Biological complexity: how we smell odors; very well (Introduction)

by David Turell , Monday, March 30, 2020, 21:03 (1488 days ago) @ David Turell

As a highly advanced species we have not lost our sense of smell, and it is not fooled like our vision can be:

https://aeon.co/essays/why-might-it-be-easier-to-fool-your-eyes-than-your-nose

Your nose is the best biosensor on the face of the Earth. This claim must sound counterintuitive since the sense of smell has acquired a rather poor reputation over the past centuries.

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"Recent scientific advances have debunked several myths about smell. First, human olfactory physiology is not in evolutionary decline. In 2017, a review in Science finally set the record straight by analysing contemporary research in olfaction. Although smaller in proportion to overall body mass, the olfactory bulb (the first cortical structure of the olfactory pathway) in humans has just as many neurons as in rodents. Further, the bulb is one of the most densely populated neuronal areas of the brain. It thus depends on how you measure size and define proportions. (my bold)

"Second, the sense of smell continues to be important to human cognition and to culture. Cross-cultural studies about language use have shown that other societies, such as the Jahai and Maniq in Southeast Asia, have extensive odour vocabularies and rites. Likewise, in the Western hemisphere, the fragrance industry has been successively expanding.

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"How good is your sense of smell, really? The human nose has been measured to detect minuscule amounts of molecules in complex chemical mixtures. Humans can sense the presence of particular odorants (smelly molecules) in dilutions of less than one part in several billion parts of air. Take the case of corked wine, which is primarily caused by the compound 2,4,6-trichloroanisole, in short: TCA. Although sensitivity to this compound varies between individuals, the threshold for its detection lies in the 10 to single-digit parts per trillion.

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"Why did the nose evolve to be so sensitive? Most of the molecules you smell could kill you in high enough concentrations, in addition to your chemical environment changing constantly. Humans, in particular, move habitats a lot. Their sense of smell thus needs to be fairly sensitive and flexible to measure the largely unpredictable chemical constituents of changing environments. So it’s good to have such a fine-tuned instrument monitoring the fluctuating chemical concentrations in your surroundings. It’s just not helpful to be aware of its workings all the time. Sitting in the backseat of conscious awareness, we simply don’t pay much attention to what the nose knows. Olfaction further does not fit well in traditionally visuocentric understandings of the nature of perception.

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"Varying sensitivities to odorants, including androstenone, have notably been linked to genetic differences in the olfactory receptor repertoire of people (most prominently in a recent series of studies). To be sure, diversity of olfactory receptor genetics is not the only causal factor explaining the variations in sensitivities to odorants. Other factors include a person’s familiarity with an odour, as well as age and sensitivity training.

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"Illusions provide us with examples of how our visual system can be fooled based on its distinct causal set-up.

"By comparison, it is much harder to trick your nose. Certainly, olfaction is much more variable in its perceptual expression than vision. But this variation doesn’t mean that the nose has been misled. Olfaction simply does not work analogously to the visual system. Crucially, feature coding in smell is not viewpoint-invariant. On the contrary, the causal principles of the olfactory system facilitate a cue-dependent interpretation in the computational integration of its neural signals.

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"Received philosophical analysis approaching the objectivity of the senses as ‘one percept matching one stimulus’ proved an ill-defined artifact of a prescientific intellectual tradition. It obscures our understanding of smell. It bypasses a lot of other sensory sensations, including the hidden senses of proprioception and interoception. And it even obscures genuine understanding of vision. In effect, it is their causal principles and mechanisms – not some naive input-output pairing that treats the sensory system as a black box – that determines how our senses grant us access to reality. To understand perception across all senses, including perceptual constancy as well as variation, requires a much more detailed look at the actual processes that connect the world with our mind. Only that way might we get to understand both."

Comment: Our nose really knows, as a very specialized part of our brain. Note my bold above.