Proximal Dendrite

Each HTM cell has a single proximal dendrite. All feed-forward inputs to the cell are made via synapses (shown as green dots). The activity of synapses is linearly summed to produce a feed-forward activation for the cell.

We require that all cells in a column have the same feed-forward response. In real neurons this would likely be done by a type of inhibitory cell. In HTMs we simply force all the cells in a column to share a single proximal dendrite.

To avoid having cells that never win in the competition with neighboring cells, an HTM cell will boost its feed-forward activation if it is not winning enough relative to its neighbors. Thus there is a constant competition between cells. Again, in an HTM we model this as a competition between columns, not cells. This competition is not illustrated in the diagram.

Finally, the proximal dendrite has an associated set of potential synapses which is a subset of all the inputs to a region. As the cell learns, it increases or decreases the “permanence” value of all the potential synapses on the proximal dendrite. Only those potential synapses that are above a threshold are valid.

As mentioned earlier, the concept of potential synapses comes from biology where it refers to axons and dendrites that are close enough to form a synapse. We extend this concept to a larger set of potential connections for an HTM cell. Dendrites and axons on biological neurons can grow and retract as learning occurs and therefore the set of potential synapses changes with growth. By making the set of potential synapses on an HTM cell large, we roughly achieve the same result as axon and dendrite growth. The set of potential synapses is not shown.

The combination of competition between columns, learning from a set of potential synapses, and boosting underutilized columns gives a region of HTM neurons a powerful plasticity also seen in brains. An HTM region will automatically adjust what each column represents (via changes to the synapses on the proximal dendrites) if the input changes, or the number of columns increases or decreases.