A Scent is Worth a Thousand Words: The Neuroscience of Smell and Memory

Hannah Koolpe

Illustrations by Kishi Oyagi

It’s a brisk fall day. Leaves fall gently at your feet as you walk around your small college town, trying to find a cafe to study in. While you search, you find yourself strolling by a bakery when a warm, buttery, slightly sweet aroma fills your nose. You smile as you realize the delightful smell is coming from freshly baked bread. Suddenly, you are transported back in time: you are six years old again, standing in your grandmother’s kitchen as your small, flour-covered hands knead a ball of soft dough. You recall the joy you felt when sitting by the oven and watching the dough rise, the same sweet aroma filling your nose as you wait for the bread to bake to a golden brown. A sense of nostalgia and warmth washes over you: you feel as though your grandmother is with you, giving you a hug as you enjoy the fresh bread you baked together. You feel immense happiness as you relive this sentimental moment. Most of us have experienced a smell that suddenly evokes an emotional memory. The powerful connection between the memory-associated areas of the brain and the olfactory system — the sensory network that controls our sense of smell — is what brings about this interesting phenomenon and allows us to relive moments from our pasts [1]. 

The Journey of the Odor Molecule 

When you inhale the aroma of the bread, odor molecules travel up your nose, first stimulating sensory receptors in the upper area of your nasal cavity [1]. In your nasal cavity, these odor molecules fit into corresponding olfactory sensory receptors. Just as a key must fit into its matching lock to open, odor molecules must bind to specific olfactory receptors to be detected. [1]. Although an odor molecule can only bind to one specific receptor, this selectivity does not apply to the receptors themselves, which can be 'unlocked' by a number of different molecules [2]. The human olfactory system is able to detect at least one trillion distinct odors despite having only 400 olfactory receptors [2,3]. To identify the smell of bread, the brain must receive an electrical signal associated with that scent. As odor molecules attach to olfactory receptors, they act as a trigger for this electrical signal to travel to the olfactory bulb in the brain via the olfactory nerve [4]. The olfactory bulb acts as both the first relay center for the electrical signal and as an intermediary between the brain and the outside world. From the olfactory bulb, the electrical signal travels further into the brain to the olfactory cortex; the cortex consolidates different electrical signals encoding unique scents, like tangy yeast and warm butter, into the rich sensory experience of smelling bread [4]. 

Signature Smells: How Odors are Stored with Memories and Emotion

After the electrical signal is analyzed in the olfactory cortex, it travels to higher brain regions that will encode the odor signal into a meaningful memory [4,5]. One of these brain regions is the hippocampus, which is responsible for storing recollections of our experiences, or our autobiographical memories [5]. Olfactory input is uniquely processed in the hippocampus, whereas most sensory stimuli like touch or vision are first received by and processed in the brain’s sensory relay station: the thalamus [6,7]. The olfactory signal bypasses the thalamus and travels straight to the hippocampus, which acts as a large filing cabinet, processing and storing memory files for us to access later [5]. Memories can be connected to specific odors, so when we inhale a familiar scent, the associated memory file is retrieved by the hippocampus [8].

The olfactory cortex and hippocampus are connected to each other through a large network of neurons; without that connection, we wouldn’t form odor-related memories or remember a previously encountered smell [8]. Even when we are not actively receiving odor stimuli, the olfactory cortex interacts with the hippocampus via these neuronal networks, more so than other sensory systems [9]. Due to this strong neuronal connection, the autobiographical memory becomes associated with an odor. From then on, that particular scent will create a powerful and vivid recollection of the memory, one that is stronger than a recollection caused by a visual cue [10]. Think about your reaction to a picture of someone baking bread versus the experience of actually smelling bread. The actual smell might fully transport you back to the time when you were six years old, standing in your grandmother’s kitchen and covered in flour as a warm, golden loaf was pulled out of the oven; merely seeing a picture of someone baking bread might only cue a faint recollection of the childhood memory.  

The interplay between the olfactory system and the hippocampus is not just involved in storing memories, but also works with a brain region called the amygdala to assign an emotional context to odor-related memories  [11]. The amygdala — our brain's attention grabbing machine — is involved in the interpretation and processing of emotional stimuli such as fear or joy [12]. Picture an event from childhood: maybe you used crayons to create a homemade birthday card or roasted marshmallows over a crackling campfire. Moments that evoke strong feelings like excitement or happiness are more likely to be remembered [13]. When an experience arouses strong emotions, the amygdala is activated and promotes the storage of the emotional memory in the hippocampus [14]. The amygdala and hippocampus work together to link a memory with its emotional significance, such as the nostalgia evoked by the smell of bread [14]. 

The Power of Scent: Why Odor Memories are Important 

Memories associated with a specific odor are often evoked before we are even consciously aware of the smell [15]. Imagine sitting by the oven with your children, watching a fresh loaf of bread rise and bake. The smell fills the house with a warm, comforting aroma as the childhood memory of baking with your grandmother is again evoked. Involuntary memories can arise from the subconscious connection of smells to particular people, experiences, or places [16]. Memories conjured by odors are instinctual, so we can recall these kinds of memories faster [15]. Smell was the first scent that animals evolved as it was necessary to engage in mating behaviors, interact with other animals, and detect potentially harmful substances [4,17]. Therefore, having strong smell-associated memories is a survival instinct, allowing us to recognize and learn what certain things smell like and then decide whether we should interact with them [4,17]. We also utilize scent for more complex interactions with our environment, such as remembering past events or information [15,18]. For example, students who smelled a scent while studying for an exam and then smell it again while taking their exam perform better [19]. The scent students smell acts as a cue to locate and retrieve the correct memory ‘file’ associated with exam content. Additionally, using odors to cue context-related memories may have therapeutic benefits for people who struggle to recall long-term memories, such as individuals with dementia [15,18]. For example, people with Alzheimer’s who are exposed to odor stimuli are able to recall memories quicker and frequently report experiencing a sense of ‘traveling back in time’ to the place where they originally experienced recalled memories [15,18]. The ability of odors to cue our past enables us to not only recall relevant information, but to revisit incredible moments in our lives.

Olfactory memories, whether we realize it or not, play an important role in our lives. As odor molecules travel up our nasal cavity to trigger olfactory receptors, the complex processes of sensory perception, memory formation, and recollection ensue. The connection between the olfactory system, the hippocampus, and the amygdala allows us to remember meaningful smells that elicit strong emotions. Smell-memory processes first evolved to assist our survival, but have since become an integral part of our memory-recollection and emotional processing systems. We can use smells to recall memories or information and to re-experience the special feelings that each memory evokes. So, the next time the smell of crayons or Elmer’s glue reminds you of your childhood, let it also remind you of the intricate systems within your brain that make that recollection possible. 

References

1. del Mármol, J., Yedlin, M. A., & Ruta, V. (2021). The structural basis of odorant recognition in insect olfactory receptors. Nature, 597(7874), 126–131. doi: 10.1038/s41586-021-03794-8

2. Patel, A., & Peralta-Yahya, P. (2022). Olfactory receptors as an emerging chemical sensing scaffold. Biochemistry, 62(2), 187–195. doi: 10.1021/acs.biochem.2c00486

3. Bushdid, C., Magnasco, M. O., Vosshall, L. B., & Keller, A. (2014). Humans can discriminate more than 1 trillion olfactory stimuli. Science (New York, N.Y.), 343(6177), 1370–1372. doi: 10.1126/science.1249168

4. Sharma, A., Kumar, R., Aier, I., Semwal, R., Tyagi, P., & Varadwaj, P. (2019). Sense of smell: structural, functional, mechanistic advancements and challenges in human olfactory research. Current Neuropharmacology, 17(9), 891–911. doi: 10.2174/1570159x17666181206095626

5. Rubin, R. D., Watson, P. D., Duff, M. C., & Cohen, N. J. (2014). The role of the hippocampus in flexible cognition and social behavior. Frontiers in Human Neuroscience, 8(742). doi: 10.3389/fnhum.2014.00742

6. Sullivan, R. M., Wilson, D. A., Ravel, N., & Mouly, A.-M. (2015). Olfactory memory networks: from emotional learning to social behaviors. Frontiers in Behavioral Neuroscience, 9(36). doi: 10.3389/fnbeh.2015.00036

7. Gutnisky, D. A., Yu, J., Hires, S. A., To, M.-S., Bale, M. R., Svoboda, K., & Golomb, D. (2017). Mechanisms underlying a thalamocortical transformation during active tactile sensation. PLOS Computational Biology, 13(6), e1005576. doi: 10.1371/journal.pcbi.1005576

8. Aqrabawi, A.J., Kim, J.C. Hippocampal projections to the anterior olfactory nucleus differentially convey spatiotemporal information during episodic odor memory. Nat Commun 9, 2735 (2018). doi: 10.1038/s41467-018-05131-6

9. Zhou, G., Olofsson, J. K., Koubeissi, M. Z., Menelaou, G., Rosenow, J., Schuele, S. U., Xu, P., Voss, J. L., Lane, G., & Zelano, C. (2021). Human hippocampal connectivity is stronger in olfaction than other sensory systems. Progress in Neurobiology, 201, 102027. doi: 10.1016/j.pneurobio.2021.102027

10. de Bruijn, M. J., & Bender, M. (2017). Olfactory cues are more effective than visual cues in experimentally triggering autobiographical memories. Memory, 26(4), 547–558. doi: 10.1080/09658211.2017.1381744

11. Felix-Ortiz, Ada C., Beyeler, A., Seo, C., Leppla, Christopher A., Wildes, C. P., & Tye, Kay M. (2013). BLA to vHPC inputs modulate anxiety-related behaviors. Neuron, 79(4), 658–664. doi: 10.1016/j.neuron.2013.06.016

12. Šimić, G., Tkalčić, M., Vukić, V., Mulc, D., Španić, E., Šagud, M., Olucha-Bordonau, F. E., Vukšić, M., & R. Hof, P. (2021). Understanding emotions: origins and roles of the amygdala. Biomolecules, 11(6), 823. doi: 10.3390/biom11060823

13. Sheldon, S., & Donahue, J. (2017). More than a feeling: emotional cues impact the access and experience of autobiographical memories. Memory & Cognition, 45(5), 731–744. doi: 10.3758/s13421-017-0691-6

14. Paré, D., & Headley, D. B. (2023). The amygdala mediates the facilitating influence of emotions on memory through multiple interacting mechanisms. Neurobiology of Stress, 24, 100529. doi: 10.1016/j.ynstr.2023.100529

15. Haj, M., Gandolphe, M. C., Gallouj, K., Kapogiannis, D., & Antoine, P. (2017). From nose to memory: the involuntary nature of odor-evoked autobiographical memories in Alzheimer’s disease. Chemical Senses, 43(1), 27–34. doi: 10.1093/chemse/bjx064

16. Heald, J. B., Lengyel, M., & Wolpert, D. M. (2023). Contextual inference in learning and memory. Trends in Cognitive Sciences, 27(1), 43–64. doi: 10.1016/j.tics.2022.10.004

17. Kücklich, M., Weiß, B. M., Birkemeyer, C., Einspanier, A., & Widdig, A. (2019). Chemical cues of female fertility states in a non-human primate. Scientific Reports, 9(1). doi: 10.1038/s41598-019-50063-w

18. Glachet, O., & El Haj, M. (2021). Odor is more effective than a visual cue or a verbal cue for the recovery of autobiographical memories in AD. Journal of Clinical and Experimental Neuropsychology, 43(2), 129–143. doi: 10.1080/13803395.2021.1882392 

19. Neumann, F., Oberhauser, V., & Kornmeier, J. (2020). How odor cues help to optimize learning during sleep in a real life-setting. Scientific Reports, 10(1), 1227. doi: 10.1038/s41598-020-57613-7