Starting with a little basic neurobiology refresher:
“Pyramidal neuron axons send many “recurrent collateral” branches laterally to neighboring areas of the cortex, the basis for both lateral inhibition and lateral excitation.”
The corticocortical axons shown above are projections to other maps in the cortex. This is how one map talks to another. In the population of neurons in the cortex the projected map-to-map axon reaches the new map and turns upward to go through all layers, possibly making contact with proximal dendrites to end up in layer 1, the dendrites of the apical layer. See the corticocortical fiber in blue shown here.
Note the green basket cell in the image above. It is mostly ignored in portrayals by Numenta, mentioned in passing as inhibitory interneurons to be simulated by k-winner. The extent shown below is roughly what would be excited by the lateral axonal projections from this cell - in a circular pattern around the cell.
Another aspect that seems to be described but seldom portrayed is how utterly dense all this is and the relative scale of a SDR to this whole structure. Let’s start with adding the basket cells:
If a lateral axon stimulates a basket cell as it goes through it goes off like a bomb, sending inhibitory signals to all the pyramidal cells within its reach. I did not put in pyramid cells from other column as the picture would be solid red vertical lines. Note that since these lateral axons start below the basket cell but reach up into them for a relatively far distance it affects many other surrounding cells but does not turn itself off. This give the classic center on/surround off that is so widely documented. All the cells receiving some stimulus from axons are firing and trying to suppress neighbors all at the same time. Only the cells with the strongest input win this competition and stay active. The rest are shut down. Note the large spatial extent of lateral axonal projections; about 3 millimeters maximum. Compare this to the much smaller reach of a dendrite where the SDR is formed. All of an SDR is formed on a single dendrite; very small in relation to the finished spatial pooler action.
@david.pfx Note that this is not just the input from the senses - this is in all cortex, in all maps. HTM is the universal computation of the entire cortex.