Introducing the brain: how odors are interpreted (Introduction)

by David Turell , Thursday, October 15, 2020, 17:17 (1498 days ago) @ David Turell

Further discussion:

http://nautil.us/issue/91/the-amazing-brain/our-mind_boggling-sense-of-smell?mc_cid=116...

"Our sense of smell offers an intriguing challenge to the mapping paradigm in vision. The nose is tailored to measure an unpredictable chemical array in our environment, allowing the brain to evaluate when minute molecular traces shift behavioral meanings to signal pleasure, danger, or novelty.

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"The reason why has to do with an intriguing feature specific to the olfactory system. That is, it takes only two synapses for information to travel from the air, through your nose, and to the core cortex, deep in the brain. To put this into perspective: two synapses won’t get you out of the retina in the visual system. You cannot find a more direct route bringing the brain in touch with the world!

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"The function of the olfactory system is not to map chemicals discretely in space. Instead, it tracks and computes the statistics of a changing chemical environment: How many, in what concentration, and how often do chemicals co-occur as a molecular cloud? (Our nose is tuned to detect blends of different chemicals—coffee aroma, for example, consists of 800 different compounds.) Simply consider the system’s mind-boggling capacity: If you calculate all possible combinations of structure-receptor interactions in smell (with one molecule hypothetically activating 100 receptors), you’d end up with a number higher than atoms in the galaxy.

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"The olfactory receptors happened to be the most structurally diverse and sizable member of the largest multigene family of protein receptors in the mammalian genome (so-called G-protein coupled receptors—or GPCRs in short). Their striking diversity and sheer size—olfactory receptor genes have more “genetic storage” than the immune system, occupying about 4 percent of the mammalian genome—caught the attention of scientists interested in mechanisms of gene recombination and the evolutionary diversification of biological processes. How did some biological entities, like proteins, evolve to facilitate a variety of functions?

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"With only two synapses straight into the cortex, a full understanding of the olfactory brain looked imminent. Yet the apparent simplicity of the olfactory system soon proved deceptive. Today, three decades after the receptor discovery, the mechanisms of odor perception continue to flummox researchers. Rather than close in on a full understanding, we merely started to appreciate the hidden complexity with which the brain makes sense of scents.

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"...odor quality is linked to structurally highly diverse chemicals. These chemicals have about 5,000 molecular features, not correlated in any straightforward manner to odor quality, that olfactory receptors (of which humans have about 400 types) pick up on to identify them.

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"All the brain “sees” (for a lack of a better term) is what receptors light up, how many of them, for how long, and in what combination and ratio. With about 400 types of receptors participating in a combinatorial coding of 5,000 molecular parameters, this turns out to be a rather sophisticated task.

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"The brain, drawing on memory, recognizes patterns in the chemical composition of the olfactory stimulus. This is one way for the olfactory system to foreground novelty, meaning the introduction of unknown (and potentially behaviorally relevant) compounds into an otherwise constant environment.

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"Such idiosyncrasies, like the weird complexity and variability of smell, now turn out vital to understanding the brain—how it maneuvers an organism through a landscape of fast-changing molecular combinations. The processing behind this is highly dynamic and radically prompt in answering how, what, and when to choose. The brain is not simply a projection screen. It is fundamentally a pattern-recognition device. Tracking the chemical statistics of an ever-changing environment needs a simple solution to a complex problem—an overly specialized map may even be disadvantageous here. Our brains evolved from our bodies, after all, not the other way around."

Comment: Amazing problem. It must involve memory of new odors. The newborn baby comes into the world with its memory system not yet formed. So the memory recognition process has to develop as the child grows older and we obviously learn how to identify odors. It is so complex, as this article shows, we still do not yet understand how the brain does its recognition job. Not by chnce. Only design will create this.