Orthonasal vs retronasal olfaction
What is flavour?
The perception of flavour is a complex process that involves both our sense of smell as well as “true taste”--that which we perceive on the tongue. Many people who lose their sense of smell say “I can’t taste anything”, or “I’ve lost my sense of taste” when they really mean that they can’t detect any flavour in their food. This is a language problem. In many languages in fact, the words for “taste” and “smell” are the same. But as we are concerned on these pages with the intricacies smell, it is worth pointing out a bit more detail about the mechanics of it. And that the combination of smell and true taste together gives us flavour.
Two ways to smell
We have, in fact, two separate ways of experiencing smell. One is via orthonasal olfaction (sniffing) and the other is via retronasal olfaction. We all understand the former. The latter is a little more complicated to explain. You can think of retronasal smelling as being most obvious when your are chewing something delicious and you say “mmm! this is so good!”. If you have ever held your nose to swallow something disagreeable (like cough medicine) you will understand what happens when retronasal olfaction is blocked.
“Retronasal olfaction is the perception of odours emanating from the oral cavity during eating and drinking, as opposed to orthonasal olfaction, which occurs during sniffing”
When we sniff something, odour molecules are taken up through the nostrils where they find their way to the patch of cells in our nasal cavity called the olfactory epithelium, where the receptors are located. This tissue then sends messages to our brain to help us understand what is happening.
How does this work for retronasal olfaction? When we chew our food, odour molecules bounce around inside our mouths, and some of them waft up the passage behind the uvula (look in a mirror to see this dangling tear-drop shaped bit of flesh), and up into the nasal cavity to reach the same bit of odour receptive tissue.
What’s the relevance of this information to those of us with an interest in smell disorders? It should not come as a surprise that some things smell very different from the way they “taste”. Coffee is a good example of this, as is strong cheese. Olfaction (experienced through sniffing) is a different experience from true taste plus retronasal olfaction.
In the article cited above, it was found that in some patients with smell dysfunction there was a difference between the perceptions of orthonasal and retronasal olfaction. This remains an under-researched area, but it is thought that obstructions in airflow account for differences reported by patients of smell/flavour perceptions in these two smell pathways.
If you are recovering your sense of smell after a virus, you might want to dig a little deeper into whether your orthonasal and retronasal perceptions are equal. You can try this even if you consider your sense of smell to be poor, as long as you have some rudimentary sense of smell, and you choose a substance that you can perceive by sniffing.
Orthonasal & Retronasal olfaction experiment
You will need:
– Some plastic coffee stirrers
– One or two of the following: vinegar, pesto sauce, mustard, coffee syrup, garlic/lemongrass paste, or any food item that is liquid or paste and has a smell strong enough for you to get a whiff of it through sniffing.*
1. Dip the coffee stirrer in a small amount of the substance. It should not be a big enough blob that it might fall off onto your tongue. You want to moisten the stirrer only.
2. Now, *avoiding your tongue* (very important!) open your mouth and hold the stirrer so that it does not touch any part of the inside of your mouth. The end of the stirrer will be above your tongue and below the roof of your mouth. Close your lips. Leave your teeth apart. You want the stirrer to be hovering in the air inside your mouth cavity, and not touching any of the surfaces of your mouth.
3. Try saying “mmm, mmm” several times, gently pumping your tongue to get the air flowing retronasally.
What do you perceive?
* I have tried this with non food substances like hand wash, which is the perfect consistency to stay put on the stirrer, but only try this if you are brave and think you can keep your tongue well away-- no one wants their mouth washed out with soap!
Being curious about your smell perceptions, as well as being a good observer of them is one important step forward in smell training. Someone once asked me this: what is the one thing that helped you the most in your journey with smell training? I’d have to say that the answer is “thinking about smell”. Not only learning to be alive to the idea that everything in my environment was an opportunity for smell training (till receipts, ball point pen ink, tennis balls, my phone case, fresh laundry, breakfast cereal, anything I could lift to my nose) but that even the most indescribably vague and indecipherable smell was an opportunity to stop and think, and try to memorise this new scent, as if I was learning a new vocabulary word.
Smell training = smell awareness.