The memory of a smell

A smell can return a memory with a completeness that a photograph rarely manages. The reason is anatomical rather than poetic.

Most sensory information travels first to a relay station in the middle of the brain and is distributed from there. Smell does not. The olfactory system connects almost directly to regions involved in emotion and memory, skipping the relay, and the memories it produces therefore arrive with their emotional content still attached rather than reconstructed afterwards.

There is a second difference that explains the vividness. Memories triggered by smell are, on average, older. Vision and hearing are rehearsed constantly throughout life, so visual memories are overwritten and updated. We encounter particular smells much less often, so a smell met again after twenty years is connected to the last time it was met rather than to a chain of intervening versions.

This also explains an experience people find strange: the memory is intensely vivid and almost impossible to describe. English has very few words for smells, so the experience arrives fully formed and finds no vocabulary waiting for it. Languages that do have rich smell vocabularies, such as Jahai in Malaysia, produce speakers who identify and name odours far more consistently than English speakers.

The effect is worth knowing for a practical reason. A smell present while studying and again while testing produces a small but measurable improvement in recall, which costs nothing to try and is one of the few study tricks with a plausible mechanism behind it.