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When mapping the different neural pathways in the brain, often pictures such as these are drawn:

map of neural pathways

Or similar versions. Clearly these sketches draw the neural pathways as being a two-sided connection, as can be judged from the double arrows. I was wondering, how such bidirectional pathways work. Specifically, I am interested to know whether individual neurons constitute such a bidirectional pathway between different parts of the brain, or if the bidirectional pathway can be thought of as a 'circuit': a pathway of neurons that eventually comes back to a certain part of the brain.

This question is especially interesting in light of abstract thoughts. Let's make some extremely generalising (and ludicrous) assumptions and assume that our conscious thoughts are governed by our hippocampus only. That is, we assume that thinking about a matter without input from sensory neurons causes the hippocampus to fire an action potential into some specific cluster of neurons (somewhere in the brain). E.g. we assume that thinking about the letter 'A' causes the hippocampus to fire an action potential into the cluster of neurons that represents the letter 'A'. Let's also assume that the memory we have regarding the letter 'A' must be returned to the hippocampus, in order to continue our thought process. For example, I might want to recite the alphabet in my head. Starting with the letter 'A', I then continue to the letter 'B', which arises in my conscious thoughts as a result of the firing of the cluster of neurons that contain information regarding the letter 'B'.

So if we assume the circuit, conscious thoughts $\rightarrow$ cluster of information fires $\rightarrow$ conscious thoughts to exist, then how would its feedback most likely work?

Let's take the example where we think of the letter 'A' and want to think of the letter 'B'. I have sketched two different theories about how the communication between these two clusters of information and the 'governing body of thoughts' (which I assumed to be the hippocampus), might be:

theories about neural pathways

The difference is the following: in the second theory, all individual neurons are expected to be able to communicate directly to the hippocampus. Whereas in the first theory, the feedback to the hippocampus arises only when we have reached our destination: the letter 'B'. Clearly, if we assume that a specific thought process is able to self-induce action potentials (of course, this is highly debatable), at least one of the two theories must be partially true. For which one exists evidence?

My first intuition would be to say that the first theory is more likely, but this is ambiguous under Dale's principle: why would only some cells be connected to the hippocampus? On the other hand, I found it hard to believe that every individual neuron is directly connected to the hippocampus.

So now that I have explained my thought process, my question can be formulated as follows:

How are clusters of neurons that are involved in abstract thoughts bidirectionally connected to the thought-governing-body (whichever actual part(s) of the brain this is)? Can we say that just a single neuron in the cluster provides the feedback to the thought-governing-body, or is every individual neuron that is involved in conscious thought potentially capable of a feedback loop?

Disclaimer: Yes, I'm not up-to-date to all the latest advances in neuroscience. Yes, I'm aware that my question might be ambiguous with respect to all the different types of neurons, connections and theory about neuronal networks and micronetworks. My question, however, is concerning the likelihood of the theories I presented. For which one do we have evidence. Are both wrong? If so, in what way? Are both right? If so, when does their difference play a role? etc.

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To give a simple answer to the first part, the feedback generally happens in circuits of at least two neurons. A synapse is generally one way, transmitting information from the presynaptic neuron's axon to the postsynaptic neuron's dendrite. I'm sure there's caveats and exceptions because biology always has that, but that's the norm. A simple feedback loop might look like this: A excites B which excites C. C inhibits A as negative feedback.

The second question relies pretty heavily on the assumption that conscious thought originates from a particular part of the brain, or that one neuron is dedicated to one concept. This doesn't seem to be true. Things like memory and thought appear to be distributed, emergent properties of several circuits and areas. They might be organized or integrated by a particular part of the brain but it's not the same as having one central executive looking at all the inputs deciding what you think.

Think of it like a person working on an Excel spreadsheet. There's all the code for the OS, the software, the formulas in the spreadsheet. Those are all important parts of the process but the user doesn't need to see any of that. All they need to see is the end products of each process, and then they can figure out what to do with those products. So circuits and neurons can be involved in lower processing without connecting directly to the area that organizes and integrates information. Also, because higher level processes like thought are so distributed, there's likely to be multiple centers organizing/integrating information and all giving feedback to each other. Does that make sense?

Side note: I think you might be misunderstanding Dale's principle. When it says that neurons have the same chemical action at every synapse, that means they release the same neurotransmitter(s) but that doesn't mean they have to be connected the same way or that the neurotransmitter release has the same results. For example, say a neuron releases the excitatory neurotransmitter glutamate. If it synapses on another neuron that releases glutamate, that will promote excitation. But if it synapses on an inhibitory neuron that releases GABA, then exciting that neuron will actually result in inhibition.

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  • $\begingroup$ Thank you for your answer. I understand that the brain processes information in different clusters but I can't grasp the theory that conscious thoughts are distributed over clusters of neurons. Taking the Excel spreadsheet example: even though we don't see the lower-level tasks, we are still presented with the lower-level's output. This is what my question is about: assuming that A and B are such lower-level processing clusters, they must report to some central place in order for the information to enter our conscious thought. Much like when I use an Excel formula, I am given the result. $\endgroup$ – Jean-Paul Nov 3 '15 at 18:56
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    $\begingroup$ The truth is that most of your brain is active most of the time. In order to get that clean picture that you see in the paper, they had to do a lot of statistics and filter out a lot of noise and hope that their baseline was decent. This is a big issue in functional brain imaging, especially fMRI, and it's pretty interesting so I'm going to paste some references at the bottom in case you want to check it out. That's not to say that localization isn't useful! But it has to be taken as one data point, not the whole picture. $\endgroup$ – K Gargiullo Nov 3 '15 at 19:57
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    $\begingroup$ ncbi.nlm.nih.gov/pmc/articles/PMC60127 An article on issues with fMRI baselines journal.frontiersin.org/article/10.3389/fnins.2012.00149/full Another on problems with reliability and data interpretation in fMRI mitpress.mit.edu/books/new-phrenology Great book about the limits of localization $\endgroup$ – K Gargiullo Nov 3 '15 at 19:58
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    $\begingroup$ Anyway, try to imagine things mapped as patterns of firing. For an oversimplified example, maybe the concept 'dog' isn't stored by cluster A, it's what you get when cluster A is firing at a low rate, cluster B is firing at a higher rate, and cluster C isn't firing beyond random baseline noise. If all 3 clusters fire at a high rate, you might get 'cat.' 'Dog' and 'cat' don't belong to any one of the clusters; they're stored by the pattern of firing from all 3. This is more complex, but it lets you store more concepts. That's why there's such an emphasis on the whole connectome thing right now. $\endgroup$ – K Gargiullo Nov 3 '15 at 20:01
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    $\begingroup$ As for where conscious thought comes from, that's kind of out of the scope of neurosceince right now. It's a fascinating question but it's difficult to impossible to study it. How do you define conscious thought? How do you measure it? We can pick at parts of it. The insula, for example, seems involved in self-awareness, and thought can light up areas involved with speech and language. But the overall concept of conscious thought is too abstract for us to handle right now. We need more information and more tools before we even know how to study the origins of conscious thought. $\endgroup$ – K Gargiullo Nov 3 '15 at 20:02

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