A postulated mechanism of internally generated neuronal sequences is the perpetual interaction among the multitude of brain rhythms maintained by cross-frequency coupling67. A prominent example of cross-frequency coupling in the hippocampus and entorhinal cortex is the theta phase-modulation of gamma power68,69, which has been shown to correlate with memory performance in both rats70 and humans71–73. This multiple-timescale organization is also evident in the spiking activity of hippocampal neurons (Fig. 5). Place cells representing the same spatial position or item form assemblies in the time window of gamma cycles54,74,75. Which neurons are considered members of an assembly is determined by their downstream ‘reader’ neurons. If the collective spiking of an upstream population occurs within the membrane time constant76 (10–30 ms) of the reader neurons, it will be classified as a single event by the readers. Spikes of other upstream neurons outside this integration window are relegated to other assemblies, thus representing separate events and resulting in the discharge of other reader neurons60. The ‘assembly window’ coincides with the time window of spike timing–dependent plasticity and the time constant of GABAA and
