As a crossroads between sensory inputs and long term memories, the hippocampus turns a plethora of information into concise episodes for us to remember. The hippocampus can employ different strategies to achieve this transformation. By selecting only notable experiences to transfer to long term memory storage, we can remember important experiences while forgetting the mundane. By encoding common principles among several experiences, we can remember appropriate general responses and predict future similar experiences. We considered ways the hippocampus might achieve these two possibilities by examining hippocampal activity while rats executed sequences for rewards.
Given that we must remember the experiences that lead to reward in order to exploit these rewards in the future, we asked if memory processes are enhanced by reward. In particular we examined hippocampal sharp wave-ripples (SWRs) because reactivation of previous experiences during SWRs is thought to be essential for event memory storage. We found that SWR activity increases when animals receive reward. This reward related SWR activity is further enhanced when animals have to learn new path-reward associations. Additionally, SWR activity reactivates neural patterns that occur as animals run to or from the reward. Because SWRs are implicated in memory consolidation, this enhanced SWR reactivation could be a mechanism to preferentially remember experiences associated with reward.
Furthermore, when navigating environments with many repeated elements, generalizing across elements can be advantageous to efficiently encode appropriate responses. Simultaneously, each element must also be differentiated from the others. To study this, we then examined hippocampal activity as animals traversed environments with many repeated elements and had to distinguish between these elements to receive reward. We found that some hippocampal cells fire very similarly on multiple repeated elements, while other cells encode the elements differently. Cells that generalize across similar elements have correlated moment to moment activity, suggesting that they are part of functional ensembles. Furthermore, this generalizing / path equivalent activity increases as animals learn new relationships between repeated elements. This generalization across repeating elements could be a mechanism to extract general principles about related experiences.