Have you ever caught a whiff of plasticine and been transported back to your first days of school? Or noticed a passing stranger's perfume only to get an instant reminder of a long-lost love?
If you have, then you don’t need telling of the link between smell and memory.
Neuroscientists have known for the better part of a century that our olfactory system has direct connections to regions of the brain responsible for memory and emotion, like the hippocampus and amygdala.
When we take a sniff, odour receptors in our noses bind with odour molecules – from that plasticine, for example – and brain cells known as olfactory sensing neurons send electrical signals to different parts of the brain. It all happens in milliseconds.
“These are very direct connections between the olfactory system and areas in the brain that relate to memory, and to emotions,” says Prof Thomas Hummel at the Technical University of Dresden, who has researched the olfactory system for decades.
The link between smell and memory is such that losing your sense of smell is considered an early warning sign for broader cognitive decline. It tracks with neurodegenerative diseases such as Alzheimer’s, as well as regular ageing.
But what if it works the other way around as well? What if, by strengthening the olfactory system, we not only improve our ability to smell the roses, but also better our memory and cognition too?
It’s an idea that’s been wafting around labs in recent years, catching the attention of scientists who think there’s something in it.
A number of papers (many of them quite small) have shown that smell training can have measurable effects on the cognition and even the physical make-up of the brain.
In 2023, a review of the 18 papers concluded that smell training can indeed improve aspects of cognitive function, such as verbal fluency and verbal learning.
It also increases the volume of some brain regions, including the hippocampus and olfactory bulb, plus the connectivity between different regions.
Interestingly, these improvements are not just seen in people experiencing cognitive decline. Smell training also seems to improve cognition in the general population too.
“It’s not a silver bullet,” Hummel says. “It’s not like you start to smell and then you become generally brighter. But when you improve in olfactory function, it’s also beneficial for some cognitive functions.
“It’s an interesting idea, because you can change something with a simple [activity] that anybody can try and there are no side effects.”
A number of mechanisms are proposed for the effect. One is that increased sensory input just leads to a healthier brain in general.
It could also be linked to our distant evolutionary past, when our ancestors navigated the world partly through smell. Or it could be direct stimulation of circuits in the brain that are involved in learning and memory, because smell has direct access to the hippocampus.
Whatever’s going on, start-up companies sniff an opportunity, with a number developing smell-training products.
One is Osmo, an AI-powered digital scent engine that raised $70m (over £52m) at a recent funding round. Researchers at UCL, meanwhile, are developing I-Smell, a digital smell-training platform.
Of course, you can just train your own nose quite easily at home. A basic smell-training protocol involves finding around four smells that are familiar to you, but different from each other.
“A strong smell is more effective than a weak smell,” Hummel says. Scents like clove, lemon, coffee and eucalyptus have all been used in research.
Spend five minutes, twice a day, sniffing the odours for at least 20 seconds each. Think about them actively as you do it. What should each one smell like? What notes are you getting? Is it stronger or weaker than you thought?
“You have to do it regularly, and every two months you should use new odours,” says Hummel. He also cautions that more and larger studies are needed, and that the effects can be subtle in some people. “What I would emphasise, though, it certainly doesn’t hurt.”
As a wise old wizard once said: “When in doubt, always follow your nose.”
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