Grading the gradient: Evidence for time-dependent memory reorganization in experimental animals

In humans, damage limited to the hippocampus produces temporally graded retrograde amnesia, with relative sparing of remote compared to recent memory. This observation forms the cornerstone of the idea that as memories mature they are reorganized in a time-dependent manner. In this paper, we evaluate evidence for similar time-dependent reorganization in experimental animals. In the majority of behavioral paradigms examined, these studies provide evidence that memories are gradually reorganized over time, with the hippocampus and cortex playing preferential (although not necessarily mutually exclusive) roles in the expression of recent and remote memory, respectively. This pattern is not observed in all tasks, however. For example, in the water maze hippocampal activity is always necessary for memory expression. Identifying situations when the hippocampus is, and is not, required for remote memory expression will help us to understand hippocampal contributions to memory, and, more generally, whether changes in memory organization lead to qualitative changes in the nature of memory.