Decoding Medial Entorhinal Cortical Dynamics Produces Planning-Like Alternations in Hippocampal theta Sequences

The hippocampus is central to memory and spatial planning. A prominent candidate mechanism supporting navigational decision making is hippocampal theta sequences-brief (~120 ms) place-cell sequences representing future trajectories-which have been interpreted as planning signals based on their alternation between maze arms in T-maze tasks. However, recent work suggests that intrinsic left-right alternation in medial entorhinal cortex (MEC) theta sequences may underlie this phenomenon. Here, we test this hypothesis using a model of MEC theta dynamics in a T-maze and show that standard Bayesian decoding yields hippocampal theta sequences that alternate between arms. We then analyzed existing simultaneous MEC-hippocampal recordings from Vollan et al. (2025). We found tight coupling between MEC and hippocampus, along with coordinated left-right alternation, while the animal was performing an alternation task. Notably, this conjoint alternation occurs not only at choice points but also during inbound and forced-turn trials. These findings suggest that left-right alternation in hippocampal theta sequences is a ubiquitous phenomenon driven by MEC dynamics, challenging the interpretation that these specifically reflect intentional decision-making or planning processes only.