Part 6

The smallest regenerative or reverberatory loop which we are at present able to devise is about 1 mm in diameter. Multiple waves, as expected, produce stable patterns in which all impulses are equally spaced. This phenomenon can be related to the slightly slower speed characteristic of the relative refractory period as compared with a more fully recovered zone.

3. Strong Coupling

If two touching pieces of iron are placed in a bath of nitric acid, a wave generated on one will ordinarily spread to the other. As is to be expected, a similar result is obtained if the two pieces are connected through an external conducting wire. However, if they are isolated, strong coupling does not ordinarily occur, especially if the elements are small in comparison with a “critical size,” σ/ρ where σ is the surface resistivity of passive iron surface (in Ω-cm²) and ρ is the volume resistivity of the acid (in Ω-cm). A simple and informative structure which demonstrates the essential conditions for strong electrical coupling between isolated elements of very small size may be constructed as shown in Figure 4. The dielectric barrier insures that charge transfer through one dipole must be accompanied by an equal and opposite transfer through the surfaces of the other dipole. If the “inexcitable” silver tails have sufficiently high conductance (i.e., sufficiently large surface area, hence preferably, dendrites), strong coupling will occur, just as though the cores of the two pieces of iron were connected with a solid conducting wire.

4. Inhibitory Coupling

If a third “dipole” is inserted through the dielectric membrane in the opposite direction, then excitation of this isolated element tends to inhibit the response which would otherwise be elicited by excitation of one of the parallel dipoles. Figure 5 shows the first such “logically-complete” interaction cell successfully constructed and demonstrated. It may be said to behave as an elementary McCulloch-Pitts neuron (15). Further analysis shows that similar structures incorporating many dipoles (both excitatory and inhibitory) can be made to behave as general “linear decision functions” in which all input weights are approximately proportional to the total size or length of their corresponding attached dendritic structures.

5. Dendrite Growth

Figure 6 shows a sample gold dendrite grown by electrodeposition (actual size, about 1 mm) from a 54% nitric acid solution to which gold chloride was added. When such a dendrite is attached to a piece of iron (both submerged), activation of the excitable element produces a field in such a direction as to promote further growth of the dendritic structure. Thus, if gold chloride is added to the solution used in the elementary interaction cells described above, all input influence “weights” tend to increase with use and, hence, produce a plasticity of function.

6. Field Effects in Locally-Refractory Regions

Our measurements indicate that, during the refractory period following excitation, the surface resistance of iron in nitric acid drops to substantially less than 1% of its resting value in a manner reminiscent of nerve membranes (4). Thus, if a distributed or gross field exists at any time throughout a complex cellular aggregate, concomitant current densities in locally-refractive regions will be substantially higher than elsewhere and, if conditions appropriate to dendrite growth exist (as described above) growth rates in such regions will also be substantially higher than elsewhere. It would appear that, as a result, recently active functional couplings (in contrast to those not associated with recent neural activity) should be significantly altered by widely distributed fields or massive peripheral shocks. This mechanism might thus explain the apparent ability of the brain to form specific temporal associations in response to spatially-diffuse effects such as are generated, for example, by the pain receptors.

Figure 6—Dendritic structures, living and non-living. (a) Cat dendrite trees (from Bok, “Histonomy of the Cerebral Cortex,” Elsevier, 1959); (b) Electrodeposited gold dendrite tree.]

SUMMARY

An attempt is being made to develop meaningful electrochemical model techniques which may contribute toward a clearer understanding of cortical function. Two basic phenomena are simultaneously employed which are variants of (1) the Lillie iron-wire nerve model, and (2) growth of metallic dendrites by electrodeposition. These phenomena are being induced particularly within dense cellular aggregates of various materials whose interstitial spaces are flooded with liquid electrolyte.

REFERENCES

1. Bok, S. T., “Histonomy of the Cerebral Cortex,” Amsterdam, London:Elsevier Publishing Co., New York:Princeton, 1959

2. Bonhoeffer, K. F., “Activation of Passive Iron as a Model for the Excitation of Nerve,” J. Gen. Physiol. =32=:69-91 (1948). This paper summarizes work carried out during 1941-1946 at the University of Leipzig, and published during the war years in German periodicals.

3. Boycott, B. B., and Young, J. Z., “The Comparative Study of Learning,” S. E. B. Symposia, No. IV “Physiological Mechanisms in Animal Behavior,” Cambridge: University Press, USA:Academic Press, Inc., 1950

4. Cole, K. S., and Curtis, H. J., “Electric Impedance of the Squid Giant Axon During Activity,” J. Gen. Physiol. =22=:649-670 (1939)

5. Eccles, J. C., “The Effects of Use and Disuse of Synaptic Function,” “Brain Mechanisms and Learning—A Symposium,” organized by the Council for International Organizations of Medical Science, Oxford:Blackwell Scientific Publications, 1961

6. Franck, U. F., “Models for Biological Excitation Processes,” “Progress in Biophysics and Biophysical Chemistry,” J. A. V. Butler, ed., London and New York:Pergamon Press, pp. 171-206, 1956

7. Gerard, R. W., “Biological Roots of Psychiatry,” Science =122 (No. 3162)=:225-230 (1955)

8. Gesell, R., “A Neurophysiological Interpretation of the Respiratory Act,” Ergedn. Physiol. =43:=477-639 (1940)

9. Hebb, D. O., “The Organization of Behavior, A Neuropsychological Theory,” New York:John Wiley and Sons, 1949

10. Hebb, D. O., “Distinctive Features of Learning in the Higher Animal,” “Brain Mechanisms and Learning—A Symposium,” organized by the Council for International Organizations of Medical Science, Oxford:Blackwell Scientific Publications, 1961

11. Konorski, J., “Conditioned Reflexes and Neuron Organization,” Cambridge:Cambridge University Press, 1948

12. Lillie, R. S., “Factors Affecting the Transmission and Recovery in the Passive Iron Nerve Model,” J. Gen. Physiol. =4=:473 (1925)

13. Lillie, R. S., Biol. Rev. =16=:216 (1936)

14. Matumoto, M., and Goto, K., “A New Type of Nerve Conduction Model,” The Gurma Journal of Medical Sciences =4(No. 1)= (1955)

15. McCulloch, W. S., and Pitts, W., “A Logical Calculus of the Ideas Immanent in Nervous Activity,” Bulletin of Mathematical Biophysics =5=:115-133 (1943)