On the Parallel Between Learning and Evolution

i) The classification of the types of learning in animals given by THORPE (1950) is taken as the starting point for an attempt to formulate in general terms the characteristics of the process. It is suggested that the most general feature of learning is the in.crease in complexity of behaviour which results. 2) The concept of complexity, which has hitherto been used by biologists without definition, is analysed in terms of the "theory of information" developed in connection with problems of communication engineering. It is shown that, although it is a purely epistemological concept, it may be applied to descriptions of systems such as the living organism on which verifiable observations can be made. It is applicable to behaviour as well as to structure by a space/time inversion, verifiability consisting in this case of a correspondence between simultaneous observations made on different parts of the structure, instead of repeatability of observations at successive instants. 3) A short account is given in general terms of the kinetics of evolutionary processes, the mathematical treatment of which (based on the theory of stochastic processes) is 100 difficult for an article of this type. 4)Evolution by natural selection of random variations is the best known example of this type of process, but differentiation, treated as the problem of the evolution of a population of "plasmagenes" (SPIEGELMANN, 1948), may be another example, and it is suggested that learning is a third example. 5) It is shown that the phenomenon of synchronization of oscillators, the general features of which are described, can lead, in a population of oscillators, to an "evolutionary" increase of complexity of rhythm in a manner analogous to the increase of structural complexity which occurs in organic evolution. A model is described which is consistent with the known features of the physiology of the central nervous system of animals, and which is capable of producing the increase of complexity found in the process of learning. The oscillators in this model consist of circular chains or "closed loops" of neurons, and it is suggested that such closed loops may generate oscillatory wave-forms more closely resembling the electrical waves found in the cerebral cortex than the impulses characteristic of peripheral axons. The coupling required to produce synchronization of these oscillators is provided by the sharing of neurons. 6) The asymmetrical inter-relationship between oscillators necessary to produce "selection" results from the non-linearity of the oscillations generated by such a closed-loop arrangement. The "variation" on which selection operates may be provided by "endogenous" variability in the frequencies of the oscillators due to random changes of excitability of the neurons, or may be provided by the variation of the stimulus pattern from outside the animal. These two cases are compared to variation by mutation and by changes in the gene-complex, on either of which natural selection can operate to produce adaptive changes in organic evolution. 7) The types of learning, as defined by THORPE (1950), are analysed in turn in a schematic manner, and it is shown how each of them is related to the properties of the proposed model. The formation of conditioned reflexes is adequately interpreted, and also the phenomena of external and internal inhibition. The difficulty of "backward conditioning" also appears to be represented in the properties of the model, being based on the hysteresis effect in the synchronization of non-linear oscillators. 8) Habituation is shown to be an example of the homoeostatic properties of a closed loop system with negative feed-back. 9) Conditioned reflexes of KONORSKY'S Type I (1948) and habituation do not demand any endogenous variability in the frequency of oscillations in the model. Conditioned reflexes of Type II, trial-and-error learning, and insight learning do, however, demand that there shall be some endogenous variability in the nervous system if they are to be interpreted in terms of the properties of this model. io The "memory" of a stimulus pattern, or the plastic change in the nervous system resulting from the application of a stimulus, has, in previous theories about the learning process, been considered to involve a change in the synaptic connections between neurons (KONORSKY, 1948) or at least a change in "synaptic resistance". It is shown that with this model the requirement is for a change in the maintained level of excitation within certain cells. It is further apparent that no localised changes are to be expected, the memory trace taking the form of a slight change in a large number of cells rather than a large change in any one. This feature of the model is consistent with the observations of LASHLEY (1950) and others that the formation of a conditioned reflex and of other associations is not accompanied by a localised change in any part of the cerebral cortex of mammals. The location of the physical events accompanying association within the cells of the central nervous system rather than b e t w e e n them introduces the possibility, which is briefly discussed, of a transfer of the events from the level of the neuron down to the level of molecular changes in the substance of the living protoplasm.