Nature Neurosci. doi:10.1038/nn.2263 (2009)

Why do some memories fade, whereas others retain their original clarity? Experiments on mice suggest that details of precise, long-term memories are stored outside the area of the brain involved in initial memory formation, the hippocampus.

Paul Frankland and his colleagues at the Hospital for Sick Children in Toronto, Canada, trained mice to expect an electric shock to their feet when placed in one type of chamber, but not when housed in a different-looking one. When returned to these chambers, the mice were more likely to freeze in the type associated with the foot shocks.

This behaviour persisted if the rodents' hippocampi were surgically damaged 42 days after training, but not if this was done just one day afterwards. So although longer-term memories may be stored outside the hippocampus, the storage process seems to be prolonged and hippocampus-dependent.