Recent progress on the relationship among navigation, memory and oscillations raises as many questions as it has answered. We list here a few pertinent issues. Perhaps the most challenging question is the meaning of the various firing patterns, including place cells, grid cells and irregularly spaced neurons, to the target networks of the hippocampus-entorhinal cortex system. Does it matter for the downstream actuator networks whether space and memory items are represented by geometrically spaced or irregular patterns? Do regularly firing neurons carry more weight than neurons with less rigorous firing patterns? Presumably, the code has time frames with clear demarcation of the beginning and end of messages, which should be coherent across neuronal populations. Is theta oscillation a critical part of this mechanism, or do other forms of coordinating mechanisms exist?Hippocampal neurons in primates rarely fire in theta-rhythmic manner, and 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
