03/25/09 - Also known as:
The Saga of the Amygdala(But not
this Modest Proposal)
In spite of what F. Scott Fitzgerald
says "The test of a first-rate intelligence is the ability to hold two opposed ideas in the mind at the same time, and still retain the ability to function."
I am still of the opinion that we are
not able to do so. The reason I hold this opinion is the result a model which I feel partially describes human neurological functioning. On the other hand, because we communicate in words, a description of this model is fraught with potential for miscommunication. However, I would be negligent if I did not even try to express these ideas. [I still contend that we need a "lexicon" in these discussions so that when we use a word (such as
cause, or
miscommunication) we all agree on its meaning.]
The basis of the proposal is similar to much of what we have discussed before. That is, that the basis for an "idea," a "thought," or any other neural construct is a collection of electrochemical structures. These structures exist in a set and definitive configuration (which can, of course, change). But they cannot, under any circumstances, be both one way AND the other at the same time. I suppose this could be said to be "deterministic" in the way that word is used in philosophy.
What I propose is constructing a simple scheme for memory with the goal of showing that Aristotle was right (and, perhaps,
MK Joseph was off base) in that there cannot be two answers to any
real question, or two routes from the current moment.
A corollary of this is that we are not able to resolve paradox because of this way that our mind/thinking is constructed (sorry, F. Scott). It may also be why we have not encountered alien beings; they will "think" differently (as Lem suggests in "His Master's Voice.) Not that their thinking will violate basic chemical principals. That is not possible. But that the underlying "structure" will be different. In much the same way, it is possible (probable?) that whales are more intelligent than we are, but their "thinking" is so concrete, that they cannot "imagine" us as being similar. BTW: The weight of a sperm whale brain is 7.8 Kg; that of a human 1.4 Kg. Whatever.......
MODEL
A neural structure is defined as a collection of neurons each at a specific resting potential.
Memory is a collection of neural structures. New memories are new collections.
A new memory may incorporate old structures.
There is a one to one correspondence between objects in the “real” world and a neural structure (or collection of structures).
Neural structures are connected by neural pathways/networks. Neural pathways are solely electrochemical, i.e. action potentials.
A sensory perception is the entry of a cotemporal series of action potentials into the brain. This is defined as a percepton.
Perceptons interact with neural structures solely on an electrochemical basis.
Sensory perception always invokes memory.
Interaction of perceptons with neural structures generate secondary perceptons. They may be a modified version of the original or an entirely novel collection of action potentials.
Perceptons are cotemporal in a very narrow sense. There may be more than one secondary percepton.
The collection of all related perceptons can terminate (in a physical and temporal sense) in the motor cortex where they initiate action (firing of motor neurons generating movement, including speech).
Neural structures, i.e. memory, act as a filter, or modulating instrument, transforming perception into action.
However, perceptons do not always initiate action.
One needs to emphasize that this scheme is entirely on the molecular level. In other words, it does not depend on quantum processes and their uncertainties. The elements of the basic structure are neurons with variable baseline potentials. One does not need to evoke quantum effects to completely describe the neuron. Neurons only do one thing. Depending on a variable threshold (and how that threshold is adjusted constitutes memory) it is either at rest, or sends an action potential down the axon. These potentials are not, for instance, dependent on unknowable spin states and are therefore entirely predictable in theory. (If asked, I could address the scenarios of
Wilczek or
Hamerof).
Enter the
Amygdala
(Please let us not have any
Isabella Rossellini whale appendage comments here)
Now this
amygdala is an interesting part of the brain. It is small organ, consisting of discrete collections of "nuclei" (collections of neurons) whose function is to serve "a primary role in the processing and memory of emotional reactions...the amygdalae are considered part of the limbic system."
Importantly:
The flow of information through amygdala circuits is modulated by a variety of neurotransmitter systems. Thus, norepinephrine, dopamine, serotonin, and acetylcholine released in the amygdala influences how excitatory and inhibitory neurons interact. Receptors for these various neuromodulators are differentially distributed in the various amygdala nuclei. Also differentially distributed are receptors for various hormones, including glucocorticoid and estrogen. Numerous peptides receptors are also present in the amygdala, including receptors for opioid peptides, oxytocin, vasopressin, corticotripin releasing factor, and neuropetide Y, to name a few.
I say importantly because the only function of neurotransmitters is to modulate ion gates emphasizing that we are dealing solely with molecular chemistry without the necessity of quantum entanglement.
.....the microcircuitry of the amygdala with a special emphasis on its relevance to fear processing and fear learning.
As we have alluded to before (and is well described in books such as Sean Carroll's
Endless Forms Most Beautiful and Krischner and Gearhart's
The Plausibility of Life) these mechanisms and reactions are very ancient, over 500 million years old, and they have been conserved in multiple species. They are present in organisms as modules. How that works out for the human brain will certainly be the scientific breakthrough of the 21st century. (More on this later)
One last bit:
"One long-standing idea is the amygdala consists of an evolutionarily primitive division associated with the olfactory system (cortical, medial and central nuclei).."
One has to wonder how big the amygdala is in Elizabeth the
bloodhound.
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Alright, stay with me. I'll get to the point in a bit.
In a recent sojourn on a blog that routinely provides tasty morsels about scientific findings (
Not Exactly Rocket Science), I ran across this review of a paper on understanding
how memories are stored in the brain. Working with rats, the researchers showed how memory of a scary experience is stored in a the
amygdala utilizing a protein called
CREB (for cAMP Response Element Binding). CREB is is one of a number of transcription factors which modulate the expression of genes. How exactly CREB stores memory in the amygdala is not known but when neurons with CREB were destroyed in the rat amygdala, the rat had amnesia for fearful experiences. As the reviewer says:
Han's goal was to understand how memories are stored in the brain. Erasing them was just a step towards doing that.
It should be noted in passing that there has been a lot of interest in the action of
propanalol on this same system in humans. This comes from researchers who are studying post traumatic stress disorder. Blocking the human's bad feeling (not necessary the memory) of the traumatic event would, of course, be something worth while. I think at the moment the jury is out on these investigations. Interestingly enough, the mainstay of this research has been functional magnetic resonance imaging (fMRI). The validity of fMRI observations has been
called into question recently causing a mild tsunami in neurobiological circles. (This was
not missed by our intrepid correspondent.)
One other quote:
"CREB proteins in neurons are thought to be involved in the formation of long-term memories; this has been shown in the marine snail Aplysia, the fruit fly Drosophila melanogaster, and in rats. They are necessary for the late stage of long term potentiation."
Here is the point. Memory for a scary experience seems to be stored in the Amygdala dependent upon a single biological molecule CREB. Thus a memory can be only one thing. The result of this memory is that it acts as a filter for the percepton in that rats who experience a scary experience ("scary" being defined as having the memory) alter the percepton as it radiates to the motor cortex (probably triggering a number of other pathways and networks) to ultimately yield the reactions associated with fright. (Just like all those kids you scare at Halloween saying "BOO"; or, more to the point, raising those
scrunchy eyebrows.)
GEEK ALERT - DO NOT READ IF SUBJECT TO FITS OF PROJECTILE VOMITING WHEN CONFRONTED WITH CRAZY SPECULATION
Now, here is an interesting
finding: The total number of neurons in human amygdala about 12,000,000. I find this interesting because the total number of neurons in the whole human brain is about 100,000,000,000, i.e. 10,000 times the number in the Amygdala. How does such a small number of neurons store such a vast amount of information? Remember, reacting to life threatening predators is one of the oldest of reflexes. I'm not sure of this, but the dinosaur's brain which is the size of the walnut must be entirely Amydgala. I will leave it to the mathematical gurus to determine how you can contain so many "memories" in so few neurons.
Musings in the last paragraph has led me to think that neurons involved in memory in addition to having 1,000 to 5,000 dendtritic inputs, can have a variable threshold for the output action potential. Thus, a computer based on this concept would not have binary code because each "gate" could exist in multiple configurations, not "0" or "1." (On the other hand, one could go intracellularly and dissect out the metabolic states that contributed to the final threshold potential and these, eventually, would come down to an on-off state.)
END GEEK ALERT
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SummaryParadox is the simultaneous entertaining of two contradictory positions, both of which appear logical in isolation. We have proposed that the reason the human brain is not able to do this is because of the underlying neurobiochemical structure of the brain and its consequent process of thinking.
A series of postulates have been made defining a model of this process.
As evidence for this model we have reviewed the recent finding of the cessation of a response in rats to a scary stimulus after ablation of CREB containing neurons in the rat amygdala.
The ramifications of the model have not been explored.
(Photo courtesy of courtesy of Gayle Reynolds, Kentucky.) 
More likely, we would think, it refers to a visit by our little critter friends, Sarcoptes scabiei in the interstices of your toes:
Which bears a certain resemblence to our first entry of today:
The essence of Crab!
Please note the duck in the upper right corner. Or, is it a bee?
This unfortunate crab either has Beckwith-Wiedemann syndrome or has been rowing in a galley.
