Consolidation of long-term memory: Evidence and alternatives

Memory loss in retrograde amnesia has long been held to be larger for recent periods than for remote periods, a pattern usually referred to as the Ribot gradient. One explanation for this gradient is consolidation of long-term memories. Several computational models of such a process have shown how consolidation can explain characteristics of amnesia, but they have not elucidated how consolidation must be envisaged. Here findings are reviewed that shed light on how consolidation may be implemented in the brain. Moreover, consolidation is contrasted with alternative theories of the Ribot gradient. Consol-idation theory, multiple trace theory, and semantization can all handle some findings well but not others. Conclusive evidence for or against consolidation thus remains to be found. Consolidation, as a concept, has a century-old history (Lechner, Squire, & Byrne, 1999). It was originally proposed as an expla-nation for retroactive interference (Muller & Pilzecker, 1900). Although interference theory soon came to rely on other constructs (McGeoch, 1932), consolidation fanned out to explain a plethora of other phenomena. A search in PsycLIT in August 2003 focusing on consolidation and memory, for example, produced 1,167 hits