Peak frequencies of the 2–5 Hz oscillation were identified in the PFC, and spectral power at these frequencies was obtained for the PFC, HC, and nRE for each stimulation episode. Contrary to the behavior of theta rhythm, PFC 2–5 Hz peak power decreased linearly with increasing stimulus intensity (r = −0.60, p <0.001). Peak 2–5 Hz power was similar in the nRE (r = −0.60, p <0.001). It was consistently lower in HC but showed a similar negative trend as RPO stimulation changed (r = −0.36, p = 0.001) (Fig. 3F). Taken together, these results demonstrated that theta oscillation power in the HC increased with greater RPO stimulation, whereas the 2–5 Hz oscillations followed the opposite trend by decreasing PFC, HC, and nRE power with increasing RPO stimulation, suggesting a possible negative relationship between the generation of the two rhythms.
