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Mechanization of Cognition 111

outputs to drive their development. So, in general, development is a one-shot process (which
illustrates the importance of getting it right the first time in childhood). Once the symbols have
been frozen, the only synaptic modifications which occur are those connected with knowledge
acquisition, which is the topic discussed next.

3.A.4 Implementation of Knowledge

As discussed in Hecht-Nielsen (2005), all of the knowledge used in cognition (e.g., for vision,
hearing, somatosensation, language, thinking, and moving) takes the form of unidirectional
weighted links between pairs of symbols (typically, but not necessarily, symbols residing within
different modules). This section sketches how these links are implemented in human cortex (all
knowledge links used in human cognition reside entirely within the white matter of cortex).
Figure 3.A.5 considers a single knowledge link from symbol c in a particular cortical source
module (lexicon) to symbol l in a particular target or answer lexicon. The set of all knowledge
links from symbols of one particular source lexicon to symbols of one particular target lexicon are
called a knowledge base. The single knowledge link considered in Figure 3.A.5 belongs to the
knowledge base linking the particular source lexicon shown to the particular target lexicon shown.
When the neurons of Figure 3.A.5 representing symbol c are active (or highly excited if
multiple symbols are being expressed, but this case will be ignored here), these c neurons send
their action potential outputs to millions of neurons residing in cortical regions to which the neurons
of this source region send axons (the gross statistics of this axon distribution pattern are determined
genetically, but the local details are random). Each such active symbol-representing neuron sends
action potential signals via its axon collaterals to tens of thousands of neurons. Of the millions of
neurons which receive these signals from the c neurons, a few thousand receive not just one such
axon collateral, but many. These are termed transponder neurons. They are strongly excited by this
simultaneous input from the c neurons; causing them to send strong output to all of the neurons
to which they in turn send axons. In effect, the first step of the link transmission starts with the
tens to hundreds of active neurons representing symbol c and ends with many thousands of excited
transponder neurons, which also (collectively) uniquely represent the symbol c. In effect, tran-
sponder neurons momentarily amplify the size of the c symbol representation. It is hypothesized by
the theory that this synfire chain (Abeles, 1991) of activation does not propagate further because

Figure 3.A.5 A single knowledge link in the human cerebral cortex. See text for discussion.