I have still not read the Numenta’s paper on the HTM spatial pooler, and the answer to my questions may actually be there. In any case, I think that this discussion could be interesting.
The potential pool of an SP minicolumn is the set of bits from the input (to the SP) which that minicolumn can possibly be connected to.
In this HTM School video (min 5:04), @rhyolight states that a minicolumn will never be connected to a bit of a specific input if that bit is not part of the potential pool of that minicolumn. But he also states that a minicolumn will never be connected to such bits “because there are no dendritic segments” connected to those bits.
If this is plausible, I suppose that
potentialRadius parameter of the NuPIC’s
SpatialPooler class can possibly depend on the number of neurons per minicolumn. However, the SP of the current implementations only works at the minicolumn level. It doesn’t take into account single neurons. Is there some experimental work which assesses these claims? Or, the default value of that parameter in the current implementations is 16. Why?
I suppose that the number of dendritic segments of each neuron may vary from neuron to neuron (or from certain “regions” to other regions).
Anyway, in the NuPIC documentation, it’s written about the parameter
This parameter determines the extent of the input that each column can potentially be connected to. This can be thought of as the input bits that are visible to each column, or a “receptive field” of the field of vision. A large enough value will result in global coverage, meaning that each column can potentially be connected to every input bit. This parameter defines a square (or hyper square) area: a column will have a max square potential pool with sides of length (2 * potentialRadius + 1).
I have actually no knowledge of neuroscience, but aren’t “receptive fields” actually associated with neurons (and not minicolumns)? I also read somewhere that a minicolumn consists of neurons that have the same “receptive field”. Is this an assumption to develop the HTM theory? But I also read that the theory that the neocortex is organized in columns is actually debatable and not a fact (at least, not yet).
So, in summary, what is the exact biological fact (or simplified assumption about facts) which justifies the existence of the potential pool?