A relatively early study investigating the link between hippocampal theta-gamma coupling and memory performance was performed in humans implanted with depth electrodes for treatment of temporal lobe epilepsy [10]. In a word recognition paradigm, coupling between theta phase and ~45 Hz gamma power was selectively enhanced when patients successfully remembered previously presented words. This finding suggested that recruitment of gamma oscillations at a particular theta phase facilitated retrieval of memories of earlier experiences. Later, consistent results were reported in rats. In an initial important study, animals learned that rewards were associated with two different stimuli in two different contexts [11]. In hippocampal subfield CA3, coupling between theta phase and ~40 Hz gamma amplitude improved as animals learned the task. This theta-gamma interaction occurred during exploration prior to the stimulus choice, a time when animals presumably retrieved their memory of the correct response associated with a particular environment. More recently, Igarashi and colleagues showed that ~20–40 Hz oscillations in hippocampal subfield CA1 became more tightly locked to theta phase as animals learned odor-place associations [12]. This cross-frequency coupling was observed during
