HONG KONG — Of our five senses, it’s often the one that we think about the least: our sense of smell.
Our sense of smell — the only sense fully developed in the womb — is managed by the olfactory bulb, which sits at the bottom of the cerebral hemisphere near the front of the brain and above the top of the nose.
Some neurological disorders illustrate the close connection between the sense of smell and the brain. For example, people with a rare condition called synaesthesia — which causes a sort of sense crossover — can “taste” shapes or “smell” colours.
British perfumer Jo Malone noticed she could identify the different colours of her artist father’s palette by their scent. Nicknamed the “bloodhound” at school for her incredibly sharp sense of smell — which was found to be as good as dogs’ for detecting scents — Ms Malone can sniff out snow before it arrives and has picked up disease in people just by the smell of them.
Our sense of smell is not just important for picking up odours; it is also intricately involved with taste.
About 80 per cent of your experience of taste relies on smell. If your sense of smell is dulled for any reason — because you’ve got a cold, say — your taste won’t be as sharp. The taste buds on your tongue only do part of the job, picking up the sweet, sour, bitter, salty and umami tastes.
As you chew food, molecules make their way back to your nasal epithelium – a thin tissue at the roof of the nasal cavity, which in adults is about 7cm behind the nostrils and informs the brain of the more complex flavours.
You can test this by pinching your nose shut when you eat something — say chocolate — and you’ll only pick up the sweet taste.
Our sense of smell is also crucial for memory.
The olfactory system has the only direct input into the memory centres of the brain and therefore has much more impact on them than the other senses, says Michael Leon, professor of neurobiology and behaviour at the University of California Irvine.
Prof Leon, with his team, Dr Cynthia Woo and Professor Michael Yassa, recently published research that showed how boosting sense of smell could significantly improve cognition — by more than double — in people aged 60 to 85.
“When there is too little olfactory stimulation, memory centres and pathways deteriorate, but when people are given olfactory enrichment, the memory centres become healthier and memory improves,” Prof Leon says.
That “enrichment” was delivered by exposing participants to seven specific scents a week, one a night for two hours. By making it possible for people to experience these smells while sleeping, they eliminated the need to set aside time for this during the day.
The atomised smells — rose, orange, eucalyptus, lemon, peppermint, rosemary, and lavender — scented only their dreams.
“People in the modern world are chronically deprived of olfactory stimulation and they need regular multi-odour stimulation to be able to maintain their memory,” Prof Leon says.
Loss of sense of smell accompanies as many as 70 neurological and psychiatric diseases. Prof Leon says it’s the first symptom of Alzheimer’s, Parkinson’s disease — and depression.
Rachel Pacyna, a University of Chicago medical student whose goal is to use neuroscience to investigate issues in women’s health, including ageing and memory, explains the link.
The sense of smell is affected in diseases such as Alzheimer’s because both smell and memory are processed in similar regions of the brain.
In particular, disease in the entorhinal cortex — described as “the gateway” for information entering and leaving the hippocampus — and in the hippocampus itself could lead to cell death in the brain that causes both impaired sense of smell and memory problems.
This happens, she says, because the build-up of amyloid plaques and tangles that are linked to Alzheimer’s leads to cell loss in brain regions associated with memory, particularly the hippocampus.
Ms Pacyna’s study showed that people with a faster decline in their sense of smell were more likely to develop dementia.
Olfactory dysfunction — reduction in the ability to smell — could predict cognitive decline up to 15 years before it manifests. It could be used as a promising — easy, accessible, affordable — early biomarker of brain health, for Alzheimer’s disease detection in particular.
It could even predict imminent death: another study of adults aged 60 to 85 found that those who had lost the ability to identify particular smells — including rose and peppermint — were more than three times as likely to die in the next five years.
Vision and hearing loss have already been linked to an elevated dementia risk.
Now it seems losing your sense of smell also poses a risk. As Prof Michael Yassa, who took part in the University of California study says, if you think about our other senses — sight, hearing — we do something about them as we get older.
But unlike vision changes which we manage with glasses, and hearing impairment with hearing aids, “there has been no intervention for the loss of smell”.
HOW TO SAFEGUARD YOUR SENSE OF SMELL
AVOID HEAD TRAUMA
The thin plate in the nose that connects to the olfactory bulb is fragile and sensitive to injury. Head trauma can pose a risk; people have lost their sense of smell after a sports injury or car accident. So wear a helmet while cycling or doing contact or extreme sports.
USE IT OR LOSE IT
Just like vision and hearing, our sense of smell isn’t immune to ageing. “The reality is that over the age of 60, the olfactory sense and cognition starts to fall off a cliff,” says Prof Leon. But because scent cells are renewed regularly — every 30 to 60 days — the olfactory system can repair and regenerate itself.
You can give it some extra help. Actively sniff out scents daily — of food, perfume, of flora and fauna in nature — and pay attention to what you can smell. The more you use your nose, the stronger the sense of smell will become.
FOLLOW A GOOD DIET
Eating well benefits us in many ways — including safeguarding our sense of smell. A lack of certain nutrients, including zinc and vitamin B12, is linked to the loss of this sense.
KEEP WELL HYDRATED
A dry mouth affects the ability to smell.
Tobacco use has been shown to kill the brain cells that help interpret scent information and it impairs the ability to smell.
TRY A SCENT ‘FAST’
To ramp up their sense of smell before a big wine-tasting event, sommeliers temporarily drop the really strong scents from their day. Our noses become so used to some smells we no longer notice them. So avoid coffee, onions and garlic, even your favourite perfume for a bit, and notice how your sense of smell comes roaring back. SOUTH CHINA MORNING POST