Resistance to forgetting associated with hippocampus-mediated reactivation during new learning

A primary reason we forget past experiences is because we acquire new memories in the interim. While the hippocampus is thought to play a primary role in acquiring and retaining memories for the past, there is little evidence linking neural operations during new learning to the forgetting (or remembering) of earlier events. Here we present novel evidence that, during the encoding of new memories, responses within the human hippocampus are predictive of the retention of memories for previously experienced, overlapping events. This brain-behavior relationship is evident in neural responses to individual events and in differences across individuals. We illustrate that the hippocampus accomplishes this function by reactivating older memories as new memories are formed—in this case, reactivating neural responses that represent monetary rewards associated with older memories. These data reveal a fundamental mechanism through which the hippocampus tempers the forgetting of older memories as newer memories are acquired.