Abstract: At the centennial anniversary of research on memory experimentalpsychology which dates back to the book "Memory" by H. Ebbinghausof Germany, the author comments on the mystery of engram whichhas been regarded as the core cf the mystery of the mind, andbriefly reviews the history of research on the brain mechanisms ofmemory. The paper also deals with the two great debates in this centuryand their representative hypotheses, such as I. P. Pavlov's conditionedreflex theory, K. S. Lashley's integration theory, M. R. Rosenzweig'scorrelation hypothesis, G. Ungar's nervous codes hypothesis, K. H.Pribram's hollogram hypothesis (1969), Jose M. R. Delgado's constellat-ions hypothesis (1981), and the hypothesis of electrical codes, chemicalcodes, and structural codes of acquired informations and engrams inthe brain.

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