Theta dominates cross-frequency coupling in hippocampal-medial entorhinal circuit during awake-behavior in rats

Hippocampal theta and gamma rhythms are hypothesized to play a role in the
physiology of higher cognition. Prior research has reported that an offset
in theta cycles between the entorhinal cortex, CA3, and CA1 regions
promotes independence of population activity across the hippocampus. In
line with this idea, it has recently been observed that CA1 pyramidal
cells can establish and maintain coordinated place cell activity
intrinsically, with minimal reliance on afferent input. Counter to these
observations is the contemporary hypothesis that CA1 neuron activity is
driven by a gamma oscillation arising from the medial entorhinal cortex
(MEC) that relays information by providing precisely timed synchrony
between MEC and CA1. Reinvestigating this in rats during appetitive track
running, we found that theta is the dominant frequency of cross-frequency
coupling between the MEC and hippocampus, with hippocampal gamma largely
independent of entorhinal gamma.