A study published in PLOS Biology by Jules Dejou and colleagues shows that neurons developing around the time of birth in the olfactory bulb granule-cell layer (a region of the brain) are the ones encoding the memories of scents associated with joy in childhood. A separate primer by Chloé Guillaume and Elisa Galliano published in PLOS Biology discusses these results. In the study, juvenile mice exposed to attractive odors while in an enriched playful environment showed a preference for that odor in adulthood more than mice exposed to the same odor while in normal housing.
Memories evoked by odors are often unusually vivid and emotionally powerful. This is particularly the case when they originate in childhood. Compared with memories triggered by visual or auditory cues, autobiographical memories triggered by odors tend to come from earlier periods of life and are more often associated with positive emotions. Repeated exposure to the same odor in a consistent emotional context can gradually create strong associations that persist for many years. Such memories may be linked not to a specific reward, but to a broader sense of comfort, happiness, or well-being associated with familiar childhood experiences.
Olfaction is especially important early in life because it is functional from birth and can guide essential behaviors before other sensory systems are fully developed. The olfactory system also has unusually direct connections with brain regions involved in memory, reward, and emotion, including the amygdala, hippocampus, orbitofrontal cortex, and related limbic structures (a network of brain regions that control emotion and memory).
Odor information is initially processed in the olfactory bulb, where granule cells (a type of small neuron) and other interneurons (neurons that transmit signals between other neurons) help refine sensory signals before they are transmitted to higher brain regions. Many of these olfactory interneurons are generated early in life and can be shaped by early sensory experience, making them plausible substrates for long-lasting odor memories.
To identify the neurons responsible for encoding olfactory memories from childhood and to understand the neural mechanism that allows them to play an important role in forming a person’s identity, the study authors started with a survey involving 647 human participants. Participants were asked to recall an odorant that was significant in their childhood, going back as far as possible in time. They were then asked to rate the pleasantness and the presence of six basic emotions (happiness, surprise, fear, disgust, sadness, and anger) associated with that smell.
Results of this survey showed that the ratings were highest for positive emotions, namely happiness and pleasantness, followed by surprise. Ratings of negative emotions were lower. Participants were also asked how many times the event that led to their earlier odor memories occurred. Seventy-three percent of participants reported that the event occurred more than five times.
Finally, participants rated the hedonic value (the degree to which something is considered pleasant or unpleasant) of their childhood odorant, which they found to be predominantly pleasant. The authors used another dataset to see whether those smells are intrinsically pleasant or whether participants only perceived them that way. The results indicated that those smells do tend to be intrinsically pleasant, indicating that individuals prefer to make pleasant odors a part of their autobiographical memory.
Based on the human survey results, the researchers developed a mouse model. They divided 23-day-old (juvenile) mice into two groups. One group was exposed up to their 33rd day of life to an attractive odor such as limonene, citronellol, or camphor while the mice were in a positive, enriched, playful environment.
This pairing between the odor and the environment was repeated five times during the 10 days of the experiment. It was done every other day for 2 hours. The second, control group of mice was also exposed to the same odor, but in their ordinary housing. The study authors assessed the emotions of the mice by registering their ultrasonic vocalizations, with more numerous and higher frequency vocalizations indicating a more positive emotional state.
Later, when the mice were 2 months old (i.e., young adults), the study authors tested whether they showed a preference for that odor. The mice that learned to associate the odor with an enriched environment and positive emotions indeed showed a greater preference for it compared to mice that experienced it in their ordinary housing. The researchers observed that this memory recall relied on increased functional connectivity in the brain’s reward system.
Investigation of the neural basis of this smell memory found that neurons born around the first day of life in the olfactory bulb granule-cell layer tended to be particularly activated by the odor the mice learned to associate with positive emotions as juveniles. When these neurons were silenced using optogenetics (a biological technique that uses light to control neurons that have been genetically modified to express light-sensitive ion channels), the preference for the odor was reduced, suggesting that these neurons are a key part of the early memory recall mechanism.
However, by 6 months of age, that memory was gone, unless the mice were periodically re-exposed to the scent. In these older mice, memory persistence was no longer dependent on the neonatal-born granule cells. Instead, the persistence of the memory was associated with a large-scale reorganization of the brain’s functional networks, shifting away from the reward system to rely on strengthened connections within the olfactory-limbic system.
The study contributes to the scientific understanding of childhood odor memories and their role in overall psychological functioning. However, it should be noted that while the initial survey was conducted on humans, the experimental manipulations were conducted on mice. While mice and humans share many physiological similarities, they are still very different species, and the exact neural network mechanisms in humans might differ.
The primer, “Neurons generated shortly after birth encode the scent of early-life happiness,” was authored by Chloé Guillaume and Elisa Galliano. The 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.