Postscript - Brain Theory from a Circuits and Systems Perspective - page 179

rest potential; this in turn attracts home with electrostatic force all positive
neurotransmitters. Back propagating pulses from a soma are quite common and
also tend to encourage positively charged neurotransmitters to leave the postsynap-
tic regions.
As pulses propagate, pulse shape remains more or less regular, similar to the soliton
of electrical theory. Regular pulse waveforms are expected in exposed membrane that
is uniform and continuous. Propagating pulses undergo dendritic logic and
also enabled logic, where enabled logic is more akin to what is accomplished by the
activation functions in the engineering field of artificial neural networks.
Plethora of Types of Neural Logic
Dendritic logic was shown in this topic to depend on the timing of pulses propagating
along dendritic branches, where pulses must arrive concurrently to accomplish the
desired logic, which may be AND, OR, XOR gates; note that NOT gates may be
derived from the XOR (exclusive OR) gates [ 6 ]. Simulations suggest that membrane
activity and geometry at a dendritic junction determine the resulting logic. Similar
logic might very well be generated by an inhomogeneous region of a dendrite,
although appropriate geometry is more difficult to envision. Thus the dendrites of
a neuron, when they are extensive, may support thousands of logic gates.
In stark contrast, enabled logic was shown in this topic to depend on a smaller
dendrite making contact to a larger body or soma. The resulting added capacitance,
when hit by pulses, thus accumulates voltage and eventually reaches a threshold for
triggering. Voltage is accumulated from standard neural pulses (not weighted
synapses) that add to produce AND, OR gates.
Enabled NOT gates are theoretically possible, but only when inhibitory
neurotransmitters are considered. The XOR function is not directly available in a
single node. The XOR may be generated using two layers of enabled logic with
some sort of complex weighting to realize the equivalent of negative weighting
factors. Alternatively, simply employ three layers using AND, OR, and NOT to
realize the Boolean expansion of the XOR function.
Enabled logic is not as dependent on timing as dendritic logic, so it has a
distinctive place in neural circuitry. Enabled logic is generally limited to neural
regions that can store adequate charge, generally the soma. This suggests only one
enabled gate per neuron. In contrast, dendritic gates may occur by the thousands in a
given neuron.
Brain System Requirements
What a brain does is based on common experience: Obviously sensory impressions
are projected into conscious short-term memory. Interspersed with these are
recalls from associative long-term memory. Recalls are extremely important to