The Science of Scent October 22, 2018 08:55

“Smell is a potent wizard that transports you across thousands of miles and all the years you have lived. The odors of fruits waft me to my southern home, to my childhood frolics in the peach orchard. Other odors, instantaneous and fleeting, cause my heart to dilate joyously or contract with remembered grief. Even as I think of smells, my nose is full of scents that start awake sweet memories of summers gone and ripening fields far away.” 
― Hellen Keller

Have you ever noticed that if you get a whiff of a particular scent it can have an incredibly powerful impact? Perhaps it reminds you of a specific time and event or maybe just influences your mood. How is it that scent can cause such a visceral response, much more than our other senses? 

Let's start with how scent works. When you smell something, the odor molecules travel to your nose where they are filtered by tiny cilia, which remove any dust and dirt. Then the scent travels through the nasal passage to the olfactory bulb, where the molecules bind to olfactory nerve cells. From a more technical perspective, the act of smelling is referred to as orthonasal olfaction or retronasal olfaction. The difference between orthonasal and retronasal has to do with the direction from which the scent enters. Orthonasal olfaction comes to the receptors via the front of the nose (how you typically think of smelling) whereas retronasal olfaction comes from the back (by means of the palate). Once the scent has bound to the olfactory nerves, it travels to the brain where it is interpreted as a specific scent.

However, it's a bit more complicated than that. The olfactory bulb is part of the limbic system, a complex group of structures that are critical to behavior, mood, and memory. Two structures in the limbic system are the amygdala and hippocampus, the parts of the brain which are respectively associated with emotion and memory. Our other senses are "wired" differently and do not have the same strong association with emotion and memory. Because of this, the science of scent is of interest to researchers in many fields including neuroscience, behavioral science, and even advertising.

The Swedish researcher Maria Larsson has studied the relationship between scent and autobiographical memories. She based her work on the research of others who had studied autobiographical memories.  Previous research had shown that autobiographical memories peaked between the ages of 15-30 and were based on visual or verbal cues. However, when it came to smells, the memories peaked around the age 5 and were more emotional and more vivid. Another interesting observation is that memories from scent aren't susceptible to retroactive interference, the phenomena whereby memories are changed when new ones mix with older ones.

But why is scent so powerful? Some researchers believe that scent is tied to a much more primordial part of our evolutionary past because it can warn us about things that are dangerous.  Scent is the first sense that is activated after we are born. However, another interesting branch of scent research has found that babies learn about scents even before they are born. This has been studied in numerous ways over the past 20 years. In one experiment, researchers gave pregnant women who were about to undergo an amniocentesis a garlic pill shortly before the test. Samples of the fluid were compared with samples from control subjects who had taken a placebo. Volunteers were able to distinguish which samples smelled of garlic and which did not. Several other studies have examined the diet of pregnant and breastfeeding mothers and found that those who had eaten a particular food in their last trimester or shortly after birth when they were nursing later had babies who either reacted strongly to the scent of that food or showed a preference for it over other foods. At the other end of the lifespan, researchers have also noted that people who have cognitive impairments such as Alzheimer's or Parkinson's also often have a dull sense of smell.

So the next time the smell of dried leaves makes you feel nostalgic but you aren't sure why just remember your nose knows.

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