Mechanization of Cognition - Biomimetics: Biologically Inspired Technologies - page 102

evaluation before being finally executed ). This is the theory's explanation for the origin of all
nonautonomic animal behavior.
As with almost all cognitive functions, actions are organized into a hierarchy, where individual
symbols belonging to higher-level lexicons typically each represent a time-ordered sequence of
multiple lower-level symbols.
Evolution has seen to it that symbols, which when expressed alone launch action commands that
could conflict with one another (e.g., carrying out a throwing motion at the same time as trying to
answer the telephone), are grouped together and collected into the same lexicon (usually at a high
level in the action hierarchy). That way, when one such action symbol wins a confabulation (and has
its associated lower-level action commands launched), the others are silent — thereby automati-
cally deconflicting all actions. This is why all aspects of animal behavior are so remarkably focused
in character. Each complement of our moving and thinking ''hardware'' is, by this mechanism,
automatically restricted to doing one thing at a time. Dithering (rapidly switching from one decisive
action (behavioral program) to another, and then back again) illustrates this perfectly.
The thought processes at the lowest level of the action hierarchy are typically carried out
unconditionally at high speed. If single symbol states result from confabulations which take
place as part of a thought process, these symbols then decide which actions will be carried out
next (this happens both by the action commands the expression of these symbols launch, and by the
influence of these symbols — acting through knowledge links — on the outcomes of subsequent
confabulations; for which these symbols act as assumed facts). Similarly for movements, as
ongoing movements bring about changes in the winning symbols in confabulations in somatosen-
sory cortex — which then alter the selections of the next action symbols in modules in motor and
premotor cortex. This ongoing, high-speed, dynamic contingent control of movement and thought
helps account for the astounding reliability and comprehensive, moment-by-moment adaptability
of animal action.
All of cognition is built from the above discussed elements: lexicons, knowledge bases, and the
action commands associated with the individual symbols of each lexicon. The following sections of
this Appendix discuss more details of how these elements are implemented in the human brain. See
Hecht-Nielsen and McKenna (2003) for some citations of past research that influenced this theory's
development.

3.A.3 Implementation of Lexicons

Figure 3.A.1 illustrates the physiology of thalamocortical feature attractor modules. In reality, these
modules are not entirely disjoint, nor entirely functionally independent, from their physically
neighboring modules. However, as a first approximation, they can be treated as such; which is
the view which will be adopted here.
Figure 3.A.2 shows more details of the functional character of an individual lexicon. The
cortical patch of the module uses certain neurons in Layers II, III, and IV to represent the symbols
of the module. Each symbol (of which there are typically thousands) is represented by a roughly
equal number of neurons; ranging in size from tens to hundreds (this number deliberately varies, by
genetic command, with the position of the cortical patch of the module on the surface of cortex).
The union of the cortical patches of all modules is the entire cortex, whereas the union of the
thalamic zones of all modules constitutes only a portion of thalamus.
Symbol-representing neurons of the module's cortical patch can send signals to the glomeruli of
the paired thalamic zone via neurons of Layer VI of the patch (as illustrated on the left side of
Figure 3.A.2). These downward connections each synapse with a few neurons of the thalamic
reticular nucleus (NRT) and with a few glomeruli. The NRT neurons themselves (which are
inhibitory) send axons to a few glomeruli. The right side of Figure 3.A.2 illustrates the connections
back to the cortical patch from the thalamic zone glomeruli (each of which also synapses with a few
neurons of the NRT). These axons synapse primarily with neurons in Layer IV of the patch, which