Consolidation of memory.

Animal studies have proven useful in addressing aspects of memory formation and consolidation that cannot be readily answered in research with humans. In particular, they offer the possibility of controlling both the extent and locus of brain lesions, and the exact nature of the experiences to be remembered. Taking advantage of these possibilities, recent studies indicated that the graded retrograde amnesia often seen after lesions to the hippocampal system is not uniform across lesion site and task, nor is it an indication that all of the remembered information available in intact subjects becomes available after hippocampal system lesions made a long time after learning. Rather, these studies support the notion that information is stored in both hippocampal and extrahipocampal sites, and that retrieval from different sites involves access to different kinds of information. The strongest evidence in support of this view is the set of findings indicating that when remote memories are retrieved, in either human or animal subjects that have suffered hippocampal system damage, these memories are not qualitatively the same as remote memories retrieved in intact subjects. In sum, memory appears to be rather more dynamic than most current conceptions allow, such that retrieval events trigger new encodings, and these new encodings engage the hippocampal system once again. As a result, older, reactivated memories become more resistant to disruption, and this mechanism helps to explain why graded retrograde amnesia is sometimes seen after brain damage. The use of new neuroimaging techniques, coupled with more sensitive neuropsychological tests in lesioned subjects, should further illuminate the complex nature of memory in coming years. It is likely that animal studies will continue to prove important in these developments.