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120 Biomimetics: Biologically Inspired Technologies

Figure 3.A.8 Learning and using the precise associations from symbols to action commands within a single
cortical module. Keep in mind that the neuron populations involved in these associations, illustrated here as small
sets, are, in the brain, extremely large sets (tens of thousands of neurons in every case). See text for explanation of
the figure.

actions and then constructing an action hierarchy). At beginning of development of each module,
the first item on the agenda is development of the module’s symbols (which was discussed in
Section 3.A.3). As this lexicon development process begins to produce stable symbols, the problem
of associating these with actions is addressed.
At first, action command neurons are randomly triggered when a particular single symbol is
being expressed by the lexicon (i.e., that symbol was the lone outcome of a confabulation operation
by the module). As this occurs, the BG monitors the activity of this lexicon (via efferents from
Layers III and V — see Figure 3.A.8). When a randomly activated action command happens to
cause an action that the basal ganglia judge to be particularly ‘‘good’’ (meaning that a reduction in a
drive or goal level was observed — which the basal ganglia know about because of their massive
input from the limbic system), that action is then associated with the currently expressed symbol via
the mechanism of Figure 3.A.8.
(Note: Reductions in drive and goal states are almost never immediate following an action. They
are usually delayed by seconds or minutes; sometimes by hours. One of the hypothesized functions
of the BG [Miyamoto et al., 2004] is that it develops a large number of predictive models, called
critics [Barto et al., 1983], that learn [via delayed reinforcement learning methods; Sutton and
Barto, 1998] to accurately predict the eventual goal-or-drive-state-reduction ‘‘value’’ or ‘‘worth’’ of
an action at the time the action is suggested or executed. It is by using such critic models that the BG
is hypothesized by the theory to immediately assess the worth of action commands produced by
Layer V outputs.)
When an action command that is randomly launched is indeed judged worthy of association
from the currently expressed symbol of a module, a special signal (the green arrow in Figure 3.A.8)
is sent (via thalamus) from the striatum of the BG to cortical Layer I of the module. This green
signal causes the synapses (blue circles) connecting axon collaterals of the neurons representing the