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Yaara Yeshurun of the Weizmann Institute of Science in Israel found that bad smells make the biggest first impression, which is likely an evolutionary defense mechanism, but early pleasant scents also make an imprint on the brain.

"We found that the first association between an object and a smell had a distinct signature in the brain, even in adults", said Yeshurun. "This ''etching'' of initial odor memories in the brain was equal for good and bad smells, yet was unique to odor."

To test smell-related memories, the researchers presented a group of volunteers with a set of objects, and then associated each with a smell and a sound. Some of the smells and sounds were pleasant, such as a pear or a guitar, and others were unpleasant, such as a dead fish or the screech of a power drill.

A week later, they asked people to recall the objects and found people tended to remember the unpleasant associations best, whether they were smells or sounds. Next, they did similar experiments while people''s brains were being scanned using functional magnetic resonance imaging or fMRI. In these experiments, they noticed that a part of the hippocampus and amygdala lit up when smells were first associated with a new object, but not sounds.

Much more study is needed, but they said the findings could lead to better ways to help improve memories, or even offer better ways to help erase early, traumatic memories.