Enhanced Theta and High-Gamma Coupling during Late Stage of Rule Switching Task in Rat Hippocampus.

Hippocampal oscillations, particularly theta (6-12 Hz) and gamma (30-90 Hz) frequency bands, play an important role in several cognitive functions. Theta and gamma oscillations show cross-frequency coupling (CFC), wherein the phase of theta rhythm modulates the amplitude of the gamma oscillation, and this CFC is believed to reflect cell assembly dynamics in cognitive processes. Previous studies have reported that CFC strength correlates with the learning process. However, details on these dynamic correlations have not been elucidated. In the present study, we analyzed local field potentials recorded from the rat hippocampus during the learning of a rule-switching task. The modulation index, an index of the CFC strength, became higher in rule-guided behavior than in the no rule condition. The enhanced coupling between theta and high-gamma oscillations (60-90 Hz) changed during the late stage of learning. In contrast, the coupling between theta and low-gamma oscillations (30-60 Hz) did not show any changes during learning. These results suggest that the coupling between theta and gamma bands occurs during rule learning and that high- and low-gamma bands play different roles in rule switching.