Research suggests that hippocampal cells fire in a particular pattern as an animal explores a novel situation, and then that same firing pattern is replayed afterwards during rest or slow-wave sleep to consolidate the encoding of that experience. This is how we learn and remember.
A study done at MIT by George Dragoi and Susumu Tonegawa, published in Nature in Jan 2011, suggests that hippocampal cells can also be observed starting to encode a novel experience before it happens. In other words, the brain is remembering something from the future or learning something that has not happened yet.
This prescient hippocampal firing happened during a rest or sleep period before the novel activity.