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

Abstract

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.