On the Elusive Permanence of Memory

That memory involves enduring physical changes in the organism has been proposed, using era-dependent metaphors, since antiquity. 1 More than a century ago, Richard Semon introduced the term engram to refer to such a change. 2 It gained popularity, however, only when a group of researchers, most notable among them Karl Lashley, embarked on a systematic hunt for engrams, using lesions to determine which parts of the brain impair the ability of animals to form and maintain memories. That search proved futile: “ This series of experiments, ” wrote Lashley in 1950 toward the end of his career, “ has yielded a good bit of information about what and where the memory trace is not . . . I sometimes feel, in reviewing the evidence on the localization of the memory trace, that the necessary conclusion is that learning just is not possible . . . Nevertheless, in spite of such evidence against it, learning does sometimes occur. ” 3