A new paper by scientists at the Weizmann Institute documents the primal connection between whiffs of smell and episodic memory. This nasal nostalgia is mediated by the hippocampus, the manufacturer of long-term memory in the brain. Here's the abstract:

Authors, poets, and scientists have been fascinated by the strength of childhood olfactory memories. Indeed, in long-term memory, the first odor-to-object association was stronger than subsequent associations of the same odor with other objects. Here we tested the hypothesis that first odor associations enjoy a privileged brain representation. Because emotion impacts memory, we further asked whether the pleasantness of an odor would influence such a representation. On day 1, we associated the same visual objects initially with one, and subsequently with a second, set of pleasant and unpleasant olfactory and auditory stimuli. One week later, we presented the same visual objects and tested odor-associative memory concurrent with functional magnetic resonance brain imaging. We found that the power (% remembered) of early associations was enhanced when they were unpleasant, regardless of whether they were olfactory or auditory. Brain imaging, however, revealed a unique hippocampal activation for early olfactory but not auditory associations, regardless of whether they were pleasant or unpleasant. Activity within the hippocampus on day 1 predicted the olfactory but not auditory associations that would be remembered one week later. These findings confirmed the hypothesis of a privileged brain representation for first olfactory associations.

As the scientists note, artists have long described the powerful linkage of smell and the past. Here's Marcel Proust, explaining the madeleine:

"When from a long distant past nothing subsists, after the people are dead, after the things are broken and scattered, taste and smell alone, more fragile but enduring, more unsubstantial, more persistent, more faithful, remain poised a long time, like souls, remembering, waiting, hoping, amid the ruins of all the rest; and bear unflinchingly, in the tiny and almost impalpable drop of their essence, the vast structure of recollection."

As I noted in Proust Was A Neuroscientist, these ornate subclauses contain some prophetic insights into how our brain works. In 1911, the year Proust began writing his novel, anatomists had no idea how our senses connected inside the skull - the brain was three pounds of mysterious mush. One of Proust's hypotheses, however, was that our senses of smell and taste bear a unique burden of memory. That's why he makes it clear that just looking at the seashell shaped cookie, which he'd glimpsed countless times in patisserie windows, brought back nothing; Combray remained lost. In fact, Proust even goes so far as to blame his sense of sight for obscuring his childhood memories in the first place. "Perhaps because I had so often seen such madelines without tasting them," Proust writes, "that their image had disassociated itself from those Combray days." Luckily for literature, Proust decided to put the cookie in his mouth. As he writes, it was "by taste and smell alone" that his childhood memories came flooding back.

Why is smell so sentimental? One possibility, which is supported by this recent experiment, is that the olfactory cortex has a direct neural link to the hippocampus. In contrast, all of our other senses (sight, touch and hearing) are first processed somewhere else - they go to the thalamus - and only then make their way to our memory center. This helps explain why we're so dependent on metaphors to describe taste and smell. We always describe foods by comparing them to something else, which we've tasted before. ("These madeleines taste just like my grandmother's madeleines!" Or: "These madeleines taste like the inside of a lemon poppy seed cake!") In contrast, we have a rich language of adjectives to describe what we see and hear, which allows us to define the sensory stimulus in lucid detail. As a result, we don't have to lean so heavily on simile and comparison.

It's also worth noting, of course, that the data doesn't quite support the strong version of the Proustian hypothesis. While olfactory associations enjoy a "privileged brain representation," that hippocampal link is less important than the unpleasantness of the smell, which is much better at predicting whether or not we'll remember the memory a few days later. This is the bleak truth of the brain: it clings to what we don't like.

For more, check out the work of Rachel Herz. And thanks to Jason Synder for the tip!