The scent of memory

June 4, 2019

Written by: Greer Prettyman

 

Maybe it’s your grandmother’s perfume or the scent of the flowers that grew in your childhood backyard— as soon as you smell it, a memory springs to mind. Like with Proust’s famous madeleines, sensory experiences can be linked to emotional memories in a powerful way.

 

This powerful association between smells and memories can be explained by the unique way that the brain processes odors compared to other sensory information. All sensory systems other than smell (vision, hearing, touch, and taste) require sensory information signals to pass through a region of the brain called the thalamus, which acts as a relay center to send the signal on to other parts of the brain. In contrast, smell bypasses this relay center and has more direct connections between cells that interact with odor molecules and parts of the brain that help to control memory and emotion.

 

Why might smell be different from other senses? Smell is considered the most evolutionarily ancient sense1. Even single-celled organisms use smell-like chemical senses to survive in their environments. Reptiles and mammals often rely more heavily on their sense of smell for survival and have a greater proportion of their brains dedicated to regions involved in olfaction compared to humans. The important role of olfaction in these more primitive species helps to explain why this sensory system is more closely connected with cortical regions of the brain.

 

Let’s take a closer look at the circuitry of olfaction to understand the link with memory. As you probably know, smell starts with the nose. We perceive odors when airborne chemical compounds enter the nose. Neurons located in a membrane within the nose called the nasal epithelium have receptors for these odor compounds. When these neurons are activated by the odor compounds, they send a signal into the olfactory bulb, a region near the front of the brain (Figure 1). From there, a type of neurons called mitral cells send signals directly to brain regions including the hippocampus, which is involved in memory, and the amygdala, which is involved in emotion. Scientists believe that these direct connections are responsible for the strong link between smell and emotional memory.

 

olfaction_circuitry
Figure 1. Schematic of the human olfactory system. In this image, we are looking at the bottom surface of the brain.  Image via Lumen Learning.

 

Additionally, part of the olfactory cortex called the piriform cortex may also play a role in storing strong memories related to smells. The piriform cortex receives information from an area called the orbitofrontral cortex (OFC), which is involved in discriminating and categorizing odors. In an experiment where the OFC of rats was electrically stimulated, long-term changes in the strength of connections between neurons (which aid in memory formation) were observed in the piriform cortex2. This result suggests a mechanism by which memories may be stored in this olfactory region after they are identified in the OFC, leading to learned associations between specific smells and specific memories.

 

In addition to animal studies like the one above, a few studies in humans have also sought to understand the relationship between smell, memory, and emotion. Think about memories you have that are evoked by odors. When you think of these memories, are they things from long ago, like the smell of your elementary school cafeteria? Researchers found that odor-induced memories are more likely to be from childhood, while memories related to other sensory systems tend to be from adolescent and adult years3. Additionally, while the emotions that odors evoke may be either positive or negative, they are often more intense than those induced by other sensory experiences. In an experiment that compared recall and emotional saliency when different sensory modalities were used as memory cues, odor-related memories were more strongly emotional than those induced by visual, auditory, tactile, or musical cues4.

 

Odor-evoked memory may be more than just a strange phenomenon— it has also been investigated as a therapeutic tool. Since memories brought on by familiar odors are often happy and pleasant, researchers have investigated inducing these memories via odors as a way to increase feelings of wellbeing. One study found that sniffing perfumes that have personal significance increased happiness and reduced anxiety compared to smelling non-significant scents and even led to changes in immune functioning5.

 

Given the ability of odors to trigger memories, some candle companies have even tried to capture smells to remind people of certain places or events. So maybe next time you’re feeling down, a comforting scent may help to bring a happy memory to mind.

 

 

 

 

References:

  1. Rowe, T. B., Macrini, T. E., & Luo, Z.-X. (2011). Fossil Evidence on Origin of the Mammalian Brain. Science, 332(6032), 955–957.
  2. Strauch, C., & Manahan-Vaughan, D. (2018). In the Piriform Cortex, the Primary Impetus for Information Encoding through Synaptic Plasticity Is Provided by Descending Rather than Ascending Olfactory Inputs. Cerebral Cortex, 28(2), 764–776.
  3. Willander, J., & Larsson, M. (2006). Smell your way back to childhood: autobiographical odor memory. Psychonomic Bulletin & Review, 13(2), 240–4.
  4. Herz, R. S. (1998). Are odors the best cues to memory? A cross-modal comparison of associative memory stimuli. Annals of the New York Academy of Sciences, 855, 670–4.
  5. Matsunaga, M., Isowa, T., Yamakawa, K., Kawanishi, Y., Tsuboi, H., Kaneko, H., … Ohira, H. (2011). Psychological and physiological responses to odor-evoked autobiographic memory. Neuro Endocrinology Letters, 32(6), 774–80.

 

 

Images

Cover image from https://unsplash.com/photos/34VYZIxPpG4

Figure 1 from Lumen Anatomy and Physiology I, Module 14: Sensory Systems, Special Senses: Smell (Olfaction); Licensed by Cenveo CC-BY-3.0, https://courses.lumenlearning.com/austincc-ap1/chapter/special-senses-smell-olfaction/

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