Memories and engrams: a clinical viewpoint.

Abstract

Memory research is based on the assumption that a more or less permanent trace of previous experience, an “engram”, is being laid down somewhere in the brain and memory research is still “the search of the engram”. However, the clinician engaged in memory research has to be aware of the fact that he is dealing with memories and behavioural change, but not with engrams. Nevertheless, clinicopathological studies as well as the introduction of animal experiments into memory research using stimulation ablation and chemical methods have led to some progress in the search of the engram. The parallelism of the clinical observations and the experimental studies seems to indicate that one has to differentiate two kinds of memory trace, a short term trace which under normal conditions becomes consolidated into a permanent trace, while pathologically, the consolidation process does not take place. There are reasons to believe that the short term trace may be initiated or even maintained for a short time electrophysiologically by reverberating circuits in certain cell assemblies. This may set into action changes at the synapses probably by means of cholinergic and/or adrenergic enzyme systems. The mode of action of the electrophysiological and biochemical changes underlying short term retention, however, is only poorly understood and this also applies to the biochemical changes which are assumed to take place during the consolidation process and which eventually form the basis of the permanent engram. The clinicopathological findings seem to indicate that the short term memory traces of human experiences, which are always multisensorial, are distributed over the various relevant neocortical areas. The specific partial short term memory traces are integrated at first by the factor of temporal contiguity and form the basis of short term personal memories which characterize the memory function in clouded states and in chronic amnestic syndromes. The cortex seems also to be responsible for the consolidation of the “partial memories” while a bilaterally intact limbic system is necessary for the integration of the short term memory traces into consolidated long term engrams of “personal memories”.