FIGURE 68. In this schematic diagram, neurons are imagined as laid out as dots in one
plane. Two overlapping neural pathways are shown in different shades of gray. It may happen
that two independent "neural flashes" simultaneously race down these two pathways, passing
through one another like two ripples on a pond's surface (as in Fig. 52). This is illustrative of
the idea of two "active symbols" which share neurons and which may even be simultaneously
activated. [From John C. Eccles, Facing Reality (New York: Springer Verlag, 1970), p.21.]
hardware can all be distinguished from each other. By this analogy, I do
not mean to go so far as to suggest that all the different symbols are just
different kinds of "waves" propagating through a uniform neural medium
which admits of no meaningful division into physically distinct symbols.
But it may be that in order to distinguish one symbol's activation from that
of another symbol, a process must be carried out which involves not only
locating the neurons which are firing, but also identifying very precise
details of the timing of the firing of those neurons. That is, which neuron
preceded which other neuron, and by how much? How many times a
second was a particular neuron firing? Thus perhaps several symbols can
coexist in the same set of neurons by having different characteristic neural
firing patterns. The difference between a theory having physically distinct
symbols, and a theory having overlapping symbols which are distinguished
from each other by modes of excitation, is that the former gives hardware
realizations of concepts, while the latter gives partly hardware, partly
software realizations of concepts.
Brains and Thoughts 357