Theta is not only coupled with slow gamma rhythms in the entorhinal-hippocampal network but also with fast gamma rhythms (~60–100 Hz) at a different theta phase [14,17,24,25]. During periods of fast gamma, CA1 is preferentially synchronized with inputs from the medial entorhinal cortex (MEC) [14]. Several recent studies suggested functional roles for such theta-modulated fast gamma coupling in the entorhinal-hippocampal network. Cabral and colleagues found that CA1 fast gamma power was more strongly locked to theta phase in mice using a place-based strategy, rather than a sequence memory-based strategy, to solve a mnemonic task [26]. Mice must pay attention to environmental stimuli, such as landmarks, when using a place-based strategy. Under these conditions, theta-modulated fast gamma may facilitate transmission of current sensory information from the entorhinal cortex to the hippocampus. The hippocampus also needs to access current sensory information when encoding new memories, and thus fast gamma coupling between the hippocampus and entorhinal cortex may support memory encoding. A recent study showed that fast, but not slow, gamma power in the MEC was reduced by the drug scopolamine, which blocks
