Neuroscientists map how early-life scent memories evolve and move through the brain

Neuroscientists have discovered how early-life scent memories develop and travel through the brain. A study on mice published in PLOS Biology suggests that these memories are initially stored by specific neurons that form just after birth. As time passes and the scent is encountered again, the memory trace shifts to broader networks in the brain.

The findings provide insight into how the brain encodes, maintains, and later reorganizes cherished childhood memories. Scent-triggered memories are well-known in psychology, with the famous example of Marcel Proust's madeleine experience. In humans, autobiographical memories tied to scents typically originate during the first decade of life and tend to have a stronger, more positive emotional impact than memories triggered by sights or sounds.

When a person or animal smells something, the information first enters the olfactory bulb, a structure at the front of the brain, where specific neurons called granule cells help process the scent. In rodents, a large number of these neonatal neurons are born on the first day of life and tend to survive for a long time, making them potential candidates for storing early-life scent memories.

Scientists have also known that memories do not remain static over time, often shifting between different brain areas as they are encoded and stored. The study's lead author, Nathalie Mandairon, a researcher at the French National Centre for Scientific Research, was intrigued by why some early scent memories persist for decades while others fade, and what brain mechanisms allow these early olfactory memories to endure or resurface later in life.

To investigate, the researchers surveyed 647 adults about their earliest scent-based memories, finding that these memories typically date back to childhood before the age of 10 and were often associated with a pleasant odor and a positive context. The team then developed a mouse model to study the brain mechanisms involved in forming and sustaining early olfactory memories.

During the mouse equivalent of childhood, the mice were placed in a stimulating environment and exposed to a specific attractive odor multiple times. As young adults, the mice showed a stronger preference for the childhood odor compared to a control group. Brain scans revealed that specific granule cells born on the mice's first day of life played an important role in forming this memory, working with the brain's reward system.

When the researchers used light to temporarily silence these early-born granule cells, the mice no longer showed a preference for the childhood scent, confirming their necessity for the memory. The researchers also found that the memory persisted only if the mice were occasionally re-exposed to the scent during adulthood. Without such re-exposure, the preference for that particular odor disappeared.

However, briefly re-exposing the mice to the scent every few weeks allowed the positive memory to persist.