Cut grass, warm cakes, and dog poo. A strange combination maybe, but smelling any of the three is likely to evoke a certain memory for you. Everyone has experienced a sudden recollection after sniffing a particularly distinctive odour, and now a team of Israeli scientist have worked out why.
Graduate student Yaara Yeshurun of the Weizmann Institute of Science suspected that this memory association is formed when we first encounter a smell in a particular context. That’s why cut grass might take you back to summer’s day in your youth, but not to a walk in the park last Tuesday.
To test her hypothesis, Yeshurun and her team got 16 volunteers to look at 60 objects, each accompanied wither either a pleasant or unpleasant smell. Next, an fMRI scanner measured their brain activity as they looked over the images again and tried to recall the associated odour. Participants then repeated the test with different smells, before coming back a week later for another round of tests.
Yeshurun found that after one week, the participants showed a distinctive brain pattern when recalling the first odour – even if they remembered both equally. The scan revealed activity in the hippocampus and amygdala, the parts of the brain involved with memory and emotion, and allowed Yeshurun and colleagues to predict participants reactions based on the data from the first day of the experiment.
Investigating further, they repeated the entire experiment with sound instead of smell. Surprisingly, they found that the first-time association was not repeated. Commenting on her result, Yeshurun said:
“As far as we know, this phenomenon is unique to smell. Childhood olfactory memories may be special not because childhood is special, but simply because those years may be the first time we associate something with an odour.”