Wall of text trigger warning
Hierarchy.
I have been reading about it in relation to the brain about as long as I have been studying biology. The bulk of what I have read anthropomorphizes this in the shape of how humans break down tasks with certain common organizing motifs. This seemed natural and I accepted it without question. You can see that I am not alone in this - older Numenta papers featured hierarchy so prominently that is is part of the name of the HTM model.
See page 9.
As I learned more about how the brain was organized I read thousands of papers and books (yes - thousands!) I started to have my doubts. Many facts and observations just did not fit this model.
I have tried to keep an open mind about how this or that model “almost” works and maybe I have one or two thing wrong about it and with a little fixing up it would be just fine. Some models start to accumulate so much cruft that you just have to put them on the shelf and quit paying any attention to them. The concept that hierarchy concentrates or consolidates to some higher order representation fits in that crufty niche for me now.
I have come to this point though several different lines. One particularly notable line is developmental biology. What kind of operations does the growing brain use to turn from a few cells into hundreds of million cells all hooked up the right way? This is all done without a schematic or wiring diagram. Check out a paper that Numenta has touted many times - in it V B Mountcastle gives a nice overview of how the cells in the cortex hook themselves up as the cortex grows.
Columnar organization of the neocortex - V B Mountcastle
https://academic.oup.com/brain/article/120/4/701/372118
See the section “The ontogenesis of the neocortex”
Cells do basic operation like migrating in waves and spreading out laterally. They can split into different types. They generate chemical markers to use as guiding scaffolds in place of schematics and road maps. These chemical markers can be gradients across a map. They can be a 2D address with markers with gradients going “top to bottom and left to right” in a map. (direction markers being an arbitrary definition here) They can make stripes and spots. See zebras and leopards for examples of this. One of the more astounding feats is the generation of inter-map axonal projections. This is roughly the same as your finger growing miles long and in the process, ending up at the right house correctly positioned for pushing the doorbell - without looking. The chemical markers are guiding the process with the growth cone on the axon sampling the chemical markers in the soup and sniffing out the target that has the exact same x/y smell as the cell that they axon started from in the target map. As it grows it add to the correct side to steer in the direction that smells right. What it does NOT have is the right details to form a complicated wiring diagram - it is part of a roughly parallel bundle of axons growing as a fleet to a target position in a distant map. (cue “flight of the Valkyries!”)
The last hurrah of the hierarchy for me was trying to use the “crazy quilting” and slight spreading of axons to pull off some sort of hierarchy concentration as the information ascends the maps.
In the discussion that followed and reading many papers on the loose ends that came up in these exchanges, while there are discontinuities, they are just that - scattered maps that do maintain topology in the multiple scattered copies of the input topology located all over the brain.
There are other lines of factoids that cast doubt on the concentrating of information as you ascend the hierarchy but I consider the developmental facts to be the most damming.
So what do the maps do with these bundles of parallel fibers? I put down some of the key factors in this post:
Which results in this schematic diagram showing the relative relationships of SDR size to spatial mapping size:
It makes little or no sense that the flow is a straight line from V1 to the association regions. There would be very little for the dendrites in the next map to use to form new SDRs; they are too far apart for even two fibers to reach a single dendrite - let alone enough to form an SDR. What must be happening is that the projecting axons from different maps come together to form new SDRs that are a combination of the output from these other maps.
Also - we have neurologists that have been giving us detailed connection maps for decades and it seems like every map is connected to many other maps. We trot this diagram out to frighten neural newbies:
I find the connectogram to be a kinder & gentler presentation but make no mistake - it has exactly the same information:
On of the basic truths of brain is that if information will be used in a certain part of the brain there will have be be a direct or indirect connection from where that information is sensed.The good news here is that almost everything is connected to everything through a low number of hops - sort of a "six degrees of Keven Bacon" thing.
I do see the information being compared and contrasted as it splits apart and rejoins going up the hierarchy. I see the maps teasing out as many relationships as possible and presenting that to the association region to form codes representing objects.
This is the essence of my hex-grid coding scheme and where I think that Numenta is going with the 1000 brains theory.