the science of smell

It’s only now that we can understand what our noses are capable of and the influence it has over our lives. As well as being an area of science where many women play leading roles, smell is the most mysterious of the senses and the most underrated. Vision, the most spectacular of our gifts, has a mechanism so straightforward it is taught in schools. Hearing is nothing more than a microphone can do. Touch is sensitive but simple. And you can literally count the number of tastes the tongue can detect on one hand. Smell however has remained stubbornly resistant to our understanding.

Our senses are the only things which connect us to the outside world. To sniff out the truth about reality we just have to inhale. Simply breathing in will draw air over the surfaces inside our nose. Of course, air is not just the stuff of life – it is a carrier for all that we smell.

As the air rushes across the membranes of the nasal cavity it deposits the small molecules which make up scents. A patch of about 10 million cells make up the part where smells are detected. For one of these molecules to be detected it must find a corresponding receptor; a sensitive protein. A receptor is like a lock for which there is only one key – each receptor only detects one type of smell: so that overpowering smell of truffle from your colleague’s lunchbox is only landing on one protein molecule in your nose, even though it might feel like way more.

Smell therefore relies on tiny fragments of the thing we are smelling reaching our nose. This takes on a wonderful metaphorical aspect when we consider the joys of inhaling the odour of our loved ones. It takes on a different hue when you consider just what is happening when you wrinkle your nose on entering a public lavatory. In all there are about 400 varieties of receptor in humans, defining our entire ability to smell. These receptors line the surface of the cells waiting for a smell molecule to bind to it.

Once it does a nerve signal is launched up to a bundle of nerves called the olfactory bulb. From there the signal is sent directly into the brain. From molecules to mind in a fraction of a second.

Much of what is described above had been known for decades but the details have only recently fully emerged. In 2004 the Nobel Prize in Medicine and Physiology was awarded to Linda Buck and Richard Axel "for their discoveries of odorant receptors and the organisation of the olfactory system." They had unravelled the mysteries of our sense of smell.

Buck and Axel cloned and identified the receptors which rats have in their nose. It turns out 3% of a rat’s DNA is given over entirely to these receptors, a large fraction. It is a similar amount in humans and shows how vital smell has been in our evolution as a species.

They also made the breakthrough in how smells are interpreted in the brain. Two very similar molecules may have completely differing smells. Add a carbon at the right point and it goes from charming to cloying. Smell is not just in the nose, it is one of the most influential senses our brains possess. Our brain detects the signals sent to it and synthesises them into an experience.

A study by Leslie Vosshall and her lab has shown that complex scents, those made up of more than one molecule, may be the key to our ability to tell odours apart. Given we have only 400-ish receptors you might think that was the limit of our ability to smell. The smell of a rose is actually a mixture of over 100 compounds, not just one molecule. A rose by any other mix of these chemicals would not smell so sweet. They estimate the human brain can take these competing odours and differentiate over a trillion (1,000,000,000,000) smells in total.

In Proust’s voluminous In Search of Lost Time he recounts an experience familiar to everyone. His character sits down to have a cup of tea, something he rarely does, and small cake, a Madeleine. No sooner has he put the cake into his mouth than the flavour sends his mind back to his youth when he last had a Madeleine and tea. The flavour, the combination of smell and taste, has sparked a long forgotten memory.

It turns out that this is not a mere fictional flight of fancy. There is good science which links the sense of smell to that of memory. The signals from the nose, unlike those from the other senses, pass right into the amygdala and hippocampus. These are areas of the brain linked to long-term memory. It seems that memory of smells, far more than any of the other senses, has shaped our brains.

Rachel Herz and colleagues found that memories triggered by odours were far more emotional than those raised by verbal cues. They also reported that participants felt ‘taken back’ to the memory associated with the smell.

When Herz used an MRI machine to scan the brains of people exposed to scents she saw the expected olfactory pathways lighting up. When given a new perfume to smell there was just activity in the normal sensory areas. Given a perfume to which they had reported a strong emotional attachment their brains sparkled with activity related to memory and feelings.

When selling a house a real estate agent may suggest you bake bread and brew fresh coffee. They are not just doing this because they fancy a nice brunch. They are looking for a quick sell via the medium of smell. Odours measurably affect how people behave. That is why shops are known to pump scents out to encourage sales.

They can also be used to our personal advantage. Research has shown that the smell of lemons improves the efficiency of those performing academic tasks, while peppermint spurs on those in athletics. Both orange and vanilla scents have been proven to reduce anxiety.

Those looking for a hit of creativity would do well to light a cinnamon candle.

While the science of smell has been driven forward in the past few decades there is still much that we do not know about this most primal of our senses. Whatever the future reveals will only serve to deepen our appreciation for the human nose. We all need to take some time to stop and smell the roses, and marvel at the wonders going on inside ourselves.