The Nonlinear Library LW - I'm confused about innate smell neuroanatomy by Steven Byrnes
Nov 29, 2023
12:34
Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: I'm confused about innate smell neuroanatomy, published by Steven Byrnes on November 29, 2023 on LessWrong.
(This post is probably only of interest to neuroscientists. I'm mostly writing it in the hopes that someone more knowledgeable will chime in and help me out. There's a comments section at the bottom, or email me.)
tl;dr
In animals, specific innate reactions are reliably triggered by corresponding specific smells - for example, odors associated with natural predators tend to trigger avoidance behavior, even in the absence of any prior experience of those odors. In order for this to work, I think odor information needs to get from the nose to either the hypothalamus or brainstem, without passing through any of a long list of regions that includes the amygdala and the whole cortex.
I'm struggling to figure out what this pathway is, if any. I offer my best current guesses as to what's going on.
Background
Why I expect direct projections of smell (like all other senses) to the "Steering Subsystem"
It's well-known that animals have numerous specific innate reactions that are triggered by specific smells. For example, odors associated with species-typical predators or unhealthy food may trigger avoidance, odors associated with species-typical healthy food may trigger approach and eating, odors emitted by conspecifics may trigger mating, aggression, or other behaviors, and so on.
Meanwhile, I continue to believe that a large fraction of the brain, which I call the "Learning Subsystem", including the whole cortical mantle, striatum, cerebellum, and some other stuff, "learn from scratch", a term that I'm using in a very specific way defined
here; and meanwhile I think the rest of the brain, which I call the "Steering Subsystem", particularly including the hypothalamus and brainstem, is a repository of innate
"business logic" such as "if I'm fertile, increase my sex drive", as discussed
here.
For sensory input processing, there's a nice story that goes along with that two-subsystems picture. The sensory input (I claim) has to split, with one copy going to the Learning Subsystem, and another going to the Steering Subsystem. The former system treats the input as input data for a learning algorithm, and the latter system uses that input to calculate specific ecologically-relevant things to trigger corresponding reactions. This split is critical, for theoretical reasons explained in
§3.2.1 here (I won't repeat it here). And this hypothesis seems to work really well for other senses: For example, visual information goes both to visual cortex in the Learning Subsystem and the superior colliculus in the Steering Subsystem; taste goes to both gustatory cortex in the Learning Subsystem and the gustatory nucleus of the medulla in the Steering Subsystem; and so on.
Relevant basics on smell neuroanatomy
…But I'm more confused about smell - particularly how it gets to the Steering Subsystem.
Let's start with some background on smell. The first step is "olfactory sensory neurons" which can actually detect odorants. "The sensory neurons are embedded in a specialized olfactory epithelium that lines part of the nasal cavity, approximately 5 cm2 in area in humans.
… The axons of olfactory sensory neurons project to the ipsilateral olfactory bulb [where they] terminate on the dendrites of olfactory bulb neurons within bundles of neuropil called glomeruli that are arrayed over the bulb's surface….
In each glomerulus, the sensory axons make synaptic connections with three types of neurons: mitral and tufted projection (relay) neurons…and periglomerular interneurons, which encircle the glomerulus.…In each glomerulus, the axons of several thousand sensory neurons converge on the dendrites of approximately 40 to 50 relay neurons. … Each glomerulus, and each mitral and tufted relay neuron connect...
(This post is probably only of interest to neuroscientists. I'm mostly writing it in the hopes that someone more knowledgeable will chime in and help me out. There's a comments section at the bottom, or email me.)
tl;dr
In animals, specific innate reactions are reliably triggered by corresponding specific smells - for example, odors associated with natural predators tend to trigger avoidance behavior, even in the absence of any prior experience of those odors. In order for this to work, I think odor information needs to get from the nose to either the hypothalamus or brainstem, without passing through any of a long list of regions that includes the amygdala and the whole cortex.
I'm struggling to figure out what this pathway is, if any. I offer my best current guesses as to what's going on.
Background
Why I expect direct projections of smell (like all other senses) to the "Steering Subsystem"
It's well-known that animals have numerous specific innate reactions that are triggered by specific smells. For example, odors associated with species-typical predators or unhealthy food may trigger avoidance, odors associated with species-typical healthy food may trigger approach and eating, odors emitted by conspecifics may trigger mating, aggression, or other behaviors, and so on.
Meanwhile, I continue to believe that a large fraction of the brain, which I call the "Learning Subsystem", including the whole cortical mantle, striatum, cerebellum, and some other stuff, "learn from scratch", a term that I'm using in a very specific way defined
here; and meanwhile I think the rest of the brain, which I call the "Steering Subsystem", particularly including the hypothalamus and brainstem, is a repository of innate
"business logic" such as "if I'm fertile, increase my sex drive", as discussed
here.
For sensory input processing, there's a nice story that goes along with that two-subsystems picture. The sensory input (I claim) has to split, with one copy going to the Learning Subsystem, and another going to the Steering Subsystem. The former system treats the input as input data for a learning algorithm, and the latter system uses that input to calculate specific ecologically-relevant things to trigger corresponding reactions. This split is critical, for theoretical reasons explained in
§3.2.1 here (I won't repeat it here). And this hypothesis seems to work really well for other senses: For example, visual information goes both to visual cortex in the Learning Subsystem and the superior colliculus in the Steering Subsystem; taste goes to both gustatory cortex in the Learning Subsystem and the gustatory nucleus of the medulla in the Steering Subsystem; and so on.
Relevant basics on smell neuroanatomy
…But I'm more confused about smell - particularly how it gets to the Steering Subsystem.
Let's start with some background on smell. The first step is "olfactory sensory neurons" which can actually detect odorants. "The sensory neurons are embedded in a specialized olfactory epithelium that lines part of the nasal cavity, approximately 5 cm2 in area in humans.
… The axons of olfactory sensory neurons project to the ipsilateral olfactory bulb [where they] terminate on the dendrites of olfactory bulb neurons within bundles of neuropil called glomeruli that are arrayed over the bulb's surface….
In each glomerulus, the sensory axons make synaptic connections with three types of neurons: mitral and tufted projection (relay) neurons…and periglomerular interneurons, which encircle the glomerulus.…In each glomerulus, the axons of several thousand sensory neurons converge on the dendrites of approximately 40 to 50 relay neurons. … Each glomerulus, and each mitral and tufted relay neuron connect...