AKA The Evolution of the Autobiographical Odor Encyclopedia
This study copied below measures how fast we detect good smells vs bad smells (spoiler, bad smells are detected faster).
But while reading through this, consider that bad smells can become good over a series of exposures matched with good feelings. Aged cheese, fermented cabbage, and burned cannabis are pretty well known examples of this. There's also people, who smell, each with our own odor fingerprint, although we may not realize it at times, as it might be below our limit of detection.
And then there's the reverse, where things (or people) that once smelled good, all of the sudden smell bad, such as with changes in birth control, or pregnancy, or after a viral infection like Covid (see the parosmia triggers study). In those cases, the whole olfactory system is rewritten, a kind of blank slate re-learning, where smells with strong odor components (like individually unique body odors, or coffee) are perceived as if for the first time, with the bad stuff up front. And all you can focus on is the bad, since you have to "re-learn" the smell, and how the good integrates with the bad to produce something that is neither good, nor bad, nor even identifiable by semantic description, but only by the name of the person.
Nonetheless, there seem to be some good millisecond metrics here:
Seeing how odor is processed in the brain
Jun 2022, phys.org
- Detection occurred before the odor was consciously perceived by the participant
- Odor information in the brain is unrelated to perception during the early stages of being processed
- Later, unpleasant odors were processed more quickly than pleasant odors
The participants wore an EEG cap while having smells shot at their face, and so that researchers could see when and where odors are processed in the brain.
"We were surprised that we could detect signals from presented odors from very early EEG responses, as quickly as 100 milliseconds after odor onset, suggesting that representation of odor information in the brain occurs rapidly"
Remember that the olfactory system has only a few synapse-steps, making it the most direct sensory system we have.
And then watch how they pretty much rehearse Proust's deep cookie immersion:
When unpleasant odors (such as rotten and rancid smells) were administered, participants' brains could differentiate them from neutral or pleasant odors as early as 300 milliseconds after onset. However, representation of pleasant odors (such as floral and fruity smells) in the brain didn't occur until 500 milliseconds onwards, around the same time as when the quality of the odor was also represented. From 600–850 milliseconds after odor onset, significant areas of the brain involved in emotional, semantic (language) and memory processing then became most involved.
via University of Tokyo: Mugihiko Kato et al, Spatiotemporal dynamics of odor representations in the human brain revealed by EEG decoding, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.211496611
Professor Robert Sapolsky Stanford Lecture - On Recognizing Relatives (with smell)
Learning to Smell: Olfactory Perception from Neurobiology to Behavior, by Donald Alan Wilson and Richard J. Stevenson, Johns Hopkins University Press (2006)