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The discussion which follows will be based upon the informal theory of CNS action introduced by Hebb in his signal 1949 work and subsequently importantly enlarged by P. M. Milner (1957) and I. J. Good (1965). I will attempt a brief recapitulation of some of the main assumptions here, with the intention of orienting the reader having some knowledge of the area. This is only one view of CNS processes and the presentation has been kept deliberately simplistic. (E.g., a more sophisticated theory would take account of substantial evidence for distinct physiological mechanisms underlying short-term, medium-term, and long-term memory.) The object is to indicate, in as simple a context as possible, the relevance of the  framework to understanding the CNS as a means of "adaptation to the environment under sensory guidance." The reader without a relevant background can gain a significant understanding by reading the papers of Milner and Good; a reading of Hebb's excellent textbook (1958) will give a much more comprehensive view. |
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The basic element of Hebb's theory is the cell assembly. It is assumed to exhibit the following essential characteristics. (Comments in parentheses in the presentation of characteristics refer to possible neurophysiological mechanisms): |
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1. A cell assembly is formed in response to repetitions of some relatively simple critical feature of the sensory input, such as pressure on a particular skin area, a simple odor, a vowel sound, an increase of brightness, a line of particular slope in the visual field, and so on. (From a more physiological point of view, a cell assembly is taken to be a collection of hundreds of neurons interconnected via synapses of high conductivity.) After some assemblies have been formed others may be formed in response to the repetitive actions of already extant (precursor) assemblies. |
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2. Whenever the critical feature causing the cell assembly's formation subsequently appears in the sensory input, the cell assembly tends to become active. Cell assemblies formed in response to precursor assemblies tend to become active when their precursors are active. (Once a significant number of neurons in the cell assembly attain a high pulse rate, the remaining neurons quickly follow suit because of the highly conductive interconnections. Because the interconnections form many loops, a reverberation results and the neurons tend to remain active for a period of time long compared to the stimulus time.) |
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3. Cell assemblies exhibit changing positive and negative associations with each other. A cell assembly which is active increases the likelihood of activity in all assemblies with which it is positively associated and de- |
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