A population oscillator in the medial septum: ring-like rotational dynamics phase-aligned with hippocampal theta

Peter Petersen, PI and Ass. Professor, Department of Neuroscience, University of Copenhagen, Denmark

The brain often relies on rhythmic activity to coordinate computation across circuits, but how a precise population clock emerges from heterogeneous single neurons remains unclear. We address this question in the medial septum, a key generator of the theta rhythm that structures hippocampal activity during navigation and memory. Although medial septal neurons appear highly diverse at the single-cell level and fire across many phases of the theta cycle, large-scale Neuropixels recordings reveal a strikingly simple population organization. At the ensemble level, septal activity evolves as a low-dimensional rotational dynamic on a ring-like manifold. Using complementary linear, nonlinear, and topological analyses, we show that this circular geometry is an intrinsic feature of the population code rather than an artifact of dimensionality reduction. The instantaneous angle on this manifold provides a compact description of septal population state and is phase-aligned with hippocampal theta. Importantly, a neuron’s contribution to this collective dynamics is not determined by whether it leads or lags the hippocampal rhythm, but by how strongly it is rhythmically coupled to the septal network. These results show that coherent theta timing does not require synchrony across individual neurons. Instead, it emerges from a distributed population oscillator with stable geometric structure. We therefore propose that the medial septum should be understood not as a collection of loosely coordinated pacemaker cells, but as a unified low-dimensional clock whose rotational phase may provide a precise timing reference for hippocampal computation.