I find the concept of stacked SPs troubling. How would that actually look in the biology?
If you compare the reach of the dendrites from the cells that make up a mini-column it is about the same or smaller than the area of a winning mini-column in the pooling competition. This cell in this mini-column has won the fight and its output axon will be the one that projects to the next map to say “we decided that what I have learned is the best match the pattern that we all sensed.” Everyone close to me must stay silent.
So what is being sent to the next layer/map is spatially sparse to the degree that the column in the next map sees at best - a few sparse points of activity. This does not sound like the cluster of axonal activity within a 40 micron stretch needed to activate a dendrite as is thought to be required.
Here is my concern in a pictorial depiction; assume for this picture that there is some pattern projected to the map on the right and it has formed the winning spatial pools shown. You can see that the projections are very sparse in relation to the scale of in size of a single SDR on a dendrite. How will these combine to form an new SDR in the target map?
If you allow that only one axon/dendrite pair is needed to fire a cell and the SP layers are stacked one-to-one (topographical aligned) then I don’t see how this is more than a bucket brigade at best. I don’t see any possible mechanism for generalization.
Now - if the target map has two or more maps projected to it things could be very different …