Caswell Barry (@caswell.bsky.social)

Decoding medial entorhinal cortical dynamics produces planning-like alternations in hippocampal theta sequences The hippocampus plays a central role in memory and spatial planning. In rodents, theta oscillations (7-12Hz) are associated with active navigation and appear necessary for planning and spatial memory. A key neural mechanism held to support navigational decision making are theta sequences - brief (∼120ms) sequential activations of hippocampal place cells representing trajectories ahead of the animal. Evidence for their role in planning includes their tendency to sample potential future paths at decision points, particularly the left–right sequences observed along the T-maze arms during alternation tasks. However, a recent study challenges this interpretation, proposing that the natural tendency of medial entorhinal cortex (MEC) theta sequences to alternate between leftward and rightward directions might account for the apparent sampling of arms on the T-maze. Given that MEC inputs drive hippocampal activity, this raises the possibility that hippocampal alternations in T-mazes simply reflect intrinsic MEC dynamics rather than active planning. Here, we test this hypothesis by modelling MEC theta sequences in a T-shaped maze and demonstrate that standard Bayesian decoding produces hippocampal theta sequences alternating between maze arms, consistent with experimental observations. Analysis of simultaneous MEC-hippocampal recordings reveals that MEC alternation consistently precedes hippocampus by ∼50ms, suggesting a feedforward, state-independent mechanism. This coordinated MEC-hippocampal left-right alternation occurs not only at choice points but also during inbound trials (where no deliberate choice is required) and at forced-turn corners (where only one path exists). 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. ### Competing Interest Statement The authors have declared no competing interest. https://www.biorxiv.org/content/10.64898/2025.12.29.696939v1