Smells connect to memories more than other senses

It’s called the Proust effect after a story in the author’s “Remembrance of Things Past: Swann’s Way.” When a character dipped a madeleine, a sweet, buttery French cake, into some lime-blossom tea, the scent suddenly transported him back in time to the moment his aunt had served him that same combination:

“Immediately the old grey house upon the street, where her room was, rose up like the scenery of a theatre to attach itself to the little pavilion, opening on to the garden, which had been built out behind it for my parents… and with the house the town, from morning to night and in all weathers, the Square where I was sent before luncheon, the streets along which I used to run errands, the country roads we took when it was fine.”

Nothing conjures up a memory so viscerally as the scent with which you associate it. While it’s been understood for some time that our olfactory system has a unique ability to vividly summon memories, the mechanism behind the phenomenon has net been well-understood. Now a study by researchers from Northwestern University’s Feinberg School of Medicine may have solved the puzzle. The olfactory system has an unusually direct connection to the brain’s hippocampus, believed to play an important role in memory.

The study’s published in the journal Progress in Neurobiology.

Credit: schankz/Adobe Stock

Previous neuroimaging and intracranial electrophysiology investigations have revealed that our senses are functionally connected to the hippocampus, if not directly. However, the new research, for which the principle investigator is Christina Zelano, is the first rigorous comparison of the strength of those connections.

It turns out that our primary olfactory cortex is a sense that’s still directly connected to the hippocampus.

“This has been an enduring mystery of human experience,” Zelano tells Medical Xpress. “Nearly everyone has been transported by a whiff of an odor to another time and place, an experience that sights or sounds rarely evoke. Yet, we haven’t known why. The study found the olfactory parts of the brain connect more strongly to the memory parts than other senses. This is a major piece of the puzzle, a striking finding in humans. We believe our results will help future research solve this mystery.”

It’s believed that during evolution, the hippocampus’ role shifted away from its original strong relationship to the sensory cortexes and toward connections with higher association cortexes. (In rodents, for example, the hippocampus maintains a powerful connection to all sensory cortexes.) It now appears that as this occurred, the olfactory cortex alone continued to be directly wired to the hippocampus.

“Humans experienced a profound expansion of the neocortex that re-organized access to memory networks,” explains Zelano. “Vision, hearing and touch all re-routed in the brain as the neocortex expanded, connecting with the hippocampus through an intermediary-association cortex-rather than directly. Our data suggests olfaction did not undergo this re-routing, and instead retained direct access to the hippocampus.”

It’s known that people who experience a loss of smell, or “anosmia,” often develop depression. “Loss of the sense of smell is underestimated in its impact,” says Zelano. “It has profound negative effects of quality of life, and many people underestimate that until they experience it. Smell loss is highly correlated with depression and poor quality of life.”

Anosmia is also associated with COVID-19. “The COVID-19 epidemic,” says Zelano, “has brought a renewed focus and urgency to olfactory research.” Lead author Guangyu Zhou agrees: “There is an urgent need to better understand the olfactory system in order to better understand the reason for COVID-related smell loss, diagnose the severity of the loss and to develop treatments.”

“Most people who lose their smell to COVID regain it,” notes Zelano, “but the time frame varies widely, and some have had what appears to be permanent loss. Understanding smell loss, in turn, requires research into the basic neural operations of this under-studied sensory system.”

She notes that, “While our study doesn’t address COVID smell loss directly, it does speak to an important aspect of why olfaction is important to our lives: Smells are a profound part of memory, and odors connect us to especially important memories in our lives, often connected to loved ones.”