It’s really hard to disprove that columns exist. For example, would three cells scattered around randomly but controlled as if in a minicolumn count as a minicolumn? I think so.
Minicolumns don’t seem necessary unless there’s a reason they’re specifically needed. You could probably use one cell per minicolumns in the temporal memory, where each activates only if it is predicted or not enough nearby cells are predicted, and it would still work as far as I know. It just requires cells with direct spike-driving inputs, biasing signals sent a little bit of time beforehand, and probably some sort of competition (which is necessary anyway for spatial pooling). Minicolumns are efficient though, since they let it pick far less spatial patterns than sequence contexts, which allocates cells better since there are way more sequence contexts.
Cortical columns are probably more complicated than has been established in HTM theory so far because there’s also the where/egocentric/how pathway. There are also neurons in the spaces between cortical columns which operate on larger scales sensory patch size-wise (and also e.g. binocularity). This space is kind of weird. For example, at least in primary visual cortex and barrel cortex, it is mostly parallel lines rather than at every border of adjacent columns. It may or may not function as a separate region.
In primary cortex, those sections between columns might be part of the where pathway because that’s where they project, at least one region does that. Also, some layers and sublayers also might not be arranged into cortical columns.
I see cortical columns as an efficiency thing like minicolumns. The fact that they exist usually but maybe not always is still evidence for treating the brain as if it looks at the world through a straw (not that everything it does is like that). The ideas that produces are applicable without cortical columns.
Unaccountably, ocular dominance columns are present in some species, but not others.
Rodents lack ocular dominance columns whereas primates and cats have them. Rats have very poor vision compared to those and their eyes point more to the sides, so they might not need them, at least not most of their visual cortex since most of it maps parts of the visual field which only one eye can see. They could still have cortical columns for places in the visual field.
Although the column is an attractive concept, it has failed as a unifying principle for understanding cortical function.
I think a dozen neuroscientists could in a couple decades summarize research on the brain sufficiently for us to know exactly how intelligence works. We have an amount of agreement proportional to how much people summarize things and draw conclusions which aren’t just educated guesses. We have many regions in many species to figure out what is essential for the cortex to work, what is essential for the cortex to do things which most of the cortex does (e.g. there’s no primary thalamus for smell), and what is just a specialization (specializations are numerous and a problem but they do exist in the context of fundamental things so they’re not useless).
Hubel and Wiesel’s classic model showing orientation columns as discrete slabs is misleading because the columns are borderless in real life.
It’s very hard to prove whether columns are quantal like they discuss. They aren’t physically exactly quantal (or they are and that’s just noisy results) but they might be functionally, and it doesn’t really matter if they aren’t exactly quantal because two cells with nearly identical orientation preferences will either both fire or not both fire, just at slightly different average rates.
First, nobody seems willing to venture how many minicolumns constitute a column; the number is arbitrary. Second, there is no evidence that short-range connections bind minicolumns into discrete, larger structural entities . . . These short-range connections do not end abruptly along fixed borders in the cortex, as they should if they actually defined the edges of a structurally distinct column.
Cells in barrel cortex (in macrocolumnar layers) have a main whisker to which they respond, which corresponds to the cortical column, but they usually also respond to a few other whiskers somewhat.
It’s not a problem for the idea of columns if it isn’t neatly organized like a man-made machine. HTM theory doesn’t require cortical columns to be so precise. It seems more about simplifying the sensory input by just considering a piece of it and figuring stuff out about objects and figuring out locations, then combining that information in other layers.
No one has demonstrated a repeating, canonical cellular circuit within the cerebral cortex that has a one-to-one relationship with the minicolumn.
If it’s in a lot of different systems, it’s probably part of the fundamental circuitry, or at least it indicates it quite well because otherwise it wouldn’t be in a lot of systems. For example, if minicolumns aren’t fundamental, they still point somewhat strongly at temporal memory, so that’s evidence for temporal memory even if it isn’t always implemented with strict minicolumns.
It remains true that nothing is known about the physiological correlates (if, indeed, any exist) of the minicolumn.
We do not know anything for certain about the fundamental cortical circuitry’s mechanisms (maybe a few things but basically nothing), and there’s no getting around that so the best we can do is show that a bunch of different things point towards a function and then show that it is powerful.
An alternative explanation is that species lacking orientation columns have relatively poor visual acuity or a small visual cortex. Perhaps orientation columns develop only in species that depend on form vision and those that have a cortical surface area sufficiently large to require subdivision into columns. This argument is contradicted, however, by a recent study of orientation tuning in the squirrel. This animal has a cone-rich retina and it surpasses the tree shrew and mink in visual acuity and visual cortical area. Although the tree shrew and mink both have beautifully organized orientation columns, the squirrel has none
That means rodents probably don’t have orientation columns. The visual stimulus is not just bars though so the minicolumns might be different, like a plain spatial pooler.