Awake Hippocampal-Cortical Co-reactivation Is Associated with Forgetting

Systems consolidation theories posit that consolidation occurs primarily through a coordinated communication between hippocampus and neocortex (McClelland and OReilly 1995; Kumaran et al., 2016; Moscovitch and Gilboa, 2022). Recent sleep studies in rodents have shown that hippocampus and visual cortex replay the same information at temporal proximity ("co-replay") (Ji & Wilson, 2007; Lansink et al., 2009; Wierzynski et al., 2009; Peyrache et al., 2009). We developed a novel TR-based co-reactivation (TRCR) analysis method to study hippocampal-cortical co-replays in humans using functional MRI. Thirty-six young adults completed an image (face or scene)-location paired associate encoding task in the scanner, which were preceded and followed by resting state scans. We identified post-encoding rest TRs (+/- 1) that showed neural reactivation of each image-location trials in both hippocampus (HPC) and category-selective cortex (fusiform face area, FFA). This allowed us to characterize temporally proximal coordinated reactivations ("co-reactivations") between HPC and FFA. Moreover, we found that increased HPC-FFA co-reactivations were associated with incorrectly recognized trials after a 1-week delay (p = 0.004). Finally, we found that these HPC-FFA co-reactivations were also associated with trials that were initially correctly recognized immediately after encoding but were later forgotten in 1-day (p = 0.043) and 1-week delay period (p = 0.031). We discuss these results from a trace transformation perspective (Winocur and Moscovitch, 2011; Sekeres et al., 2018) and speculate that HPC-FFA co-reactivations may be integrating related events, at the expense of disrupting event-specific details, hence leading to forgetting.