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 memory encoding but not memory retrieval [24]. Indirect support for the hypothesis that fast gamma is involved in memory encoding was provided by a study of place cell firing during theta-modulated slow and fast gamma states [22]. During theta-modulated fast gamma, individual place cells fired late in their place fields, and place cell ensembles preferentially coded locations in the recent past. Such ‘retrospective’ activity in the hippocampus may occur in response to persistent firing in entorhinal cortex [27,28] and may provide the repetitive activation necessary to drive memory encoding [29].
