10.3 A Neural-Darwinist Perceptual-Motor Hierarchy

    In Chapter 6 we used Edelman’s theory of Neural Darwinism to explore the nature of neural
analogy. However, we did not suggest how the "lower-to-intermediate-level" details discussed
there might fit into a theory of higher-level brain function. It is possible to give a partial Neural-
Darwinist analysis of the perceptual and motor hierarchies. This entails wandering rather far
from the biological data; however, given the current state of neuroscience, there is little choice.
    Assume that the inputs of certain neural clusters are connected to sensory input, and that the
outputs of certain clusters are connected to motor controls. The purpose of the brain is to give
"appropriate" instructions to the motor controls, and the determination of appropriateness at any
given time is in large part dependent upon the effects of past instructions to the motor controls —
i.e. on using sensory input to recognize patterns between motor control instructions and
desirability of ensuing situation.
    In order to make sense of this picture, we must specify exactly howappropriateness is to be
determined. Toward this end I will delineate a hierarchy of maps. Maps which are connected to
both sensory inputs and motor controls (as well as, naturally, other clusters) we shall call level 1
maps. These maps are potentially able to "sense" the effect of motor control instructions, and
formulate future motor control instructions accordingly.
    One question immediately arises: How are appropriate level 1 maps arrived at and then
maintained? In the simple Hebb rule discussed in Chapter 6, we have a mechanism by which any
map, once repeatedly used, will be reinforced and hence maintained; but this says nothing about
the problem of arriving at an appropriate map in the first place. Rather than proposing a specific
formula, let us dodge the issue by asserting that the appropriateness of level 1 maps should be
determined on the basis of the degree to which the levels of certain chemical substances in their
vicinity are maintained within biologically specified "appropriate" bounds. This cowardly
recourse to biological detail can serve as the "ground floor" of an interesting general definition of
    Define a map which is not a level-1 map to be appropriate to the extent that the maps or motor
controls to which it outputs are appropriate. The idea is that a map is appropriate to the extent
that the entities over which it has (partial) control are appropriate. The appropriateness of a level
1 map is partially determined by the extent to which it directs motor controls to make appropriate
actions. And in the long run — barring statistical fluctuations — this is roughly equivalent to the
extent to which it represents an emergent pattern between 1) results of motor control and 2)
appropriateness as measured by sensory data and otherwise. This is the crucial observation. In
general, the appropriateness of a map is determined by the extent to which it directs other maps
to effect appropriate actions, either directly on motor controls or on other maps. And, barring
statistical fluctuations, it is plain that this is roughly equivalent to the extent to which it
represents an emergent pattern between 1) results of outputs to other maps and 2) inputs it
obtains from various sources.
    It is important to remember that we are not hypothesizing the brain to contain distinct "pattern
recognition processors" or "analogical reasoning processors" or "memory cells": our model of
mind is schematic and logical, not necessarily physical; it is a model of patterns and processes.
We have hypothesized a neural mechanism which tends to act much like a pattern recognition
processor, and that is all that can reasonably be expected.
    Now, let us go beyond the level 1 maps. Define a degree 2 map as a map which outputs to
level 1 maps (as well as possibly inputting from level 1 maps and other maps and outputting to
other maps). Define a degree 3 map as one which outputs to degree 2 maps (as well as possibly
level 1 maps, etc.). One may define maps of degree 4, 5, 6,.. in a similar way. The level of a map
is then defined as the highest degree to which it possesses. If a level k map accepted inputs only
from maps of level k-1 or lower, the network of maps would have a strictly hierarchical
structure. There would be a "top" level n, and our definition of appropriateness would define
appropriateness on all levels less than n in terms of top-level appropriateness, but say nothing
about theappropriateness of a map on level n. But in fact, the maps of the brain are arranged in a
far less orderly fashion. Although there is a bottom level — the level of perception and action —
there is no distinct top level.
    The nonhierarchical interconnection of the maps of the brain implies that the evaluation of
appropriateness is a very tricky matter. If A and B both input and output to each other, then the
appropriateness of A is determined in part as an increasing function of the appropriateness of B,
and vice versa. The hierarchy of maps does bottom out at level 1, but it also re-enters itself
multiply. In a very rough way, this explains a lot about human behavior: our internal definition
of "appropriateness" is determined not only by low-level biological factors but also by a subtle,
probably unstable dynamical system of circularly reinforcing and inhibiting patterns.
    We have not yet specified what exactly happens to inappropriate maps. Clearly, an
inappropriate map should be dissolved, so that it will no longer direct behavior, and so that a new
and hopefully better map can take its place. The easiest way to effect this would be to inhibit the
connections between clusters of the map — to decrease their conductances (roughly speaking,
proportionally to the lack of appropriateness). Naturally, if a connection belonged to more than
one map, this decrease would be mitigated by the increase afforded by membership in other
maps, but this effect need not rob inhibition of its effectiveness.
    Biologically, how might such a system of inhibition work? It is known that if a once-
frequently-used connection between clusters is unused for a time, its conductance will gradually
revert to the level of other infrequently-used neurons. Naturally, the presence of inhibitory
connections between individual neurons plays a role in this. However, it is not presently known
whether this effect is sufficient for the suppression of inappropriate maps.
   At this point, just as we are beginning to move toward the higher levels of mental process, we
must abandon our discussion of the brain. We have already left our starting point, Neural
Darwinism, too far behind. On the positive side, we have constructed a hierarchy of neural maps
which is, intuitively, a combination of the perceptual and motor control hierarchies: it recognizes
patterns and it controls actions, using a simple form of analogy, in an interconnected way.
However, we have accounted for only a few of the simpler aspects of the master network to be
described in Chapter 12 — we have spoken only of pattern recognition and a simple form of
structural analogy. We have said nothing of induction, deduction, Bayesian inference, or
modeling or contextual analogy, or even more general forms of structural analogy. It is difficult
to see how these subtler aspects of mentality could be integrated into the Neural Darwinist
framework without altering it beyond recognition. It seems to me that they may require
somewhat more structure than the self-organizing network of maps which Neural Darwinism
Kaynak: A New Mathematical Model of Mind

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