Consolidation and the medial temporal lobe revisited: methodological considerations

It is widely believed that new memories are stored in the medial temporal lobe structures in the short term, but then are reorganized over time as the neocortex gradually comes to support stable long-term storage. On this view, the medial temporal lobe structures play a time-limited role in information storage. This putative process of reorganization, known as consolidation, is supported by some clinical findings in humans and by some data from nonhuman animals. Here we review prospective studies of retrograde memory in nonhuman animals, with particular emphasis on experimental design. In considering the evidence for a time-limited role for the medial temporal lobe in information storage, we note that there are alternative interpretations for at least some of the findings typically cited in support of the consolidation process. In addition, we suggest that some studies arguing against the consolidation view should probably be given more weight than they have so far received. Finally, we observe that different structures in the medial temporal lobe are unlikely to operate together as a single functional unit mediating a single consolidation process. Although evidence for a time-limited role for medial temporal lobe structures in memory is at present equivocal, future studies that consider some of the alternative accounts we and others have identified will provide a clearer picture of the mechanisms underlying information storage and retrieval in the brain.