Our noses contain hundreds of odour receptors, each primed to interact with specific molecules. Photo / 123rf.
The ability to discern odours is intrinsically linked with mood, cognition and general well-being.
In 2022, researchers asked a group of 407 American students a question. If they had to give up one of their senses, which would they most willingly live without? Fewer than 2% said they would want to lose their sight; 13% suggested their hearing should be the first to go.
More than eight in 10 suggested they’d be happiest to give up their sense of smell. So little did they value this function that a quarter of respondents told researchers they would forfeit their sense of smell if they had to choose between that and being able to keep their mobile phones.
For Professor Barry Smith, the founding director of the Centre for the Study of the Senses at the University of London, this decision is misguided and scientifically wrong-headed.
“A study published in the International Journal of Neuroscience showed that people who have lost their sense of smell get more depressed than if they lose their vision,” Smith says. “Losing your smell – otherwise known as anosmia – can have a devastating effect on your life and can seriously affect your health.”
Smith is not alone in this assertion. An increasing body of evidence is showing that our mood, cognition and general well-being are intrinsically linked with how well we are able to sense our environment through our olfactory receptors.
“Everyone has historically talked about hearing, touch and vision, but in neuroscience, less attention has been spent on smell and taste,” Smith says. But the indications are that this is starting to change.
While we may not consciously dwell on it, our sense of smell is our most evocative. Even decades later, a certain scent can bring memories back to life: the brand of washing powder your grandmother used, or a perfume you wore as a teenager. For Smith, it’s Ambre Solaire, bringing back family holidays. For this reporter, Germolene, as applied to childhood cuts and bruises.
Scientists describe these moments as “like a key being inserted into a lock”. Our noses contain hundreds of odour receptors, each primed to interact with specific molecules. When those molecules enter your nose and bind to their matching receptors, sensory neurons fire electrical signals which whizz to different parts of your brain.
These are the areas involved in learning and emotion: the olfactory or piriform cortex, which identifies smells; the amygdala, which is involved in generating emotions; and the hippocampus, which stores and organises memories.
If the hippocampus thinks the smell is important or connected with a particularly emotional moment, it can “file” the information and store it indefinitely.
Smith began his academic career in philosophy, studying the area of mind and perception. After a stint as a wine columnist, he founded the Centre for the Study of the Senses at the University of London’s School of Advanced Study in 2011, alongside eminent neuroscientists such as the late Professor Colin Blakemore. Their brief was to study multisensory perception.
The idea is that our senses don’t just exist in isolation, but that they inform one another. “We don’t just hear or see – everything is interconnected,” Smith says. “The most multisensory experience is tasting, and most of tasting is due to smell. It’s the combination of the two that creates flavour.”
It wasn’t until the pandemic that the rest of the world sat up and started to take notice. “Mainstream science didn’t pay much attention until Covid,” says Smith. “But when anosmia emerged as one of the symptoms, online searches spiked. Respiratory doctors started getting in touch with my colleagues to ask for advice. Since then, our organisation has started to do more work with clinicians and patient groups.”
In the past few years, more evidence has appeared to suggest that anosmia is a symptom of serious illness or injury.
“You can lose your sense of smell after a head injury, when nerves to the brain are severed,” Smith says. “Sinusitis, or a virus, can also cause anosmia. If there is no discernible cause, we call it ‘idiopathic’ anosmia. Research has shown that a gradual loss of smell can be a two-year advance warning of conditions such as dementia or Parkinson’s. It may even give clues as far as 12 years in advance.”
Researchers have been working on this area for a while. In a 2014 study held in Leipzig, Germany, 7000 participants were given tests on their sensitivity to different smells, their capacity to distinguish between them, and their mental abilities. Those who had the keenest sense of smell also did better on the cognitive tests. Conversely, the weaker their “nose”, the worse they scored in verbal fluency, memorisation and attention span.
A 2021 study at San Diego State University in California found that people’s performance on odour sensitivity tests appeared to predict who would go on to develop mild cognitive impairment and who would develop Alzheimer’s disease. These tests were seen to be a better predictor of the condition’s progression than the mental state examination used by GPs and neurologists.
Scientists are also finding links between olfactory dysfunction and depression. A 2022 study by the Dresden University of Technology in Germany found a correlation between patients’ olfactory ability and symptoms of depressive illness: when their olfactory function improved, so did their mood.
Brain scans seem to agree: olfactory dysfunction is accompanied by loss of grey matter and a shrinking of the hippocampus. Such changes may explain both the cognitive decline and the rising depression in people with dementia: “Smell loss interferes with the affective and emotion control systems,” says Professor Thomas Hummel of Dresden University, one of the leading experts in this area.
Hence, scientists believe olfactory testing could become more useful in early screening for cognitive impairment. “As the new dementia drugs rely on early intervention, this could be important,” says Smith.
If we know that losing your sense of smell is a sign of serious illnesses such as Alzheimer’s disease, can we reverse the equation and train our senses to slow down or even ward off dementia? For now, to claim this would be a stretch, though those working in the field are optimistic that it may one day be shown to be the case.
There is more consensus around the theory that paying attention to your sense of smell – “training it”, as it were – can add to a person’s enjoyment of life, and perhaps enhance their cognitive skills.
Smith is convinced of this, pointing to a number of studies showing that smell training can keep you cognitively more active. “One study followed people in their 70s, some who did Sudoku, and others who did daily smell training for three months,” he says. “The ones who did Sudoku just got better at Sudoku. But the ones who did smell training had better recall.”
It’s important to point out that these studies are correlative – as in, no “cause and effect” between smell and a sharper brain has been proved. But Smith feels this does not detract from the validity of their outcomes. “It’s a case of use it or lose it,” he says.
“Unlike your hearing and vision, your sense of smell is underused. This is also in contrast to other animals. Therefore, we have spare capacity. There was even a study published in 2016 in the Frontiers of Neuroscience suggesting that perfumers and wine experts have more grey matter than people in other occupations.”
As scientists debate the potential of using smell in a clinical setting, Smith remains a flag-bearer for the use of aromas to boost your well-being and stimulate your brain. “Smell is bound up with memory, emotion and arousal,” he says. “If you lose arousal, your emotions are flattened and the world becomes dull and grey.”
What is smell training?
In 2009, Professor Hummel ran a study to investigate whether repeated short-term exposure to odours over a period of three months would have any effect on the olfactory ability of a group of anosmia sufferers. One group of volunteers was given four essential oils and told to sniff each one every day, morning and evening for 10 seconds at a time over a period of 12 weeks. The other was not.
Has it been shown to help?
At the end of 12 weeks, researchers found that 30% of the patients who had sniffed the oils had had an improvement in olfactory function, compared with the group who didn’t participate.
“Smell training definitely won’t harm and it might help,” says Professor Smith. “I do this every night, and every morning, and I feel more alive than ever before.”
What were the essential oils in the trial?
Rose, eucalyptus, clove and lemon.
How should I do this if I want to try?
According to Smith, the best way to try this is to put three drops of each essential oil onto a piece of cotton wool in a mini jam jar. “Keep them by your bed and sniff night and morning,” he says. “During the day, you will pay more attention to the scents around you: coffee, tar, and flowers – you’ll simply feel more alive.”