Neural reactivation reveals mechanisms for updating memory.

Our ability to remember new information is often compromised by competition from prior learning, leading to many instances of forgetting. One of the challenges in studying why these lapses occur and how they can be prevented is that it is methodologically difficult to "see" competition between memories as it occurs. Here, we used multi-voxel pattern analysis of human fMRI data to measure the neural reactivation of both older (competing) and newer (target) memories during individual attempts to retrieve newer memories. Of central interest was the following: (1) whether older memories were reactivated during retrieval of newer memories; (2) how reactivation of older memories related to retrieval performance; and (3) whether neural mechanisms engaged during the encoding of newer memories were predictive of neural competition experienced during retrieval. Our results indicate that older and newer visual memories were often simultaneously reactivated in ventral temporal cortex--even when target memories were successfully retrieved. Importantly, stronger reactivation of older memories was associated with less accurate retrieval of newer memories, slower mnemonic decisions, and increased activity in anterior cingulate cortex. Finally, greater activity in the inferior frontal gyrus during the encoding of newer memories (memory updating) predicted lower competition in ventral temporal cortex during subsequent retrieval. Together, these results provide novel insight into how older memories compete with newer memories and specify neural mechanisms that allow competition to be overcome and memories to be updated.