people not exposed to an odor manipulation. People who worked in the presence of a pleasant smelling air freshener also reported higher self-efficacy, set higher goals and were more likely to employ efficient work strategies than participants who worked in a no-odor condition. Pleasant ambient odors have also been found to enhance vigilance during a tedious task and improve performance on anagram and word completion tests."

But smell-induced behavior can also go wrong. A 2011 study found that if people were introduced to a new smell (nice or not) during a gambling task, they did significantly worse than those people who had no such aroma "interruption." And scents have been shown to be able to manipulate our moods and impressions of others even if they aren't obviously detectable in the environment.

There's also a lot of interest in whether or not smell can interfere with or mediate our perceptions of pain. It certainly seems like there may be something promising in it, but more to do with the familiarity of a pleasant scent than anything innate in the scent molecules themselves. In 2013, 135 newborns who had to have an injection were either prepared for the event by sleeping next to a vanilla-scented pad or not. Then they were either given the vanilla smell on a pad during the procedure or given other types of care. The newborns who'd been familiarized with the smell and given it again during the pain were much less likely to cry than the other two groups, who either smelt it for the first time or had nothing to smell. This may help to explain why familiar "comforting" scents like lavender help to reduce anxiety during IUD insertion, for instance; we associate them emotionally with an unthreatening situation, and our response is calmer. If you're going through something stressful, program your brain to associate a scent with good, calm times, and then take it everywhere.