The study of electrical fields in densely-packed cellular media is
prompted primarily by a desire to understand more fully the details
of brain mechanism and its relation to behavior. Our work has
specifically been directed toward an attempt to model such structures
and mechanisms, using relatively simple inorganic materials.
The prototype for such experiments is the “Lillie[1] iron-wire nerve
model.” Over a hundred years ago, it had been observed that visible
waves were produced on the surface of a piece of iron submerged in
nitric acid when and where the iron is touched by a piece of zinc.
After a short period of apparent fatigue, the wire recovers and can
again support a wave when stimulated. Major support for the idea that
such impulses are in fact directly related to peripheral nerve impulses
came from Lillie around 1920. Along an entirely different line,
various persons have noted the morphological and dynamic similarity of
dendrites in brain and those which sometimes grow by electrodeposition
of metals from solution. Gordon Pask (17), especially, has pointed to
this similarity and has discussed in a general way the concomitant
possibility of a physical model for the persistent memory trace.
[1] For review articles see: Lillie (13), Franck (6).
By combining and extending such concepts and techniques, we hope to
produce a macroscopic model of “gray matter,” the structural matrix of
which will consist of a dense, homogeneously-mixed, conglomerate of
small pellets, capable of supporting internal waves of excitation, of
changing electrical behavior through internal fine-structure growth,
and of forming temporal associations in response to peripheral shocks.
A few experimenters have subsequently pursued the iron-wire
nerve-impulse analogy further, hoping thereby to illuminate the
mechanisms of nerve excitation, impulse transmission and recovery,
but interest has generally been quite low. It has remained fairly
undisturbed in the text books and lecture demonstrations of medical
students, as a picturesque aid to their formal education. On the
outer fringes of biology, still less interest has been displayed;
the philosophical vitalists would surely be revolted by the idea of
such models of mind and memory, and at the other end of the scale,
contemporary computer engineers generally assume that a nerve cell
operates much too slowly to be of any value. This lack of interest
is certainly due, in part, to success in developing techniques of
monitoring individual nerve fibers directly to the point that it is
just about as easy to work with large nerve fibers (and even peripheral
and spinal junctions) as it is to work with iron wires. Under such
circumstances, the model has only limited value, perhaps just to the
extent that it emphasizes the role of factors other than specific
molecular structure and local chemical reactions in the dynamics of
nerve action.
When we leave the questions of impulse transmission on long fibers
and peripheral junctions, however, and attempt to discuss the brain,
there can be hardly any doubt that the development of a meaningful
physical model technique would be of great value. Brain tissue is
soft and sensitive, the cellular structures are small, tangled, and
incredibly numerous. Therefore (Young (24)), “ ... physiologists hope
that after having learned a lot about nerve-impulses in the nerves they
will be able to go on to study how these impulses interact when they
reach the brain. [But], we must not assume that we shall understand
the brain only in the terms we have learned to use for the nerves.
The function of nerves is to carry impulses—like telegraph wires. The
functions of brains is something else.” But, confronted with such
awesome experimental difficulties, with no comprehensive mathematical
theory in sight, we are largely limited otherwise to verbal discourses,
rationales and theorizing, a hopelessly clumsy tool for the development
of an adequate understanding of brain function. A little over ten years
ago Sperry (19) said, “Present day science is quite at a loss even
to begin to describe the neural events involved in the simplest form
of mental activity.” This situation has not changed much today. The
development, study, and understanding of complex high-density cellular
structures which incorporate characteristics of both the Lillie and
Pask models may, it is hoped, alleviate this situation. There would
also be fairly obvious technological applications for such techniques
if highly developed and which, more than any other consideration, has
prompted support for this work.
Experiments to date have been devised which demonstrate the following
basic physical functional characteristics:
(1) Control of bulk resistivity of electrolytes containing
closely-packed, poorly-conducting pellets
(2) Circulation of regenerative waves on closed loops
(3) Strong coupling between isolated excitable sites
(4) Logically-complete wave interactions, including facilitation
and annihilation
(5) Dendrite growth by electrodeposition in “closed” excitable
systems
(6) Subthreshold distributed field effects, especially in
locally-refractory regions.
In addition, our attention has necessarily been directed to various
problems of general experimental technique and choice of materials,
especially as related to stability, fast recovery and long life.
However, in order to understand the possible significance of, and
motivation for such experiments, some related modern concepts of
neurophysiology, histology and psychology will be reviewed very
briefly. These concepts are, respectively:
(1) Cellular structure in the central nervous system
(2) Short-term or “ephemeral” memory
(3) The synapse
(4) Inhibition
(5) Long-term memory traces or engram
(6) Spatially-diffuse temporal association and learning.