#LEARNABOUT: VOL 8.2. Four Things You Should Know About Smell

Smell is one of the earliest-evolved senses, directly linked to our memories and emotions, and crucial to our perception of flavor. Smell, ...

Smell is one of the earliest-evolved senses, directly linked to our memories and emotions, and crucial to our perception of flavor. Smell, or olfaction, is often given less than deserved attention when it comes to food. In this issue of #LEARNABOUT, we will continue the topic of smell+taste from last issue, and offer you some additional scientific information on why smell is so important to us.

Olfaction and Emotion/Memory

The thalamus in the brain is often viewed as a “relay center”-- every other sense must first go to the thalamus, where it must be filtered, and then is relayed to other parts of the brain. However, olfaction has the choice of both relaying through the thalamus or skipping this step altogether and going straight to brain areas such as the amygdala and hypothalamus, which both process memories and their affiliated emotions. The entire perfume industry is built upon this: a certain scent will trigger desires of wanting and pleasant memories. This is why when you smell the scent of your old teddy bear, all of your childhood memories flood back, and you likely would experience a déjà vu of that bright midday summer sun shining through the leaves while you are on a picnic with your grandma.
This is why we are so inclined to treasure grandma's recipes, because the taste means something more than just the flavors. There are only 8 types of olfaction receptors, but we can smell up to 10,000 scents with training. Emotions affiliated with a certain smell is highly selective and therefore can only be triggered by a very specific type of smell. This is why that flavor has to be "exactly" right for you to experience the same emotional satisfaction as before. Also, unpleasant odors are remembered more than pleasant ones, and that's why you will probably never smell that durian once again.
Below is a picture of a transition zone between the human olfactory epithelium (bottom) and the respiratory epithelium (top).
Olfactory epithelium

Olfaction and Sleep

smell and sleep

Our brain is not completely restless during sleep. In fact, it's almost the complete opposite: some areas of the brain are more active during sleep than during the waking state. Such areas include the olfactory cortex, hippocampus, and amygdala; and one thing that the brain is always doing during sleeping/dreaming is memory consolidation, where new synapses form in the hippocampus to associate a form of event (or odor) with a certain memory.
Scientists also discovered that if something is learned in the context of an odor, and if the odor is presented again during slow wave sleep, there is improved retention of the memory. This is why if you sleep with a scented candle or a slow-cooking crockpot of food, you will be more likely to perceive this odor as "familiar" and "memorable." In addition, olfactory memory activates unique areas in the hippocampus that is not activated by auditory memory, showing that the strong connection again between olfaction and memory.
Scents can be used to change the emotional aspects of dreams. Negative olfactory stimulus (a rotten egg smell) leads to more negative dreams, while a positive stimulus, like roses, leads to more positive toned dreams. You can read this paper for more details.

Sex differences in Olfaction

gender differences in smell

Ever wonder how your mom (or girlfriend) is able to pick out that one dirty sock that was tucked away in the corner of your room? Yes, women might have be better at smelling than men! However, the previous sentence should come with two huge disclaimers: 1. There are huuuuuge individual differences in the ability to smell, and 2. The ability of smell can be trained to improve. But in general, women outperform men in odor discrimination tasks, and this is true across cultures. So what this probably means is that women, in general, have a head start when it comes to wine tasting and telling wines apart. I am sure my sommelier friend is very happy to hear this. 

Olfaction and Damage

Loss of smell, or anosmia, can be partial or complete, temporary or permanent, depending on the cause. Usually, anosmia isn't a concern if the cause is simply a congested nose, as it will only cause loss of appetite, or in an extreme cases, weight loss/malnutrition. However, in some cases, anosmia can also be an early warning sign of many neurodegenerative disorders, including Parkinson's Disease and Alzheimer's Disease. So don't overlook any symptom of anosmia, especially when one is elder in age.
Olfaction abnormalities can also be a result of head trauma, as the olfactory nerve is very fragile. So the symptoms can occur even after minimal head trauma and can begin months after the moment of injury. The abnormalities often include decreased taste acuity (hypogeusia), distortion of taste acuity (dysgeusia), decreased smell acuity (hyposmia), and a distortion of smell acuity (dysosmia), and they are diagnosed with the University of Pennsylvania's Smell Identification Test (UPSIT), which is comprised of four booklets of “scratch-and-sniff” odorants, which subjects must identify from a list of alternatives, and this is the same test researchers performed on the sex differences in olfaction study. 
So now that you have learned a lot about the human olfaction system, we hope that you have a better understanding and appreciation of your sense of smell. Really, it's hard to appreciate what you have until you no longer have it. So take a good sniff of that glass of wine or the flower around the street corner, and enjoy what you have!
Special thanks to UMN neuroscience PhD student Amy Nippert on the help with this article (for doing all of the scientific literature search). For the next issue of #LEARNABOUT, we will get back to food, and okra is coming up!

(image credits: smell and emotions:https://daks2k3a4ib2z.cloudfront.net/580e5333bd9da8b9577709ec/583d7e29783f64f67f5d39ac_smell-sense-olfactory-image.jpg (© Dianka | Dreamstime.com); olfactory epithelium: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3429117/; smell and sleep: http://www.nytimes.com/2012/08/28/science/study-shows-learning-of-smells-and-sounds-in-sleep.html (© Chris Cash ); gender differences in smell: http://www.dailymail.co.uk/sciencetech/article-2617977/You-smell-persons-SEX-Humans-subconsciously-identify-gender-using-subtle-odour-pheromones.html; UPSIT: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3429117/)

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