Systems consolidation and the content of memory

Systems consolidation is the process by which memories become independent of the hippocampus and stored in regions of the neocortex. This process is commonly studied in rodents using context fear conditioning. It is becoming increasingly clear, however, that context memories do not always undergo systems consolidation. To explain this fact, the current review describes a number of factors that determine whether or not context fear can be retrieved without the hippocampus during remote memory tests. These include neurogenesis, the presentation of reminder cues after learning, the quality of the memory that is retrieved during testing and the method that is used to inactivate the hippocampus. Based on these data, we propose that remote context fear memories can be retrieved by either the hippocampus or the neocortex. Tests of memory quality (e.g. context discrimination) can typically be used to determine which system is engaged during retrieval. The same is not true of recently formed context fear memories, which appear to always require the hippocampus during retrieval.