PROJECT SUMMARY/ABSTRACT Hippocampal-neocortical interaction during sleep is critical for consolidation of newly acquired information into long-term memory. There is substantial amount of evidence supporting that neocortical regions including the anterior cingulate cortex (ACC) store long-term memories. However, it remains unclear how the hippocampus, particularly the dorsal hippocampus (dHPC), communicates with the ACC during the memory consolidation process – one main objective of the PI’s R01 grant. One puzzle and challenge we face is that there is almost no direct projection from the dHPC to ACC, or vice versa; thus, the dHPC communicates with the ACC via a relay region, likely the retrosplenial cortex (RSC) which receives a dense projection from the dHPC and is reciprocally connected with the ACC. This Supplements proposal intends to extent the PI’s R01 project by investigating dHPC–RSC interactions as a potential mechanism underlying memory consolidation. Our preliminary results revealed that a subset of RSC layer 5 neurons (~25%), termed ‘silent assembly’, display a high probability of activation during short periods of cortical silence and precedes the activity of other RSC neurons, ACC neurons, and dHPC neurons. The central objective here is to test our hypothesis that a coordinated dHPC–RSC communication at the assembly level is critical for memory consolidation: first, activation of RSC layer 5 silent assembly during cortical silence initiates the memory consolidation process; second, dHPC assembly sends information to the RSC layer 2/3 for transforming hippocampus-dependent memory into cortex-dependent memory. Supported by considerable preliminary data, we propose to pursue this objective through the following three specific aims. Aim 1 investigates the input that drives the RSC silent assembly activity during cortical silence. Aim 2 investigates a causal relationship between RSC silent assembly activity and memory consolidation. Aim 3 investigates how the communication of dHPC and RSC assemblies evolve after a new learning experience. Results from this study will advance our understanding of memory formation associated circuits and provide new insight into the mechanism of initiation of memory consolidation. Meanwhile this study will provide the candidate an excellent opportunity to master advanced data analysis and visualization skills that are critical for understanding dynamic brain region interactions and information exchanges.