Curved geometry shapes the observability of hippocampal oscillations

Curved neural architectures reshape the spatial distribution of field oscillations, complicating the identification of their underlying generators. To test these principles experimentally, here we resolve the paradoxical poor expression of theta oscillations in the hilus of the folded dentate gyrus (DG), a region that otherwise exhibits some of the largest FPs in the brain. Using laminar recordings across both hippocampi and computational modeling, we show that theta is locally masked by cancellation of phase-coherent contributions. Theta generators in CA1 and DG produce volume-conducted fields that, due to the curved geometry of the DG, exhibit opposite dipolar orientations in the hilus, leading to regional cancellation. In contrast, beta and gamma generators are only weakly coordinated across hippocampal regions and therefore remain prominently expressed despite the same geometry. These results demonstrate that theta amplitude, local observability, spatial segregation and apparent translocation emerge from the interplay between geometry and temporal coherence among distributed generators, with tissue geometry acting as a spatial filter for coherent activity.