Human sense of smell is considered to be underdeveloped, but this is groundless. The events of the past two years, in which millions of people lost their sense of smell to COVID-19, have put the spotlight on research emerging from the world of olfaction.
The experience of a smell sparking a vivid memory will be familiar to many people. The smells of things remain suspended a long time, causing us to time travel in our mind and evoke past emotion. How it is possible is something that scientists are trying to understand.
The study of how smells influence memory in humans has long been an interesting area of research.
Rodents are able to navigate mazes guided by memories of odours; it has allowed to scientists to get a sense of how neurons in the grain store this information, and some insights into phychological elements of odour memories in human.
It is considered by no few scientists that Smell is very deeply ingrained in our emotional memory, and that architecture of the brain itself is a clue to how tightly connected odours are to memories. While sound generates a signal which is conveyed from our ears to the brainstem, then up to a part of the brain called the thalamus, before finally reaching the auditory cortex, sensing smells connection to the brain is less circuitous: smell-sensing neurons in the nose extend directly to the olfactory bulb of the brain, from which they can be passed on to other brain regions, including areas involved in memory.
The sense of smell is specific, which helps to explain how our smell memories can be so precise. Humans have more than 400 types of olfactory receptor. This affords us a tremendous amount of olfactory detail, and our nervous system needs to categorize all of that smell input. It has been suggested 1 that just as there are five senses of taste (sweet, salty, sour, bitter and umami), there are ten basic dimensions of smell, such as fruity, nutty, woody and citrus 1; and maybe there are more, because it was just assayed a tiny fraction of the full spectrum of smells.
It has been demonstrated the existence of two memories function in the brain: short-term and long-therm odours memories.
Experiments with mice 2, showed that the gene-activity of their olfactory sensory neuron have different patterns after two hours after exposing to the odour, each olfactory sensory neuron (OSN) harbors a distinct transcriptome whose content is precisely determined by interactions between its odorant receptor and the environment, being this transcriptional variation systematically organized to support sensory adaption: expression levels of different genes relevant to transforming odors into spikes continuously vary across OSN subtypes, dynamically adjust to new environments over hours, and accurately predict acute OSN-specific odor responses. The sensory periphery therefore separates salient signals from predictable background via a transcriptional rheostat whose moment-to-moment state reflects the past and constrains the future. Exposure to odours would trigger smell-sensing cells to boost the activity of genes that attenuated their responses to those same odours.
In other words, when neurons pick up a scent, they become less sensitive to it in the short term — “filtering out the expected to emphasize the new”. Many people experience this as getting used to a smell in their environment and becoming temporarily unaware of it.
1.Castro JB, Ramanathan A, Chennubhotla CS (2013) Categorical Dimensions of Human Odor Descriptor Space Revealed by Non-Negative Matrix Factorization. PLoS ONE 8(9): e73289. https://doi.org/10.1371/journal.pone.0073289
2.Tsukahara T, Brann DH, et al. (2021). A transcriptional rheostat couples past activity to future sensory responses. Cell 184(26)6326.
3.Pashkovski, S.L., Iurilli, G., Brann, D. et al. (2020). Structure and flexibility in cortical representations of odour space. Nature 583, 253–258. https://doi.org/10.1038/s41586-020-2451-1