Yaara Yeshurun and colleagues at the Weizmann Institute of Science in Rehovot, Israel, showed volunteers an object such as a chair or pencil that was unlikely to already be associated with a smell at the same time as exposing them to an odour or sound. An hour-and-a-half later, they showed them the same object with a different odour or sound.

One week later, the researchers showed the volunteers the object again and asked them which odour or sound they associated with it, while scanning their brains using functional magnetic resonance imaging.

Volunteers were more likely to mention the first odour – and when they did, their brains showed a characteristic pattern of activity in the hippocampus. This pattern did not appear if volunteers plumped for the second odour, or if they had been exposed to sounds rather than smells.

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Yeshurun concludes that the brain reserves a special pattern of activity for memories that represent the first time we have associated a smell with a particular thing – and that such pairings are most likely to be laid down in childhood.

The brain may have evolved to lay down these privileged memories because it enhanced our ability to sense danger. "This is especially true for unpleasant odours," says Yeshurun.

This makes sense, says Rachel Herz, author of The Scent of Desire: Discovering our enigmatic sense of smell and visiting professor at Brown University in Providence, Rhode Island. "The evolutionary implication is that the situation in which you first encounter an odour is likely a reliable maker for its meaning, and it is highly adaptive to learn that meaning so that the odour can be responded to appropriately in the future."