What happens in layers 5a and 6a?



By comparing these two cortical circuits it looks like displacement cells exist only in L5b and grid cells exist only in L6b.

What happens in L5a and L6a? Also, why do L2/3 merge?


Not directly answering your question but adding a different and related question:

The attention I see is strictly to “displacement” or “object” cells and how the L2/3 fits into this picture.

Why are there no questions about how this fits into map-2-map connections, thalamic-2-cortical connections, attention mechanisms, wave activation, and the other functions that are known to happen in the cortex?

For that matter - how about all the “other” cells types in the cortex - why is nobody asking how they fit into this pictures?


I’ve briefly read some of your posts in other threads addressing that problem in the overall HTM theoretical model that they’ve developed

do you think they are missing something in their L2/3 model that would prevent them from extending the thalamic-2-cortical connection for optimal performance between the layers?


Most of the cortical-2-cortical map connections involve L2/3.

As far as the thalamic-2-cortical connections please note that there are multiple circuits.

A major one terminates in L4. Speculation is that this is involved in attention and synchronizing waves of activity. HTM theory involves a synchronized “before” and “after” to work and these waves fit the bill perfectly.

The other circuit is involved in the lower layers and seems to be involved in hierarchy originating from the connections from the forebrain, and through that, to the sub-cortical circuits. Mixing in a huge level of speculation would suggest that this is orders/requests/filters from the dumb boss being distributed to the smart cortical adviser. in this model the smart adviser responds by providing a greatly digested version of the senses to the temporal lobe and through that, back to the EC/hippocampus/amygdala.


Are there any clues that something similar is happening in lower mammalian brains also? Would a brain be capable of “working” if it didn’t have that temporal like lobe highway through the EC/hippocampus/amygdala


What are your thoughts on why the sensory input passes through the thalamus? Do you think it does something different from the CTC connection to L4?
I see the feedback signal from higher order thalamus and non-lemniscal thalamus (matrix?) to primary cortex non-L4 (L2/3, L5 ST cells, or septa in L4) as better suited to attention because it can be facilitating for some of those, like L6 CT signals, which are thought to be involved in attention.

Which connections are you referring to specifically?
I’m starting to think that one type of L6 CT cell corresponds to a thalamic pathway which serves the initiation of the where pathway in primary cortex, tightly tied to motor copy signals, proprioception, or similar. This part of the thalamus might be modulated by the sensory input the same way that L6 CT cells modulate all thalamic cells.

For the primary cortex, maybe the dumb boss is subcortical where/how-related structures and the smart adviser is the sensory input and cortical feedback. A point on a map of saccade targets is a lot simpler/dumber than a representation of lines and whatever else, but needs advising on which points on that map are good targets for saccades.


I am only familiar with a limited set of critters (rat, cat, primate, corvid) in regards to the mammal and avian brains. As far as I know, this is a common layout for all of them; the limbic system is well preserved though the mammal family.

While we are at it - how about the amygdala?


@gmirey may be able to shed more light on this question.

As far as “working” this is an interesting question. Patient HM lost his hippocampus after forming a working set of memories. He was not able to form new memories but from what I have read he was able to talk and reason without a functioning hippocampus. I imagine that this would not have worked as well in an infant without working memories to draw on to survive.

  1. The thalamus is an older structure; earlier this was all there was.

  2. One of the properties that I mentioned has a prior post that goes into this in some depth, see numbered references # 4 though # 8, with a focus on #8 for my statement:

I have trouble pointing to a single reference on the attention thing. I have read a bunch of papers since the original “searchlight of attention” and most of them have refined this by patching up the missing or wrong bits. Of course - there are the counter argument papers that point out the problems without proposing any alternatives.


I could try to work through the top-down (reverse?) stream in the sub-cortical structures but Randall O’Reilly does a great job of it with this paper - I don’t seen any need to duplicate the effort:


Sorry about the confusion. The figure on the left is an anatomy diagram. It doesn’t say what the layers do. The figure on the right illustrates what we mean by saying a cortical column has grid cell modules and displacement cell modules. It isn’t meant to be super accurate anatomically.

Notice in the figure on the right I didn’t label L5a vs. L5b and L6a vs. L6b. It is implied by the figure but unintentionally so. I was only trying to show that displacement cell modules are in L5 (somewhere) and grid cell modules are in L6 (somewhere). This figure is from a talk. The main point of the talk was the functionality of a cortical column and less about specific anatomical details. We are working on these details and we hope to write a paper in the coming months where we can be more specific about layers, what they represent, and how they function.