Slow Potentials During Long-Term Memory Retrieval

Abstract

Almost half a century ago, Karl Lashley summarized his decade-long effort on the localization of the engram with the following, somewhat pessimistic statement: “it is not possible to demonstrate the isolated localization of a memory trace anywhere in the nervous system. Limited regions may be essential for learning or retention of a particular activity, but . . . the engram is represented throughout the region” (Lashley, 1950). As far as neocortical structures are concerned, this position seems to be still valid. Neither experimentally induced lesions in animals nor those which occur naturally in human subjects support the idea that specific mnemonic contents can be narrowly localized anywhere in the cortex. A localization of memory functions could be established only insofar as that specific anatomic structures were found to be essential for the process of storage and retrieval but not for the engram itself. Moreover and surprisingly, these structures, which may be seen as relay stations within larger functional circuits, are for the most part not localized in the neocortex but in more ancient regions of the brain, for example, in the diencephalon, the basal forebrain, the hippocampus or the amygdalae (see, for example, the summarizing theories of (Mishkin and Appenzeller, 1987, or Markowitsch, 1985)). How these relay stations communicate with neocortical regions during storage and retrieval and where in the neocortex specific memory contents are held are still open questions.