Some aromas get etched into our memories since childhood, and now researchers at Weizmann Institute have found the scientific basis of this connection.
According to graduate student Yaara Yeshurun, along with Profs. Noam Sobel and Yadin Dudai of the Institute's Neurobiology Department, the key might not necessarily lie in childhood, but rather in the first time a smell is encountered in the context of a particular object or event.
In other words, the initial association of a smell with an experience will somehow leave a unique and lasting impression in the brain.
For the study, scientists devised an experiment, in which they first made the subjects in a special smell laboratory to view images of 60 visual objects, each presented simultaneously with either a pleasant or an unpleasant odor generated in a machine called an olfactometer.
Next, the subjects were put in an fMRI scanner to measure their brain activity as they reviewed the images they'd seen and attempted to remember which odor was associated with each.
The test was repeated with the same images, but different odors accompanying each.
After one week the subjects were scanned in the fMRI again. They viewed the objects one more time and were asked to recall the odors they associated with them.
The scientists found that after one week, even if the subject recalled both odors equally, the first association revealed a distinctive pattern of brain activity.
The effect was seen whether the smell was pleasant or unpleasant.
This unique representation showed up in the hippocampus, a brain structure involved in memory, and in the amygdala, a brain structure involved in emotion.
The pattern was so profound, it enabled the scientists to predict which associations would be remembered just by looking at the brain activity within these regions following the initial exposure.
The scientists could look at the fMRI data on the first day of the experiment and predict which associations would come up a week later.
When the experiment was repeated with sound, it was found that the results were specific to the sense of smell.
"For some reason, the first association with smell gets etched into memory and this phenomenon allowed us to predict what would be remembered one week later based on brain activity alone," said Sobel.
"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 odor," said Yeshurun.
The study is published in the latest issue of Current Biology.