Gamma oscillations coordinate different theta rhythms in the hippocampus

Theta-gamma cross-frequency coupling (CFC) is thought to route information flow in the brain. How this idea copes with the co-existence of multiple theta rhythm generators is not well understood. We have analysed multiple theta and gamma activities in the hippocampus to unveil the dynamic synchronization of theta oscillations across hippocampal layers, and its differential coupling to layer-specific gamma frequency bands. We found that theta-gamma CFC is stronger between oscillations originated in the same hippocampal layer. Interestingly, strong CFC was linked to theta phase locking across layers in a behaviourally related manner, being higher during memory retrieval and encoding. Systematic analysis of cross-frequency directionality indicated that the amplitude of gamma oscillations sets the phase of theta in all layer-specific theta-gamma pairs. These results suggest, contrary to an extended assumption, that layer- and band-specific gamma-oscillations coordinate theta rhythms. This mechanism may explain how anatomically distributed computations, organized in theta waves, can be bound together.