The modulation of our sense of smell

May 19, 2012 4:20 AM

How and why do we become accustomed to smells? What makes some odours personally super-pungent one day and unnoticed 'background noise' later on, despite the smell remaining objectively unchanged?

We've all got our own examples. The peculiar odour from some houses that the occupants don't recognise but visitors do; our incredulity at how someone could work in a place like a fish gutting factory; the reduction in the ability to notice, say, stale, fetid air in a room we occupy for long hours, or how the sickly sweet air nearby a cake factory becomes less noticeable over time, if you live in the neighbourhood. Maybe not the best examples perhaps, pulled out of the air on the run, but I think you know what I mean.

Is this process/ability 100% psychological? Or is/are there physical mechanism(s) at play? Do we innately 'muffle' familiar smells to the background as a protection system, so we're more able identify the odours of potentially dangerous and unfamiliar products/agents? Does any of this point towards it being an evolutionary element? Or is it all a kind of a modified thinking process, where we basically innoculate ourselves through repeated exposures and semi-consciously decide not to let it bug us??

posted by peacay to science & nature (7 answers total)

The psychological term for this is habituation. Here's a brief explanation.


Is this process/ability 100% psychological? Or is/are there physical mechanism(s) at play?

We think think, feel and remember with our brains, so the answer to questions like this almost always 'both'.
posted by nangar at 4:52 AM on May 19, 2012

More specifically, this is called olfactory fatigue. There are a bunch of nice experiments and explanations.
posted by goggie at 5:50 AM on May 19, 2012 [1 favorite]

It's the same basic mechanism by which you become accustomed to, say, the feeling of your clothes against your skin. The nerves fire a whole bunch initially, but after a relatively short time (depending on the type of nerves being stimulated and how many there are) the rapid-fire immediate response nerves basically run out of energy and stop going off. If they cease being stimulated for a short time, they recover fully and fire off with gusto when stimulated again. There are also mechanisms in the brain itself that shunt constant or repeated stimuli into almost immediate forgetting, and much of that process takes place outside of our conscious awareness (almost by definition, really; imagine willing yourself to forget something.)

The hypothesis is that these systems of nerve fatigue and auto-forgetting are part of the human body's method for dealing with the truly ridiculous amount of input constantly coming in. I like to think of my brain as the incompetent nominal ruler of a massively corrupt banana republic. Filter it out, narrow it down to the handful of "most urgent" actions, and give the sound-bite report to El Presidente, who can rubber-stamp a few of the decisions (that are mostly already moving forward) and thus feel like he's in charge.
posted by Scattercat at 6:28 AM on May 19, 2012 [10 favorites]

From a programmers point of view, I often think the brain just runs 'diff' on the world around you, and focuses all it's efforts on the differences that are highlighted.

I assume it's a survival instinct, if you've survived a few days with this object, smell, temperature, whatever in your life, it's fair to say it's safe, so your brain just pushes it into the shadows. So you start to not see that painting that's hanging above your stairs, the slight buzzing sound off your dimmer switches, or the sound of your air conditioning.

I often thing, probably incorrectly, that people with autism don't have the innate ability to do that filtering, they just get the full hose pipe of data streaming in and that's one of the reasons it's so hard for them to deal with the world.
posted by Static Vagabond at 9:58 AM on May 19, 2012

The technical term for this occurring in sensory systems is neural adaptation. It is basically how we separate irrelevant stimuli from those that actually make a difference. In general, organisms are hugely responsive to change in their environment, but not to a constantly present stimulus -- because this tends to be evolutionarily advantageous, so that we can actually pay attention to the stimuli that matter and ignore those constant background irritants.
posted by peacheater at 12:23 PM on May 19, 2012 [1 favorite]

Yup, probably adaptation/desensitization, and as others have pointed out there is definitely a molecular component to it. Even single-celled bacteria use this trick, actually: to find food, they have to swim up chemical gradients, but the absolute concentration of the chemical they're following might vary up to 100,000x. So they use feedback to turn down the sensitivity of chemical receptors on the cell surface as the absolute concentration increases. You can imagine that if they didn't do this, the receptors would saturate and they wouldn't be able to continue moving towards the food. By "adapting" they can continue to tell whether things are getting better or worse.
posted by en forme de poire at 8:08 PM on May 19, 2012

From the bottom of my olfactory nerve to the tip of my nose, I thank you all.
posted by peacay at 2:04 AM on May 20, 2012