Fig. 4.

Reverberatory circuits and the cell assembly. These figures show the evolution of the reverberatory circuits that Hebb developed into his cell assembly. (a) A diagram of the pathways connecting interneurons among themselves and with the ocular motoneurons. V = vestibular nerve; I to 6 = cells in the primary vestibular nuclei; 7, 8, 9 = cells in the reticular formation in the medulla (Med.) and pons (P.); 10, 11, 12 = cells in the reticular nuclei in the midbrain (M.b.); Oc.n. = oculomotor nuclei; Fl, F2 and Col. = positions of the stimulating electrodes. The response of the motoneurons can be recorded with electrodes (R) from the trochlear or oculomotor nerve (III). Delivery of a shock to these nerves outside the brain stem through electrodes (A) causes the arrival of antidromic impulses at the motoneurons. The diagrams below illustrate the two types of chains, M = multiple and C = closed, that are found in the internuncial system. The closed loop system (C) represents the reverberatory circuit. ([125], page 407). (b) Schematic arrangement of neurons to account for conditioning by closed chains. The assumption is made that activation of a nerve cell occurs only when the cell receives excitation from two axon endings simultaneously. Impulses in CS are ineffective with respect to neurons a and b. If impulses in UncS and CS arrive simultaneously, however, they summate to excite b. This sets the closed chain b-c in continuous activity, and impulses in CS now summate with collaterals from fibers in the chain to activate a and RuncS. Simultaneous excitation of b will occur by chance if CS and UncS are stimulated together repeatedly. The chances of simultaneity are increased if the frequency of impulses in CS and UncS is greater; i.e., if the intensity of the stimuli is greater. While the speed of conditioning in any single neuron unit is thus largely a matter of chance according to this scheme, the sum of the changes in many such units would result in a gradual increase in the number of RuncS fibers activated. This scheme is not elaborated here to account for extinction, or other phenomena of conditioning. Inhibitory effects might be introduced by the addition of specific inhibitory collaterals, or by consideration of temporal relations resulting in refractory period decrement. CS = Conditioned stimulus; UncS = Unconditioned stimulus; Rcs = conditioned response; RuncS = Unconditioned response. (From [91], page 331). (c) Diagram of neurons and their synaptic connections illustrating the principle of recurrent (reverberatory) nerve circuits as seen in the IIIrd nerve nucleus (After Lorente de Nó). (From [139], page 64). (d) Hebb’s diagram of his cell assembly. Arrows represent a simple “assembly of neural pathways or open multiple chains firing according to the numbers on each (the pathway “1, 4“ fires first and fourth, and so on), illustrating the possibility of an “alternating“ reverberation which would not extinguish as readily as that in a simple closed circuit. (From Hebb [77], page 73). (e) An illustration of the way in which learning might modify the functioning of cortical circuits and establish a cell-assembly. It is assumed that A-B-C and D-E-F, in association cortex, are excited by the same sensory event (axons from the sensory cortex are not shown, but it is assumed that they excite these cells separately). If A then delivers impulses to B at the moment when B is being fired by axons from sensory cortex, the synapse A-B will be “strengthened”, and similarly with the other synapses. As a result of this strengthening the excitation of one cell may become able to set up reverberation in the circuit. Broken lines show possible connections between the two circuits, which would permit them to function as one system. (From [80] page 104). a Reprinted from Lorente de No R. Transmission of impulses through cranial motor nuclei. J Neurophysiol. 1939;2:402–64 [125]. Copyright (1939), with permission from the American Physiological Society. b Reprinted from Hilgard ER, Marquis DG. Conditioning and learning. New York and London: Appleton-Century Company; 1940 [91]. Copyright (1940), with permission from Appleton-Century-Crofts. c Reprinted from Morgan CT. Physiological Psychology. New York and London: McGraw-Hill Book Company Ltd; 1943 [139]. Copyright (1943), with permission from McGraw Hill. d Reprinted from Hebb DO. The organization of behavior; a neuropsychological theory. NY: Wiley; 1949. [reprinted 2002 by Lawrence Erlbaum Associates, Mahwah, New Jersey] [77] Copyright (2002), with permission from Lawrence Erlbaum Associates. e Reprinted from Hebb DO. A textbook of Psychology. Philadelphia and London: W. B. Saunders Company; 1958 [80]. Copyright (1958), with permission from Mary Ellen Hebb