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This month, we dis­cuss the sci­ence and psy­chol­ogy of smell with Bar­ney Shaw, au­thor of The Smell Of

Fresh Rain

Freshly-mown grass, wood smoke, sea­side air… smells have an un­canny abil­ity to un­lock emo­tions and mem­o­ries. We dis­cuss the sci­ence and psy­chol­ogy of smell with Bar­ney Shaw, au­thor of The Smell Of Fresh Rain

When we get a whiff of some­thing, what is it we’re ac­tu­ally smelling?

It’s the light, or­ganic mol­e­cules given off by the sub­stance. But it’s al­ways a clus­ter of mol­e­cules that you’re smelling, not a sin­gle mol­e­cule. When you smell a cup of cof­fee, you’re breath­ing in around 800 dif­fer­ent mol­e­cules, of which about 30 con­trib­ute to the odour – it’s a kind of ‘chord’ of smell.

You of­ten find the same mol­e­cules re­oc­cur­ring in un­ex­pected places. There’s a mol­e­cule in cof­fee that you also find in the smell of fresh bread, cu­cum­ber and old peo­ple’s skin.

How do th­ese mol­e­cules get trans­lated into smell?

The mol­e­cules are de­tected by the mil­lions of re­cep­tor nerves inside our noses. There are a num­ber of com­pet­ing the­o­ries for how ex­actly th­ese re­cep­tors recog­nise the mol­e­cules – they might be read­ing the shape, vol­ume or even the vi­bra­tions of the mol­e­cules – but the sig­nals get passed on to the brain’s ol­fac­tory bulb, just be­hind the eye­brows. Here, the dis­parate in­for­ma­tion from our re­cep­tor nerves is com­bined into a sin­gle pat­tern, which the rest of the brain – espe­cially the pir­i­form cor­tex – trans­lates into a recog­nis­able smell.

Why do smells have such a spe­cial power to evoke mem­o­ries?

The ol­fac­tory bulb is bang next door to two im­por­tant brain struc­tures: the hip­pocam­pus, which is crucial for the for­ma­tion of long-term mem­o­ries, and the amyg­dala, which plays an im­por­tant role in emo­tions. The close con­nec­tion be­tween th­ese re­gions prob­a­bly ex­plains the un­usu­ally evoca­tive na­ture of smells. In con­trast, the other senses take a longer path into the brain, via a struc­ture called the thal­a­mus, which is not part of our brain’s emo­tional cen­tre.

When you ask peo­ple for their favourite smells, the same ones crop up time and again – this is of­ten be­cause they’re linked to our child­hood mem­o­ries. But it’s also cul­tural. Ger­mans adore the smell of marzi­pan; Ja­panese peo­ple think it smells of oil or saw­dust. If you ask an Ethiopian herder for their favourite smells, they will prob­a­bly in­clude cow dung, while the most at­trac­tive fra­grance for Mali’s Do­gon peo­ple is the scent of onions.

There are also un­pleas­ant smells that we come to en­joy be­cause we as­so­ciate them with de­li­cious food, such as pun­gent cheese. If ba­bies

are given a whiff of Camem­bert, they’ll wrin­kle their noses. It’s some­thing we learn to ap­pre­ci­ate over time.

You also talk about the links be­tween smell and mood…

The con­nec­tion be­tween emo­tion and smell works both ways. If you smell some­thing dis­gust­ing, it tends to make you feel more hos­tile, and if you’re a nat­u­rally hos­tile per­son, you tend to be more sen­si­tive to dis­gust­ing smells. Sim­i­larly, re­search has shown that peo­ple who are given some­thing fishy to smell are more likely to be sus­pi­cious, while nat­u­rally sus­pi­cious peo­ple are more sen­si­tive to fishy smells.

This link prob­a­bly goes back to our evo­lu­tion­ary past, when a fishy smell might be a warn­ing sig­nal that some­thing was ran­cid or ‘off’.

Has hon­ing your own sense of smell changed how you ex­pe­ri­ence the world in any way? Be­ing able to recog­nise a broader range of smells def­i­nitely gives the world an ex­tra di­men­sion. I have a son who is blind, and so he’s not ac­cess­ing the world through what is most peo­ple’s pri­mary sense. He would fre­quently ask me ques­tions like, “What does three o’clock in the morn­ing smell of?” Writ­ing this book has helped me to put what I smell into words. It’s also taught me that some­one who re­lies more on their sense of smell than their sense of sight is get­ting a more emo­tional view of their sur­round­ings, as well as one which has more po­ten­tial to frighten, dis­gust and de­light.

Fi­nally, what’s your favourite smell?

Grilled ba­con. But my sec­ond favourite is per­haps more un­usual: the smell of hot tar be­ing laid on a road. It’s a rich, thick smell – a blend of choco­late, bad egg and petrol. And it’s also got an edge of sul­phur, which of course makes it all the more ex­cit­ing.

THE SMELL OF FRESH RAIN BY BAR­NEY SHAW

OUT NOW

Smells can con­jure up mem­o­ries due to the ol­fac­tory bulb’s lo­ca­tion in the brain

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