SENSE OF SMELL: THE HIDDEN GEM

INTRODUCTION

“Smell is a potent wizard that transports you across thousands of miles and all the years you have lived”- Helen Keller

Hellen Keller the famous writer and author, also called ‘miracle worker’ by none other than Mark Twain, was born as a normal child but due to an infection at the age of 19 months, she lost the sense of sight and hearing. She had the ability to differentiate roses on the basis of smell and could smell literally everything from the trees to flowers to soil and even burning wood. She could even differentiate garments belonging to different members of the family just on the basis of smell. She rued in one of her writings the respect not given to smell among all the senses.

When one thinks of the sense of smell it evokes powerful memories. Be it the smell of freshly mowed grass transporting you to your childhood, the smell of the earth after rain, the smell of freshly baked bread at your uncle’s bakery, the smell of cologne taking you to your high school days, or the particular smell of the flower transporting you to your grandparents garden.

MECHANISM OF OLFACTION/SMELL

Firstly, for a smell to be perceived by our noses, it has to be in a gaseous state, ie when we smell flowers we are actually inhaling a mixture of volatile odorants that make their way up the sinus to the olfactory epithelium. The human olfactory epithelium is 9cm2 while that of the canines is about 20 times bigger than humans meaning that they can smell a whole world of smells in their one sniff as compared to ours. The smell gets converted into a signal at the olfactory receptor neurons which then transmits the signal to the olfactory bulb.

SMELL AND MEMORY

The limbic system is the place where we store emotional memories associated with a particular smell. Olfactory memory is key to our survival instinct and is stored in the amygdala like the smell of something burning. The amygdala is also involved with the formation of emotional memories especially those associated with fear or fight-or-flight. For example, the memory of running helter-skelter when being chased by a dog during childhood. This data is then forwarded to the hippocampus which files and stores it away for long term use to be retrieved when required.

NOSE: THE PATHFINDERS

The brain connects smell to memories through an associative process where neural networks are linked through brain waves of 20-40Hz. It was proven by a research conducted by the Kavli Institute for Systems Neuroscience. In the research, a maze was designed for rats where they had to poke their nose through a hole. On the other end two distinct smells were provided alternatively with one smell associated with the food being kept on the left side while the other smell with food being kept on the right side. The rats soon learned the smell associated with food on the particular side with 85% accuracy.

Furthermore, 16 electrodes were inserted in the hippocampus and the entorhinal cortex to study the pathway in the brain. Immediately after the rat is exposed to the smell there is a burst in the activity of 20Hz waves between the lateral entorhinal cortex and the hippocampus.

AGE DISTRIBUTION OF ODOR MEMORIES

The age distribution of memories evoked by verbal information (recollection of past events with date) follows a pattern having three components: The childhood amnesia, the bump, and the recency. Childhood amnesia is the dramatic reduction of memories in early childhood. In contrast, a significant number of memories are recalled from ages of 10-30 years, phenomena called the bump. The third component recency reflects better retention of events in the past 10 years.

A bump has been seen at an earlier age for odor-evoked memories(<10 years) compared to verbally cued memories(10-30 years). Therefore the oldest memories we have are more often associated with the sense of smell than the other senses.

I would like to end the topic by some excerpts from the writing of the great author Helen Keller, again showing the power of smell-

“From exhalations, I learn much of people—the work they are engaged in and the places they have been in, for instance, the kitchen, the garden, or the sick-room. I know when certain friends approach me by the cosmetics they use or the cigarettes they smoke. The dear odors of those I love I would not part with for all the perfumes of Arabia.”