For each stimulation episode, peak theta frequencies were identified in the HC, and peak spectral power at these frequencies was obtained for all three signals and normalized across experiments (see Methods). Peak theta power increased linearly in the HC (r = 0.63, p <0.001) with increasing stimulus intensity. There was little effect of stimulus intensity on spectral power at theta frequency in the PFC (r = 0.11, p= 0.36) or nRE (r = 0.06, p = 0.59), i.e. on average, peak power of theta elicited by RPO stimulation did not change significantly as stimulus intensity increased. However, in individual experiments PFC and nRE theta often showed increasing trends and specifically, in a subset of animals (n=7) with strong HC-PFC relationship (see below), PFC theta, although weak, did show significant increase with stimulus intensity (r=0.36, p=0.03) (Fig. 3E, dashed line)”.
