What purpose might separate slow and fast gamma oscillations serve in the mPFC–hippocampal network? Fast gamma oscillations in the hippocampus are coherent with fast gamma oscillations in the medial entorhinal cortex [31••], an area that transmits information to the hippocampus about the current environment [33,34]. It is possible that mPFC fast gamma oscillations are coherent with fast gamma in both the hippocampus and the entorhinal cortex. If so, coherent fast gamma oscillations in the entorhinal–hippocampal–mPFC network could coordinate information flow across the three areas during processing of information related to the external environment. Considering that mPFC plays an important role in attentional selection [7], one possibility is that fast gamma oscillations selectively activate neurons that code information related to attended stimuli. On the contrary, slow gamma oscillations in CA1 are coherent with slow gamma in CA3 [31••], an area that is required for memory retrieval [35,36]. Thus, coherent slow gamma oscillations between mPFC and the hippocampus could serve to coordinate interactions between the hippocampus and mPFC during operations such as memory retrieval and working memory, in which neuronal codes can be evoked or maintained internally after the represented information is no longer present in the external environment.
