Theta rhythms coordinate the activity of neurons on a relatively slow time scale. Slow oscillations are capable of coordinating activity across widespread networks of neurons because neurons in areas that are separated by long conduction delays can still be activated within the same oscillatory cycle [14]. Monosynaptic delays between the hippocampus and mPFC have been reported to be ~15 ms [15]. Thus, the ~150 ms period of a theta cycle would certainly be capable of coordinating direct activations between the two regions and plausibly could tolerate polysynaptic interactions as well. In line with this idea, several studies have recorded simultaneously from the hippocampus and mPFC and observed coherent theta rhythms [16,17,18••,19••], as well as correlations between spike times of mPFC neurons and hippocampal theta phase ([16,18••,19••,20–22,23••,24,25•]; Figure 1a).
