Dynamical Modulation of Hippocampal Replay Sequences through Firing Rate Adaptation

During periods of immobility and sleep, the hippocampus generates diverse self-sustaining sequences of "replay" activity, exhibiting stationary, diffusive, and super-diffusive dynamical patterns. However, the neural mechanisms underlying this diversity in hippocampal sequential dynamics remain largely unknown. Here, we propose such a mechanism demonstrating that modulation of firing rate adaptation in a continuous attractor model of place cells causes the emergence of different types of replay. Our model makes several key predictions. First, more diffusive replay sequences positively correlate with longer theta sequences across animals (both reflecting stronger adaptation). Second, replay diffusivity varies within an animal across behavioural states that affect adaptation (such as wake and sleep). Third, increases in neural excitability, incorporated with firing rate adaptation, reduce the step size of decoded movements within individual replay sequences. We provide new experimental evidence for all three predictions. These insights suggested that the diverse replay dynamics observed in the hippocampus can be reconciled through a simple yet effective neural mechanism, shedding light on its role in hippocampal-dependent cognitive functions and its relationship to other aspects of hippocampal electrophysiology.