Distal Dendrites. Each HTM cell maintains a list of distal dendrite segments

Each HTM cell maintains a list of distal dendrite segments. Each segment acts like a threshold detector. If the number of active synapses on any segment (shown as blue

dots on the earlier diagram) is above a threshold, the segment becomes active, and the associated cell enters the predictive state. The predictive state of a cell is the OR

of the activations of its segments.

A dendrite segment remembers the state of the region by forming connections to cells that were active together at a point in time. The segment remembers a state that precedes the cell becoming active due to feed-forward input. Thus the segment is looking for a state that predicts that its cell will become active. A typical threshold for a dendrite segment is 15. If 15 valid synapses on a segment are active at once,

the dendrite becomes active. There might be hundreds or thousands of cells active nearby, but connecting to only 15 is sufficient to recognize the larger pattern.

Each distal dendrite segment also has an associated set of potential synapses. The set of potential synapses is a subset of all the cells in a region. As the segment learns, it increases or decreases the permanence value of all its potential synapses. Only those potential synapses that are above a threshold are valid.

In one implementation, we use a fixed number of dendrite segments per cell. In another implementation, we add and delete segments while training. Both methods can work. If we have a fixed number of dendrite segments per cell, it is possible to store several different sets of synapses on the same segment. For example, say we have 20 valid synapses on a segment and a threshold of 15. (In general we want the threshold to be less than the number of synapses to improve noise immunity.) The

segment can now recognize one particular state of the cells nearby. What would happen if we added another 20 synapses to the same segment representing an entirely different state of cells nearby? It introduces the possibility of error because the segment could add 8 active synapses from one pattern and 7 active synapses from the other and become active incorrectly. We have found experimentally that up to 20 different patterns can be stored on one segment before errors occur. Therefore an HTM cell with a dozen dendrite segments can participate in many different predictions.