The above-described studies provide evidence that theta-gamma coupling facilitates memory operations in the entorhinal-hippocampal network and suggest a mechanism for how such coupling arises. But the question remains as to why strong coupling between theta phase and gamma amplitude would improve memory processing. If theta-modulated slow and fast gamma serve separate memory retrieval and encoding functions, as hypothesized above, then ensuring that slow and fast gamma episodes occur on different theta phases may prevent interference between memory retrieval and encoding [37]. It will be important to determine whether slow and fast gamma play important roles in retrieval and encoding processes or merely correlate with retrieval and encoding due to their relationships to theta phase. In any case, locking fast gamma to a particular theta phase could ensure that the entorhinal cortex and hippocampus are in the fast gamma mode at the same time, considering that theta is phase-synchronized across the regions [38] (Figure 1). Maximal firing probability of cells in the superficial layers of entorhinal cortex occurs immediately after the theta phase associated with maximal gamma power [39]. CA1 place
