A new study published in PLOS Biology shows that neurons born around birth in the olfactory bulb’s granule-cell layer are the ones that encode memories of childhood scents associated with happiness. A linked primer by Chloé Guillaume and Elisa Galliano discusses the same results. In the core experiment, juvenile mice exposed to appealing odours within an enriched, playful environment developed a stronger preference for those smells in adulthood than peers exposed in standard housing.
Memories triggered by odours are often unusually vivid and emotionally potent, particularly when formed in childhood. They tend to be autobiographical and linked with positive emotions more often than memories sparked by other senses. Repeated exposure to the same scent within a stable emotional context can forge durable associations that endure for years, sometimes encompassing a general sense of comfort or well-being rather than a single reward.
Olfaction plays a crucial role early in life because it functions from birth and helps guide essential behaviours before other senses are fully developed. The olfactory system has especially direct connections with brain regions involved in memory, reward and emotion, including the amygdala, hippocampus, orbitofrontal cortex, and related limbic structures.
Odour information is initially processed in the olfactory bulb, where granule cells and other interneurons refine sensory signals before transmission to higher brain areas. Many of these interneurons are generated early in life and can be shaped by early sensory experience, making them plausible substrates for long-lasting odor memories.
In a human component, 647 participants were asked to recall a childhood odour that was meaningful to them and to rate its pleasantness and associated emotions—happiness, surprise, fear, disgust, sadness and anger. Results showed the highest ratings for positive emotions, particularly happiness and pleasantness, with negative emotions rated lower. Seventy-three per cent of participants said the event behind the memory occurred more than five times.
Participants also rated the hedonic value of their childhood odour as predominantly pleasant. A separate dataset indicated that these smells tend to be intrinsically pleasant, not merely perceived as such by the participants.
Researchers then translated the human findings into a mouse model. Twenty-three-day-old juvenile mice were divided into two groups. One group was exposed to attractive odours—such as limonene, citronellol or camphor—while in a positive, enriched environment. The pairing was repeated five times over ten days, with two-hour sessions every other day. A control group experienced the same odours in ordinary housing.
Emotions in the mice were assessed by measuring ultrasonic vocalisations, with more frequent and higher-pitched calls interpreted as signs of a more positive emotional state.
When the mice reached two months of age, they were tested for a preference for the odour they had learned to associate with the enriched environment. Those with the enriched pairing showed a greater preference for the odour than the controls, and the researchers linked this memory recall to increased functional connectivity within the brain’s reward system.
Neuronal analysis revealed that neurons born on the first day of life in the olfactory bulb’s granule-cell layer tended to be particularly activated by the odour associated with positive emotions during youth. When these neonatal-born neurons were silenced using optogenetics, the odour preference diminished, suggesting these cells play a crucial role in early memory recall.
By six months of age, the memory had largely faded unless the scent was periodically re-exposed. In older mice, memory persistence no longer depended on the neonatal-born granule cells; instead, it correlated with a broad reorganisation of the brain’s functional networks, shifting from the reward system to strengthened connections within the olfactory-limbic circuit.
The study advances understanding of childhood odour memories and their potential impact on psychological functioning. It is important to note that while the initial survey involved humans, the experimental work was conducted in mice, and the exact neural mechanisms in people may differ.
The primer, Neurons generated shortly after birth encode the scent of early-life happiness, was authored by Chloé Guillaume and Elisa Galliano. The main paper, 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.
