Why Do Some Smells Trigger Memories?

Maybe it’s the paint or the Play-Doh that you used to use in kindergarten. Or maybe it’s the laundry detergent your grandparents always used, or the disinfectant you used to use in the coffee shop where you had your first job. Or maybe it’s something from your past that you had long forgotten about or shelved away in your mind.

No matter what it might be, this is likely something you’ve experienced at some point: A single particularly noticeable or recognizable smell suddenly brings back a vivid memory of a moment or episode from your past. But why are smells so effective in jogging our memory—even of things we’ve long forgotten about?

The Olfactory Memory Phenomenon

This phenomenon is known as olfactory memory. At least one reason why it is so potent is that smell is our oldest sense. In fact, even the most rudimentary bacteria have the ability to sense chemical signals around them, and it was this most basic capacity—which our primordial ancestors developed long before the likes of sight or sound—that gradually became our sense of smell. 

Our sense of smell is remarkably well furnished in our bodies, too. What you perceive as a smell actually involves the breathing in of many millions of tiny odor molecules, which trigger a complex response system inside our nose. There, tens of millions of smell-detecting sensory neurons line a passage known as the olfactory epithelium, at the tips of which are clusters of receptors that bind, in varying combinations, to odor molecules as we breathe them in.

Different olfactory receptors are aligned to respond to different odor molecules (and not all those that do so respond in quite the same way). And it is this combination of triggers of varying types and quantities—sent on to the brain by so many millions of olfactory neurons—that allows us to discern so great a number of different smells.

In fact, with an estimated 450 different types of olfactory receptors lining our noses, humans are capable (potentially, at least) of discerning as many as one trillion distinct odors. 

All the smells that we do respond to, however, are interpreted by a part of the brain known as the olfactory bulb, and it’s there that this mishmash of neural signals is decoded and interpreted, and ultimately perceived as a smell. And now that neuroscientists have at long last decoded how this complex chain of events unfolds, the prevailing theory is that it is the location of the olfactory bulb inside our brain that is responsible for the vivid memories that smells can evoke. 

The olfactory bulb is at the very front of the brain, just above our nose, and it is there, located by and connected to the hippocampus—the region of our brain that we know to be chiefly responsible for storing and accessing memories.

Scientists believe that it is the connection between these two regions of the brain (as well as the nearby amygdala, which deals with emotional responses and processes) that is responsible for the seemingly deep-seated association between smell and memory. The triggering of the smell receptors in our noses, therefore, sends messages to a smell-decoding region of the brain that, thanks to a quirk of our brain’s anatomy, happens to connect them also to our mental store of memories.

Admittedly, to date, much of this train of thought remains somewhat circumstantial. But rather tellingly, our other major senses—including touch, sight, and hearing—do not pass through this same emotionally sensitive region of our brain, which would at least explain why it is apparently smell alone, out of all of our senses, that seems so uniquely evocative.