Another prominent rhythm, gamma oscillations, plays a role in local processing in both regions. In the hippocampus, slow (40–60 Hz) and fast (80–120 Hz) gamma are associated with different phases of theta oscillations, and are thought to mediate internally driven representations arising from CA3 and externally driven representations via entorhinal cortex, respectively [32,57]. Outside of theta states, slow gamma is also linked to SWR replay, possibly influencing attractor states that govern replay events [58–60]. In PFC, gamma oscillations are prominently linked to cognitive processes, including attentional processing, rule representation, and flexible rule switching [11,14,23,61]. Critically, there is evidence indicating the existence of hippocampal-cortical gamma interactions: PFC gamma oscillations are coordinated with hippocampal theta [29], and perturbing ventral hippocampal connections to PFC selectively impairs PFC gamma oscillations in a memory task [30]. Thus, activity during gamma oscillations must be coordinated between the regions, although this still remains to be shown.
