The Nose Knows

Hi Friends!

There are so many scents in Paris that I can talk about, ranging from lovely floral fragrances to freshly baked baguettes in patisseries. My olfactory (another word for smell) senses are overwhelmed with all the new odors that I’m exposed to! Last week, during a class excursion, we went to the Fragonard Fragrance Museum. We got to tour the museum and learn about the history of fragrance. Apparently back in ancient times, they used fragrance to preserve the dead so that the people could mourn the bodies during ceremonies. It’s bizarre to think how much the usage of fragrance has changed since the beginning of time! Now the perfume industry is all about making yourself appear attractive. When I was at Fragonard, my favorite perfume scents were floral mixed with fruity. It was an exciting experience to find the perfect perfume for myself (and my family) and it was extremely fun to try to distinguish all the different scents. I learned from one of my classes here that the human olfactory system can distinguish more than a trillion olfactory stimuli (Bushdid, 2014). One trillion?! That’s a ton of different smells!

I got these three perfumes at the Fragonard Museum for me and my family! Hopefully we all smell nice 🙂

Even though there are many pleasant scents here, I also found some that are cringe-worthy. For example, I noticed that many Europeans smoke cigarettes and the streets are constantly polluted with people smoking. Cute bistros will have tables outside where people smoke, which makes my dining experience quite unpleasant when I’m eating in a haze of smoke. I guess I am never really exposed to this much smoking in the states, so I spent the first couple of weeks here adjusting to the smell of cigarettes. After thinking about all these pleasant and unpleasant smells, I started to wonder…how does this relate to neuroscience? Surely my field of study can explain how my olfactory senses adjust to unpleasant smells!

Typical tables outside of a French bistro. This is my favorite one called Bistrot d’Edmond!

In a study done by Ferdenzi et al. in 2014, the researchers found that repeated exposure to odors induces “affective habituation” of perception and sniffing. “Affective habituation” was a phenomenon described by Cain and Johnson in 1978 that states repeated exposure to a scent shifts odor pleasantness ratings toward neutrality. In other words, this theory says that we will start to like a pleasant smell less and feel more neutral to it if we are exposed to it repeatedly. Cain and Johnson’s “affective habituation” phenomenon happens with unpleasant smells, too. Previous studies, however, only focused on self-reported ratings and did not investigate variations of physiological responses among individuals. Thus, Ferdenzi et al. aimed to analyze both self-reported ratings and changes in sniffing patterns to pleasant and unpleasant smells.

Ferdenzi et al. first recruited twenty-six young adults at a French university (what a coincidence this study was done in France!). The researchers split these people up into two groups – the “likers” and the “dislikers” when presented with an odor such as chocolate. They kept track of these groups and repeated this step with 8 different odors through a nasal mask. The “olfactometer” was connected to a nasal mask and measured how much each person breathed, which calculated the levels of sniffing. Lastly, study participants were instructed to rank how pleasant each smell was on a computer.

This picture shows the experimental device and how study participants would wear a nasal mask in order to smell the odor and rank its pleasantness.

Ferdenzi et al. found that pleasantness significantly decreased with time in “likers” while unpleasantness tended to decrease with time in “dislikers.” Increase in sniffing also correlated to a shift in greater pleasantness ratings, meaning that people would sniff more when they were repeatedly presented with an unpleasant smell. In general, their findings support the “affective habituation” hypothesis both at the self-reported level and at the olfactomotor level. This is interesting because I find this to be congruent with my life experiences. I notice that I find an unpleasant smell less aversive when I’m repeatedly exposed to it. However, I wonder if 26 young adults is a representative sample of the general population. I wish they had more test subjects and an equal amount of males and females in this study so that the researchers can have a more comprehensive analysis. Previous research has shown that females have better odor identification abilities than males (Doty et al., 1985). I wonder what the results would have been if they had an equal number of males and females.

I now understand why I am becoming more immune to the strong smell of cigarettes here in Paris. Affective habituation and neuroscience can definitely help explain this weird phenomenon! Maybe one day I’ll get accustomed to the pungent smell of body odor on the metro (ha ha!). Let’s just hope that people will wear deodorants on hot days so that I don’t have to deal with smelling and feeling body sweat on the train (eek!). I hope you enjoyed this blog post about smell and neuroscience. Sending love from Paris! 🙂

-Kimi

References

Bushdid C, Magnasco MO, Vosshall LB, Keller A (2014) Humans Can Discriminate More than 1 Trillion Olfactory Stimuli. Science 343: 1370-1372.

Cain WS, Johnson F Jr (1978) Lability of odor pleasantness: influence of mere exposure. Perception 7(4):459-65.

Doty R, Applebaum S, Zusho H, Settle R (1985) Sex differences in odor identification ability: A cross-cultural analysis. Neuropsychologia 23(5)667-672.

Ferdenzi C, Poncelet J, Rouby C, and Bensafi M (2014) Repeated exposure to odors induces affective habituation of perception and sniffing. Front Behav Neurosci 8:119.