Figure 4. The rat hippocampus displays a low-frequency oscillation, which is entrained to neocortical SO, but not to respiration, during deactivated states. A, Representative nasal pressure and LFP signals recorded during large-amplitude SO in the neocortex (nCTX). B, Power spectra of the nasal signal (blue) and nCTX LFP (dark gray) (left y-axis). Blue and black dashed lines indicate the respiratory and SO peak frequencies, respectively. Thick blue line indicates the coherence spectrum between nCTX and the nasal signal (right y-axis). C, Power spectra of OB and hippocampal LFPs, which show a power peak ∼1 Hz coinciding with the nCTX SO peak frequency. D, Top, nCTX-LFP coherence spectra, revealing coherence peaks at the SO frequency. Bottom, Nasal-LFP coherence spectra. There is low coherence at all frequencies. Light gray lines reproduce the power spectra in C. Results in B–D were obtained from a 60 s representative data segment of deactivated state, which included the raw signals depicted in A. E, Mean respiratory and SO peak frequencies (n = 4 rats). *p < 0.01 (paired t test). F, Left, Mean nasal-LFP coherence at respiratory and SO peak frequencies. Right, Mean nCTX-LFP coherence at SO peak frequency (n = 4 rats). *p < 0.001, compared with nasal-LFP at SO frequency (paired t tests). Error bars indicate SEM.