Bottled Guinness has gone out of fashion nowadays in favour of draught. I recently found a bottle of Guinness at the back of a press at home where it had lain forgotten for years.
I opened it that night and, to my great surprise, I was plunged into a vivid and emotional memory of my teenage chore of bottling Guinness in the cellar behind our family pub.
The memory was triggered by the characteristic smell of the bottled Guinness. I could picture the contents and layout of the cellar, but it was disquieting to feel my boyhood grievance again at the disparity I perceived between the work I did in the cellar and my weekly pocket money. The power of smell to trigger intense memory is described by Rachel Hertz in the July/August 2000 edition of The Sciences. Smell is the sense by which we perceive odours, and the nose with its olfactory nerves, is the organ of smell. In addition, most sensations that appear introspectively as tastes are really smells. For example, I understand the taste we get from mushrooms is due to smell.
There are seven primary odours: camphor-like, musky, floral, peppermint-like, ethereal (e.g. dry cleaning fluid), pungent (vinegar-like), and putrid. Chemicals emitted by odourous substances interact with smell receptors in the nose. Impulses are sent along the olfactory nerve producing a perception of odour in the brain.
Olfaction (sense of smell) can be found in single-celled organisms, reflecting the need of all organisms to sense the chemical make-up of their surroundings.
In mammals, odours are detected by a layer of cells in the nasal cavity called the olfactory epithelium. An average person has about 10 million odour receptors, whereas a typical dog, can have up to two hundred million receptors. The phenomenon of odour-triggered memories was described in Marcell Proust's novel Swan's Way and the term "Proust phenomenon" used to indicate a powerful odour-evoked memory.
Many people believe that Proustian memories are different to others triggered by sights or sounds, claiming that the Proustian ones are more vivid, accurate and emotionally intense.
Research carried out by Rachel Hertz shows memories evoked by odours are no more accurate than memories evoked by sight, sound or touch. Hertz confirmed emotional intensity is characteristic of odour-induced memories, but the vividness of these memories and the impression of accuracy is an illusion created by that rush of emotion.
The feeling that memories and reflections are accurate when coloured by emotional experience is similar to what happens during eyewitness testimony in court. Despite the confidence shown by eye witnesses that what they recollect is accurate, psychological research shows such memories are often quite inaccurate. The emotional intensity on the witness stand produces vivid memories but are not necessarily any more accurate than other recollections.
There is good evidence that the ability to experience and express emotion grew directly out of the brain's ability to process smell. Our sense of smell involves our ability to detect chemicals in our environment. The ability to "suss out" the chemical environment is fundamental for survival.
Even the one-celled creature needs to know what and what not to eat. In more complex animals, smell is the key form of communication for the most important aspects of behaviour finding mates, recognising kin, finding food, and knowing whether another animal or object is dangerous. The first sense to evolve was smell. Humans no longer rely primarily on smell for survival.
Visual and verbal communications are our main tools for navigating the world. Nevertheless, the sense of smell retains some basic functions. Our response to a smell is to like or dislike, approach or avoid. Our emotions convey what is joyful, loving and good, and avoid what is fearsome, bad or likely to cause grief.
Emotion and olfaction are analogous in function. Both enable an organism to react to the environment in an appropriate manner, optimising chances for survival and reproductive success. Hertz suggests the human emotional system is an evolved cognitive version of basic behavioural motivations instigated by the olfactory system in animals. The intimate connection between olfaction and emotional experience is illustrated when olfaction is lost, through disease or injury.
One might imagine that loss of smell would have little deep impact on one's life, but this is not the case. Sensual experiences such as eating, sex or walking in the countryside , become greatly diminished. People who lose their smell often report a loss of intensity of emotional experience in general.
(William Reville, is a Senior Lecturer in Biochemistry and Director of Microscopy at UCC).