Fragmented replay of very large environments in the hippocampus of bats

The hippocampus is crucial for memory. Memory consolidation is thought to be subserved by hippocampal “replays” of previously experienced trajectories. However, it is unknown how the brain replays long spatial trajectories in very large, naturalistic environments. Here, we investigated this in the hippocampus of bats that were flying prolonged flights in a 200-m-long tunnel. We found many time-compressed replay sequences during sleep and during awake pauses between flights, similar to rodents exploring small environments. Individual neurons fired multiple times per replay, according to their multiple place fields. Surprisingly, replays were highly fragmented, depicting short trajectory pieces covering only ∼6% of the environment size—unlike replays in small setups, which cover most of the environment. This fragmented replay may reflect biophysical or network constraints on replay distance and may facilitate memory chunking for hippocampal-neocortical communication. Overall, hippocampal replay in very large environments is radically different from classical notions of memory reactivation—carrying important implications for hippocampal network mechanisms in naturalistic, real-world environments.