The myths and realities of odour psychology

We make judgements about what we smell, where odour certainly influences the proximity of our relationships with others. Body odours tell us a lot about a person’s disposition. Odour influenced public policy about sewage and rubbish collection and was one of the main reasons the world’s population is no longer subject to great plagues that hit earlier societies through the ages. The ability to smell was one of our most primal senses, which was important in helping us find food, give warnings of danger, and identify spoiled food, etc. Odour also masked our body odours and gave us a sense of pleasure and sensuality. The importance of odour has declined in our age as most city buildings and homes are relatively sterile or neutral in their odour. The majority of odours we come across in the urban environment are commercially produced in the bakery, from an air freshener, through the air conditioning system, or from a colleague wearing an eau de toilette. Relatively few people now directly appreciate the odour of new mown hay, the changing odour of hanging jasmine on a trellis, or the smell of ozone after a thunder storm. We must first understand some of the basic concepts about the physiology and cognitive aspects of our olfactory architecture to understand the myths and realities about the psychology of smell.

Our olfactory architecture

How the olfactory system works has been well explained by other literature and this will only be briefly recanted here. When we sniff something or just breathe in through our nose, we also inhale aromatic molecules with the air. These molecules are bundled together in their millions and diffuse from their concentration according to Graham’s law (the rate of effusion of a gas is inversely proportional to the square root of the mass of particles) towards areas of lesser concentration where the olfactory mucosa, a membrane with a mucous surface contains the olfactory receptors at the top of the nasal cavity. Odour molecules are slightly water soluble and can pass through the lipid rich mucous that immerse the receptors at the epithelium surface and interact with olfactory receptor neurons. The odorants bind with the olfactory receptor neurons and change the shape of proteins, creating ion flows which pass along the axons of the olfactory nerve through the cribriform plate or skull to reach the olfactory bulb (alomeruli) which is an outcropping from the inferior (bottom) side of the pre-frontal cortex of the brain. The olfactory mucosa also contains another sensory system made up of trigeminal nerve receptors which are able to sense tactile pressure, pain, and temperature variation. They also exist around the mouth and eyes. These receptors can sense hot, cold, tingling, and irritability, in a similar manner to the skin. It is through these receptors that we can sense the coldness of menthol and the warmth of methyl salicylate. Up to 70% of our odour perception travels through this system of receptors.1 As mentioned above, aromatic molecules must possess some water solubility characteristics, have a high vapour pressure, and have the ability to dissolve in fat. Molecules above a molecular weight of 300 cannot pass through the mucous membrane, thus our inability to smell any molecule with a molecular weight more than that. Each neuron has only one type of receptor, of which there are about 1,000 different types. Even aromatic molecules with slightly different chemical structures activate different types of receptors, so octanal smells like orange will be differentiated from the similar octanal acids which are sweeter. Larger amounts of aromatic molecules tend to bind a wider variety of receptors than do smaller amounts of the same molecule. This may explain why concentrated amounts of indole tend to smell horrible but lower concentrations smell somewhat sweet and florally. Although the rate of odour flow does not affect the intensity of the odour, a concentrated odour molecule that attaches to most receptors may prevent the receptors sensing other odour molecules bringing on a condition that a perfumer calls olfactory fatigue, where the sense of smell is lost to other odours for a short period of time until the receptors are free again. It is estimated that a human can differentiate between 5,000-10,000 odours.2 This is probably possible through some method of combinational recognition and processing scheme to encode the identities of different odours, and a single odour may be recognisable by a single receptor and different odourants are recognised through different combinations of receptors.3 However, just as a person can recognise slurred vowels spoken by someone or read sentences that are spelt incorrectly, the brain through the process of categorical perception simplifies our perception loads into convenient odour groups or categories.4 As we know, similarly structured molecules do not necessarily smell the same, and differently structured molecules may smell the same, thus it is not easy to relate smell with the physiochemical structure to our sense of smell. Our categorisation of odours actually puts limits and biases upon how we define them through the various classification systems that have been developed over the years. It is only when brilliant innovations are recognised that our restricted vocabularies are expanded. Fragrance was categorised into nicely restrictive compartments until Martin Gras wrote his two seminal papers The Overdose and The Overdose II that we could see beyond the compartmental definitions we devised for ourselves.5 Unlike the receptors in our sight and hearing, olfactory receptors regenerate.