We all have at least one – whether it’s coffee, baking cookies, hot tar on a sunny afternoon, freshly mowed grass, the salty ocean tang or the smell of our mother’s perfume – one smell that instantly transports us to a long-distant childhood moment.
But how and why does smell trigger memories, how has smell become devalued compared to our other senses and can we be trained to enhance our abilities to detect and identify smells? This is the work being undertaken by STIAS fellow Jonas Olofsson of the Department of Psychology, Stockholm University and his colleagues at the Sensory Cognitive Interaction Laboratory. Olofsson has been a Wallenberg Academy Fellow since 2018 and is working on a book he hopes to publish in 2023.
“Among the five major senses, smell – olfaction – remains the least appreciated,” he said. “In fact, a 2012 survey of young adults reported that half of responders would rather lose their sense of smell than their smartphone. I argue that despite this common disregard for smell, it is of utmost importance to human life. We need to better understand and reappraise human olfaction.”
Understanding smell is a long-term research interest of Olofsson’s – ignited when he worked as a part-time nursing assistant with dementia patients while studying. Smell loss as a predictor of early Alzheimer’s Disease became the topic of his PhD.
“Most of us are not conscious of how much we use smell on a daily basis,” he said. “Of course, it came to the forefront during the COVID-19 pandemic when we realised that sudden smell loss was the best symptom predictor of infection and most common long-COVID symptom.”
Smell has been the neglected sense according to Olofsson. Tracing some of the history, he pointed out that things were different in ancient times when trading in spices on the Silk Route and therefore olfactory sensitivity was important. Smells were also used to communicate with the Gods – part of ritualistic offerings – and the long-discredited miasma theory linked disease transmission with unhealthy or polluted vapours or smells which is why Plague Doctors wore beaked masks stuffed with dried herbs for protection.
“There was a long cultural shift from smell to vision – vision became more culturally important from the modern period.”
French physician, anatomist and anthropologist Pierre Paul Broca described smell as a ‘bestial sense’. Broca is best known for his research on Broca’s area, a region of the brain’s frontal lobe. The rise of evolutionary theory ignited fears about what brain research and anthropology would find out. “It was deemed important that human brains were not too similar to animals. Broca’s work looking at mouse and human brains – where one of the biggest differences is the olfactory bulbs which are prominent in mouse but hidden under the frontal lobe in humans – argued that humans had expanded their frontal lobes which is where intelligence lies and smell had atrophied as intelligence predominated.”
Olofsson also pointed out that as modern science developed there was emphasis on tools of optics – like telescopes and microscopes – which continued to enhance visual domination. Even the cognitive science field remains visually dominated – “something we want to change by introducing smell”.
“And,” pointed out Olofsson, “contrary to widely held dogma, human olfactory capabilities are extraordinarily sensitive. Historically, smell was devalued relative to ‘higher’ senses such as vision, and human olfaction was considered poor – recent neuroscientific and psychological research has led to a reconsideration.”
“Far from being a simplistic mode of engaging with the environment, smelling engages our deep emotions as well as our intellect. Emotional responses to smells, especially body odours, may also reflect our ideological attitudes about other people and society.” He pointed to studies looking at body odours and voting preferences which show that body odour disgust may be associated with right-wing voting and xenophobia.
In the 1990s Richard Axel and Linda Buck (winners of the 2004 Nobel Prize in Physiology or Medicine) undertook pioneering studies that clarified how our olfactory system works. They described a large gene family – 1000 genes – that give rise to olfactory receptor cells, which occupy a small area in the upper part of the nasal epithelium and detect inhaled odorant molecules. We have about 6 million of these in each nostril.
Research since then has shown that the human smell genome is very sensitive – in fact humans are more sensitive to most molecules than non-humans with the exception of dogs. “Humans can probably discriminate millions of olfactory stimuli,” said Olofsson. “The notion that it has decreased is also not true – the olfactory structure is well preserved in the mammalian evolutionary line.”
We also know a lot more now about how the brain works. Being able to monitor brain activity and measure the communication between different parts of the brain allows us to understand why smell is linked to memory and emotion. Various studies have shown that the smell cortex has strong connectivity to the Hippocampus – a structure in the temporal lobe that acts as the memory hub of the brain.
Smell and declining cognition
Smell also provides an important window into aging brains.
“The brain actively makes predictions about what we are about to smell, and losing this ability is often an early marker of dementia of the Alzheimers’ type,” said Olofsson.
Loss of smell may be therefore be a warning of cognitive decline. Work on datasets including 1600 participants in Sweden has shown that you might be able to predict future cognitive decline, dementia onset and mortality risk.
‘Of course, we are not suggesting that smell training could ward off neurodegenerative disease. The studies are not conclusive so we don’t have strong evidence. It’s hard to improve the health of the brain but maybe there is a role for brain training using smell as opposed to only visual.”
“Smell loss is an invisible impairment,” continued Olofsson. “One in 20 people have no ability to smell, and 10 to 20% have poor smell. Smell is also very vulnerable – the neurons dangling in the nasal cavity are vulnerable to infection. This accumulates over time so that by old age about half of us have some smell impairment.”
And loss of smell (particularly sudden loss) results in a reduced quality of life – “70% of people report reduced enjoyment of food – saying everything tastes like cardboard, and that they can’t cook for others”. Loss of smell can also cause social anxiety because “people are afraid that they smell”.
On the other end of the spectrum are the about 10% of people who have a high sensitivity to smell and trials are ongoing to see if you can enhance this using cognitive behaviour therapy.
Not leaving a bad smell
Olofssen pointed out that “our sense of smell is highly flexible, which enables it to be improved and recovered following injury. During the pandemic, many millions lost their sense of smell. Although this impairment was only temporary, it had a severe impact on health and well-being, and highlighted the important roles that smell plays in daily life. My research team is developing novel smell-training methods for rehabilitation, and for enhancing smell in wine experts.”
With the heightened interest in smell, Olofsson and his colleagues established the Global Consortium for Chemosensory Research – involving 763 members from 71 countries. It undertakes survey-based research that has involved 30 000 participants across 31 countries thus far and provided early data on loss of smell due to COVID.
They have also developed an online app (https://smelltrainingapp.com) which can retrain your sense of smell and recover smell sensitivity as well as a Virtual Reality Nosewise Wine Game (https://www.youtube.com/watch?v=G8ye5Npn3JU) which in 10 minutes a day for 40 days can teach you to tell smells apart almost as well as a wine expert. They are also looking at brain activity before and after training to further understand brain connectivity.
The good news is that most patients with smell loss will eventually recover. And a post-pandemic survey has shown that now “only 19% of adults would sacrifice their sense of smell” said Olofsson.
“Our sense of smell is good but neglected,” concluded Olofsson. “It’s now being reappraised. Smell holds huge insights into the brain and psychology.”
Michelle Galloway: Part-time media officer at STIAS
Photograph: Anton Jordaan