Synchronous Activity of Event and Place Representations in Hippocampal CA1

Integration of spatial and event information in episodic memory is thought to be supported by hippocampus. However, it is unknown how an event such as receipt of reward is represented together with spatial information at the cellular level. To address this issue, we performed longitudinal calcium imaging in hippocampal CA1 of mice performing a spatial delayed reward task in virtual reality. In this task, reward was given when mice stayed for 2 sec at one of the three different colored zones in a virtual linear track. After learning, about 13% of cells demonstrated time-locked activity while the mice waited for reward, and the proportion was correlated with the task performance. When the delay before the reward was omitted, the onset of activity shifted accordingly. In addition, 1.4% of cells continued to respond to the previous reward zone after reward delivery was moved to a new zone. Interestingly, a subset of cells that showed time-locked activity participated in multiple synchronous activity with place cells during the stationary, and the occurrence of such coactivity persisted over multiple days. These results suggest that formation and persistence of neuronal assemblies that involve cells encoding event timing and place cells may be implicated in a hippocampal memory process that associates events with space.