continuous local field potential theta rhythm is observed only infrequently30. Place and grid cells have been described in bats without apparent theta modulation28,29. Can the temporal coordination role of internal rhythms be substituted by other means, such as echolocation emission in bats, as occurs with saccadic eye movements in the visual system in primates? Neurons in the hippocampus and entorhinal cortex can be ‘driven’ by external cues and may persist in showing position-dependent firing even after theta oscillations cease24,99. Grid cells with either strong or no theta modulation often coexist, even at the same location. Yet in the presence of theta oscillations, place and grid cells are more pronounced, have higher peak rates and are more stable. Is theta coordination more important for memory than navigation, as memory depends primarily on internally generated neuronal sequences?Are independent changes in grid maps of different modules instrumental in generating new representations in the hippocampus, and if so, by what mechanisms? Are there intrahippocampal mechanisms for further segregation of representations? How many representations can be generated, and what mechanisms determine their independence? Is the hippocampal output important for generating and maintaining grid formation in the entorhinal cortex and subicular complex, and if so, how
