The sense of smell in humans is highly complex and not completely understood. Many studies have shown that scents can alter our perceptions of experiences and influence human behaviour ”” such as buying behaviour and visiting nicely scented places, but the reasons for this are not well understood. It is easier to show that something happens than explain why it happens!
Our nasal capacity
Humans can recognise and remember about 10,000 scents, but the basic principles of how we can do this were not understood until recently. In addition, our basic sense of smell is mixed with other physical, chemical and mental processes that all combine to make up an overall perception of smells that is even more complex.
We have 5-10 million scent receptors in a small patch of tissue deep inside the nose (compared to 220 million in a dog) that are directly connected to the brain. This patch of tissue is only about 10cm squared in humans. These receptor cells can have about 1000 ”˜receptor proteins‘ that bind to specific molecules that make up scents ”” determined by their physical shape and chemical characteristics.
This is likened to a lock and key fitting together to open the lock, in this case to trigger sending a signal to the brain. Mucus in the nose dissolves the scent molecules from the air and bathes the receptor cells with these molecules.
Each receptor cell contains only one type of receptor protein, but can detect a small number of different scent molecules. Each of the proteins is encoded in one gene, so we have 1000 genes that determine the different types of receptors and the sense of smell in our nose ”” 3% of our genes. We have lost some smell genes that are used in other mammals because we use colour vision to sense our environment.
The scientists that discovered these basic principles of the mechanism of smell in the nose (the ”˜olfactory system‘) and the genes responsible for encoding each of the proteins, Richard Axel and Linda Buck, won the 2004 Nobel Prize in Physiology or Medicine.
More sensory perceptions
That is not the end of our smelly story, however. The human perception of smell is different when we breathe in and when we breathe out. ”˜Orthonasal‘ smell is what we detect when breathing in, straight from the air around us, while ”˜retronasal‘ smell comes from breathing out, adding air from the mouth (and whatever it contains) and mixing the sensations of flavour (from the tongue: salt, acid, sweet, bitter, umami), texture, hearing and muscle activity.
The perception of smell from the olfactory system is also complemented by sensory cells in the moist surfaces of the eyes, nose, mouth and throat ”” called the ”˜common chemical sense‘. These can detect substances such onion, or menthol and this is the reason why people with a smell disorder can still detect many types of scent.
In the brain, the perception of smell becomes even more complex. The signals that come from the receptor cells in the nose go into nearby parts of the brain called olfactory bulbs, which transmit signals on to other parts of the brain responsible for a range of functions:
- the piriform cortex, which is involved in perception of scents;
- the entorhinal cortex pairs specific scents to specific memories;
- the olfactory tubercle is involved in the reward behaviours associated with scents; and
- the amygdala is involved in emotional and non-conscious (autonomic) responses to scents.
Learning before birth
The learning of scents begins before birth, when compounds in foods eaten by the mother are passed into amniotic fluid and ingested by the developing foetus. Studies have shown that when a mother consumed substances such as garlic, alcohol or cigarette smoke during pregnancy, their infants preferred these smells. These early experiences also influence preferences later in life for food and flavours. (Scientific American, 2002, Rachel Herz)
Association and tradition
In choosing smells, it is important to know some cultural background of new customers. As an extreme example, the Dassanetch tribe of Ethiopia, whose life revolves around cattle, and wealth and social status is associated with cattle, have a different perception of valuable and desirable scents. The men wash their hands in cattle urine and smear their bodies with manure, while the women rub butter onto their bodies to smell more attractive. Another African tribe, the Dogon of Mali, are said to rub fried onions on their bodies as a scent”” both men and women. (The Smell Report).
Sense of smell may predict lifespan
A University of Chicago study of older people, which was part of the US National Social Life, Health and Aging Project, found that losing the ability to identify scents is a strong indicator of lifespan. In fact, the loss of smell was better at predicting mortality than a diagnosis of heart failure, cancer or lung disease. The reason for the association is not clear, however. The loss of smell is not the cause of the increased mortality but an early warning that something has gone wrong in the body, according to the researchers.
The researchers say that one explanation may be that as the olfactory nerve is exposed to the environment and directly connected to the brain this may expose the central nervous system to airborne toxins and pathogens.
This does, however, show how closely connected the sense of smell is to general body functions. One of the researchers, associate professor Jayant Pinto said, “Of all the human senses, smell is the most undervalued and underappreciated ”” until it is gone.”
Scents cause illness?
In ancient times it was recognised that body odour could indicate illness. Both the Persian philosopher Avicenna and the Greek philosopher Hippocrates used this method to diagnose illnesses. In Europe up to the 19th century, however, some doctors mistakenly believed that these odours were the causes of illnesses, which lead to the belief that perfumes could be used to treat diseases.
During outbreaks of the plague and typhus people carried scented pouches and torches to protect themselves against the diseases. Even into the 19th century perfumes were used by the medical profession to treat a wide range of both diseases and mental disorders.
An army of scientists
This complexity of our sense of smell is the reason why understanding its effects on human behaviour involves a pharmacopeia of disciplines, including: biochemists, molecular biologists, radiologists, physiologists, surgeons, behavioural psychologists, cognitive neuroscientists, neuropharmacologists, anthropologists, aromachologists, food scientists and others. Combined, they provide a battery of evidence of the power of scents.
Our understanding so far does show that scents can affect emotions and behaviour, and can be used to give positive effects for businesses. These include better customer ratings for products and services, intent to purchase, longer linger time, good memories, feelings of being relaxed and comfortable ””powerful tools for businesses to elevate their brands.
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