Theta-gamma coupling in the entorhinal-hippocampal system

Abstract

For decades, theta rhythms (∼5-10Hz) have been thought to play a critical role in memory processing in the entorhinal-hippocampal network. However, recent evidence suggests that successful memory performance also requires coupling of ∼30-100Hz gamma rhythms to particular phases of the theta cycle. Recent insights imply ways in which theta-gamma coupling may facilitate transfer of information throughout the entorhinal-hippocampal network. Activating gamma-modulated cell assemblies at a particular theta phase may allow the network to produce a more powerful output by ensuring that distributed cells fire closely in time. I hypothesize that such a mechanism would serve to facilitate either memory encoding or memory retrieval, depending on which type of gamma rhythms are recruited.

Figure 1

A cartoon illustrating how coordinated theta-gamma phase coupling between MEC and CA1 could affect information transfer across the regions. Note alignment of gamma activity across the regions when strong theta phase-gamma power coupling is seen in CA1 (left). Under these conditions, effective interregional communication is expected, considering that cells in the regions are highly active immediately after ~80 Hz gamma episodes [14,30]. Weak theta-gamma coupling in CA1, as depicted in the panel on the right, would be expected to detrimentally affect communication between the regions.