Theta-gamma coupling in the hippocampus is thought to be involved in cognitive processes. A large body of research establishes that the hippocampus plays a crucial role in the organization and maintenance of episodic memory, and that sharp-wave ripples (SWR) contribute to memory consolidation processes. Here, we investigated how the local field potentials in the hippocampal CA1 area adapted along with rats' behavioral changes within a session during a spatial alternation task that included a 1-s fixation and a 1.5-s delay. We observed that, as the session progressed, the duration from fixation onset to nose-poking in the choice hole reduced as well as the number of premature responses during the delay. Parallel with the behavioral transitions, the power of high gamma during the delay period increased whereas that of low gamma decreased later in the session. Furthermore, the strength of theta-gamma modulation later in the session showed significant increase as compared to earlier in the session. Examining SWR during the reward period, we found that the number of SWR events decreased as well as the power in a wide frequency range during SWR events. In addition, the correlation between SWR and gamma oscillations just before SWR events was higher in the earlier trials than in the later trials. Our findings support the notion that the inputs from CA3 and entorhinal cortex play a critical role in memory consolidation as well as in cognitive processes. We suggest that SWR and the inputs from the two areas serve to stabilize the task behavior and neural activities.
Keywords: Gamma oscillations; Hippocampus; Rat; Sharp-wave ripples; Spatial alternation; Theta rhythm.