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Why Do You Stop Noticing Smells After A While?

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Our brains lie to us. Not maliciously – they think they’re helping. Which is why smells seem to disappear: It’s olfactory adaptation. How it happens is complicated, but Lauren breaks it down.

Learn more at HowStuffWorks.com:
http://health.howstuffworks.com/mental-health/human-nature/perception/smell.htm

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Hi there, internet. I’m here today to tell you that your brain lies to you. Not maliciously. It thinks it’s helping. Which brings us to our question: “Why do you stop noticing smells after a while?” They’re still there. Why does your brain tell you that they’re not?

The “why” is actually simple. Experts in biology and psychology and volatile aroma compound physics (aka the science of smells) all pretty much agree that you stop noticing a smell after a short while because your brain wants to concentrate on scanning for new (and potentially hazardous) smells.

Sharp, “gross” scents may indicate that a dangerous predator is in the area, or that there’s a disease at work, or that the thing you’re about to eat should under no circumstances be eaten.

If you stick around a particular scent, your brain figures it’s already done warned you about that one and thus frees up its processing power for logging new scents and changes in scent intensity. This is called olfactory adaptation.

How your brain accomplishes this type of sensory adaptation is more complicated. When you notice a smell, a molecule of a volatile aroma compound (that is, a gaseous, smelly thing) has entered your nose. Our nasal passages are lined with somewhere around 10 million neurons. You can think of each of those neurons as a tiny tree designed to pick up on a single type of scent.

They have branches called dendrites, each covered in smaller structures (like leaves, going with the tree concept) that are called cilia. The cilia are studded with odor receptors. A molecule -- of whatever kind of scent that neuron specializes in sensing -- can bind to each receptor.

When that binding happens, it sets off an electrochemical chain reaction. In the end, the neuron sends an electrical impulse through its axon (metaphorically, its root), up into your olfactory bulb. That’s the part of your brain that processes scent stimuli and sends the information on to other parts of the brain.

Very basically, the more molecules that bind to a given type of scent receptor throughout your nasal passages, the stronger the signal to your brain will be, and the stronger you will perceive the scent to be.

But we can adapt to a scent’s presence within a few breaths. Researchers think that there are a couple things going on here. First, in those tree-like neurons in your nose, at least one chemical plays double-duty in both helping send electrical impulses to the brain and in stopping those electrical impulses. The culprit here seems to primarily be calcium ions, if you were curious.

The second thing going on here is that there’s some kind of feedback loop among your nasal neurons and your olfactory bulb. Researchers aren’t precisely sure what's going on here, but we do know it happens because they’ve conducted experiments where they’ve only exposed only one nostril to a scent. Neurons in the other nostril start adapting to it.

But there are more questions here. How do different scents and different lengths of exposure lead to different adaptations? Why can you never smell your own home the way other people experience it?

“Shenanigans” is the answer science has to give us right now. Because the perception of scent isn’t just physical; it’s psychological. What the other parts of your brain do with the scent information that the olfactory bulb sends them absolutely factor in, in ways that researchers are still trying to suss out.

SOURCES:

http://chemse.oxfordjournals.org/content/25/4/487.full

http://chemse.oxfordjournals.org/content/25/4/473.full

https://www.researchgate.net/publication/8487297_Making_scents_olfactory_adaptation

http://health.howstuffworks.com/mental-health/human-nature/perception/smell.htm/printable

http://www.ncbi.nlm.nih.gov/books/NBK10896/

http://nymag.com/scienceofus/2014/08/why-cant-you-smell-your-own-home.html

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