The scent of a particular perfume or a childhood home can often bring back vivid, emotionally charged memories from a person’s earliest years. A recent study on mice suggests that these early-life scent memories are initially stored by specific neurons that develop just after birth, but as time passes and the scent is encountered again, the memory trace relocates to broader networks in the brain. The research, published in PLOS Biology, provides evidence for how the brain encodes, sustains, and eventually reorganizes our most cherished childhood memories.
Scent-triggered memories are a well-documented psychological phenomenon, famously described by Marcel Proust as a flood of vivid nostalgic feelings brought on by the smell of a madeleine cake. In humans, autobiographical memories linked to scents tend to originate from the first decade of life. These memories also tend to carry a much stronger, more positive emotional tone than memories triggered by sights or sounds.
Supporting this idea, a 2016 review indicated that scents tied to positive personal memories have a uniquely powerful impact on a person’s mood, often increasing positive emotions and reducing physiological signs of stress. Despite this well-known emotional potency, the exact ways the brain creates and stores these long-lasting scent memories have remained somewhat mysterious.
When a person or animal smells something, the information first enters the olfactory bulb, a small structure at the front of the brain. Within this structure, specific neurons called granule cells help process the scent. In rodents, a massive wave of these granule cells is born on the very first day of life. Because these neonatal neurons are highly active during childhood and tend to survive for a long time, scientists suspected they might be ideal candidates for storing early-life scent memories.
Memory researchers also know that the brain’s storage systems do not remain static. According to a framework discussed in a 2018 review, memories often shift between different brain areas as time passes. A memory might initially rely on specific regions, like the hippocampus, before being reorganized and transferred to broader networks in the brain’s outer layers.
“I have been interested in odor perception and in the mechanisms of brain plasticity underlying olfactory learning since my PhD in Lyon, and later during my postdoctoral research at Cornell University,” said study author Nathalie Mandairon, a director of research at the French National Centre for Scientific Research (CNRS) based at the Lyon Neuroscience Research Center. “This study therefore follows naturally from my previous research.”
“The question that particularly intrigued me was this: during childhood, we are exposed to a multitude of odors,” Mandairon continued. “Why do some of them remain embedded in our memory for decades, while others disappear? And what brain mechanisms allow these very early olfactory memories to persist or resurface later in life?”
To find out, the research team began by surveying 647 adults about their earliest scent-based memories. “We found that this memory generally dated back to childhood, before the age of 10,” Mandairon explained. “They were most often associated with a pleasant odor, but also with a particularly positive, sometimes joyful, context.”
“In most cases, the experience had also been repeated several times during childhood,” she added. “Based on these observations, we developed a mouse model to investigate the brain mechanisms involved in the formation and long-term maintenance of this early olfactory memory.”
During the mouse equivalent of childhood, the researchers placed the animals in a large, enriched cage filled with toys, tunnels, and opportunities for social interaction. They paired this playful environment with a specific attractive odorant over five separate sessions. Microphones recorded the mice emitting a higher number and frequency of high-pitched squeaks, known as ultrasonic vocalizations, which indicates a positive emotional state.
When the mice reached young adulthood, at two months of age, the researchers tested their reactions to the childhood odorant. The mice that had experienced the scent in the playful environment spent more time investigating it compared to a control group that had smelled the same scent in a standard, non-playful cage.
To understand what was happening in the brain at this two-month mark, the researchers looked at granule cells born on the mice’s first day of life. “Our results revealed an important role for neurons in the olfactory bulb that are generated at birth,” Mandairon said. “These neurons are highly plastic and, together with the brain’s reward system, contribute to the memory of an odor learned very early in life.”
To test whether these neurons were strictly necessary for the memory, the scientists used optogenetics, a technique that allows researchers to turn specific brain cells on or off using light. When they used light to temporarily silence these early-born granule cells, the mice no longer showed a preference for the childhood scent.
The researchers also examined broader brain activity in the young adult mice. They found that smelling the childhood odorant triggered highly synchronized activity between the brain’s reward centers and memory systems. This network included areas like the dorsal hippocampus, which is involved in detailed memories, and the medial prefrontal cortex, which handles complex decision-making.
Next, the team investigated how this memory held up as the mice aged into later adulthood, at six months old. “We also found that, later in life, mice retained the memory of this odor only if they had been occasionally re-exposed to it during adulthood,” Mandairon told PsyPost. “Without such re-exposure, their preference for that particular odor disappeared.”
However, if the researchers briefly re-exposed the mice to the scent every three weeks, the positive memory persisted at six months of age. Surprisingly, this long-lasting memory no longer relied on the neonatal granule cells in the olfactory bulb. The light-based silencing technique that worked at two months had no effect at six months.
The researchers had expected a brain structure so directly involved in processing odors to continue playing a central role in olfactory memory. “Instead, over time, other brain circuits, particularly those associated with emotions, seem to take over,” Mandairon said.
“What is particularly interesting is that the brain circuits supporting the memory appear to change over time,” she explained. “In adulthood, the contribution of the olfactory bulb and the reward system decreases, while regions of the limbic system, which is strongly involved in emotions, become more important. In other words, the memory can persist, but the brain mechanisms supporting it seem to evolve throughout life.”
These network shifts are in line with a study covered by PsyPost in 2016, which found that early-life memories in young rats persist into adulthood as latent traces that can be reactivated by later reminder cues. The findings also align with another 2016 study covered by PsyPost, which found that worms form lifelong olfactory memories using specialized neural circuits. It is worth noting, however, that both of those previous studies examined the formation of aversive memories in response to threats, whereas the current study measured the development of positive odor associations in mice.
One detail to keep in mind is that modeling human autobiographical memory in rodents comes with inherent limitations. “Although the anatomical and functional organization of the olfactory system shares many similarities between mice and humans, the study was conducted using a mouse model,” Mandairon cautioned. “Our study primarily allows us to identify general principles and brain mechanisms that can then be investigated further in humans.”
Moving forward, the research team hopes to uncover the specific conditions that make some scent associations permanent while others fade. “We would like to determine whether there is a common ‘recipe’ that allows an odor encountered during childhood to become a memory that stays with us for a lifetime: what the odor was, the context in which it was encountered, who we were with, the emotions we experienced,” Mandairon said.
The study, “Positive early-life olfactory memory is rooted in the olfactory bulb and triggers large-scale changes beyond the olfactory system,” was authored by Jules Dejou, Anna Athanassi, Théo Brunel, Marc Thevenet, Anne Didier, and Nathalie Mandairon.