Cortical computation: some complications

Developing Intelligence provided a helpful summary of a fun and thought provoking paper by Douglas and Martin that addresses the question, "What is the basic cortical computation?" with special emphasis on the traditional answer: whatever it is that a column does.

The paper points out that the traditional functional columns (as seen in rat whisker barrels and ocular dominance columns in V1) don't align neatly with minicolumns, which are often taken to be the fundamental computational unit (minicolumns are 'vertical cylinders' of cells about about 50 microns in width (the above image is an example from this review)). Columns, on the other hand, are about 500 microns in diameter and the cells within a column are typically taken to have similar functional properties (e.g., similar receptive fields).

Douglas and Martin say, regarding this mismatch between minicolumns and columns, 'This means that even in a highly specialized “column” such as those evident in the rodent barrel cortex, one cannot simply clip out a cylinder of tissue that contains the whole local circuit, for later reconstruction “in silico”'. They may be overstating things a bit: while the minicolumn and the column don't align neatly, that doesn't mean the columns aren't the fundamental unit of computation. So, the fundamental unit isn't the minicolumn, it's a whole bunch of minicolumns organized into a column via horizontal connections. What's the problem?

On the other hand (and Douglas and Martin might agree) even larger-scale structures like barrels are suspect as the basic computational units. When you stimulate an individual whisker in awake rats, often ten or more whisker barrels are activated, on average more than five (review here). A recent study by Ferezou et al. showed that when a single mouse whisker is stimulated in awake mice, about ten milliseconds after the initial localized response most of the barrel cortex is activated, and this activity sweeps even to motor cortex (image to left is taken from Figure 3 of this paper). While it would certainly be inconvenient if cortical representations, even of simple spatially punctate stimuli, were much more distributed than previously thought, it is starting to look like an inconvenient truth.

I don't want to overstate these results. After all, the initial response is fairly localized in that study! However, the above results are at least consistent with the claim that the simplest anatomical (i.e., columns) and functional (i.e., receptive fields determined in anesthetized animals) maps may not generate an accurate functional decomposition of the cortex in all its complicated glory.

To make matters worse, some think that we shouldn't even talk about the basic functional decomposition of the cortex, but given the tight feedback between cortical and thalamic systems, we should think more about the fundamental computations being carried out by thalamocortical loops (for example, see this review).

These speculations about thalamocortical loops mostly apply to the "back" of the brain. In the frontal lobe, we have to consider the importance of the role of the basal ganglia, which then speaks back to the cortex via the thalamus (for example see this review). For all we know, we may end up having to describe two functional basis sets, the thalamocortical computational units and the striato-thalmo-cortical units. One loop geared toward processing things sensory, the other geared toward processing things motor. Wouldn't that be convenient.