As a child in school, I remember learning about the five senses. As part of our lesson, we were asked to pick a sense that we thought we could live without. This question has resurfaced numerous times since then and without thinking I always say smell. This week I spotted some new research that has made me realise smell should be higher in my estimation.
Being one of the most underrated senses; you might be surprised to learn that there are more than 1,000 olfactory genes in mammals. These genes make up the largest gene family in the mammalian genome. This fact alone conveys how important smell is to our survival. It warns us of danger (bad food, fires, chemicals), enhances the taste of our food and even stirs up emotional responses.
The smell of sun-cream and petrol remind me of childhood memories… the beach and cutting the grass. So how do we associate memories with odours? The area in the brain responsible for processing the odours we encounter is the olfactory bulb. It is one of just two structures in the adult brain where new neurons (nerve cells) are constantly born through a process called neurogenesis. Research published last month in the Journal of Neuroscience tested the role these newborn neurons play in long-term memory.
A series of experiments involving mice were conducted. To find if newborn neurons are involved in memory, they first trained the mice to associate an odour with a treat (smell equals treat). They were then retrained to associate an object with the treat (red cap equals treat). To deceive the mice even more during retraining, the odour was placed with the treat at random. The adaptable mice soon ignored the odour and learnt the food was beside the red cap. The mice had one more test to face. Could they remember the original smell equals treat lesson? It turned out they weren’t very good students as they had no memory of it at all.
But the mice weren’t failures! This experiment in conjunction with others showed that newborn neurons were created to store the odour-reward memory. When the odour was no longer associated with the reward, the newborn neurons died and the memory was removed. Interestingly, erasing odour memories seems to play a vital part in our ability to adapt to new environments.
The importance of adaptability was evident when the death of the newborn neurons was prevented in a later experiment. The neurons survival resulted in the mice maintaining the smell equals treat memory. Since the odour was no longer associated with the treat, it left the mice completing a pointless activity.
Neurogenesis and newborn neurons are just pieces of the overall olfactory system’s picture but their potential role in memory creation and retention is intriguing. Empowered with this knowledge, this leads me to wonder is it possible to rewire your own brain? Replacing bad associations to certain odours with good ones? Going by this research, we most likely do this subconsciously on a regular basis. A subject for another day perhaps. What I can say for definite is I won’t be as quick to write off smell as the fifth sense in future!
Sultan S, Rey N, Sacquet J, Mandairon N, & Didier A (2011). Newborn Neurons in the Olfactory Bulb Selected for Long-Term Survival through Olfactory Learning Are Prematurely Suppressed When the Olfactory Memory Is Erased. The Journal of neuroscience : the official journal of the Society for Neuroscience, 31 (42), 14893-8 PMID: 22016522
Top image: Rama / Wikimedia Commons