What's the difference between layers and levels in the context of the HTM theory?

[Bitking]

@nbro

As I understand it, the sheet of the cortex is composed of what is generally recognized as six layers.
The outer layer (one furthest from the center of the brain) is called layer one
The bottom is generally called layer six. These are not hard and fast divisions as the whole concept of what makes up a layer is pretty loose at this time. There are some that consider the thalamus as the 7th layer.

From my previous readings (and not going too much in detail), a biological cortical column (debatably) contains 6 layers. So, is the “sheet of the cortex” (as you name it) composed uniquely of these “vertical columns” of (mostly pyramidal) neurons (and connections between them)? If yes, I suppose this is the reason why HTM theorists are focusing on studying and modeling a single biological cortical column. Is this supposition correct?

There is some confusion about what makes up a column or micro-column.

A vertical cluster of cell bodies runs vertically through the entire six layer stack and are spaced on 30 μm centers; these are frequently called a micro column and are actually visible in some parts of the cortex under magnification.

These column work in concert and are thought to group in columns of about 300 μm in diameter. I am not aware if there are physical features that can be discerned on inspection but scientists that measure the behavior of collections of cells do see this grouping in the activities of the cell populations.

The HTM algorithms enforce sparsity at the level that mimics this column density.

You are correct that HTM theory focuses on this at the basic unit of function repeated endlessly throughout the cortex.

@Paul_Lamb has done a nice job of explaining the basic distinction between the layers of the HTM model and relations to both the basic HTM theory and maps/regions above - I won’t try to duplicate this work as he has covered it.

I should note that the population of cells in the cortical sheet contains a bunch of other cells types that are not currently part of the HTM canon and will likely end up having to be addressed as the theory progresses.

My own work on Hex-grids uses the layer 2/3 to join columns into larger assemblies inside a single map.
Please see this post for more details:

HTM Mini-Columns into Hexagonal Grids!

Or at least some wild conjectures to provoke thought! This post assumes that you are familiar with basic HTM theory and are conversant with the concepts of SDRs and HTM minicolumns. May I be so bold: [HTM%20and%20GRIDS] First - let’s be clear what I am and am not talking about. This is about cells that work by forming regular hexagonal grid structures and are not necessarily the same thing as the “grid cells” that are coding some spatial aspect of the surrounding environment. These grid-fo…