Low-frequency amplitude modulation of hippocampal theta rhythm in rats during treadmill running

The hippocampus is the main structure involved in the generation of the theta rhythm (4–12 Hz), which is among the most regular neural oscillations in electroencephalography (EEG) signals recorded during locomotion. However, whether changes in hippocampal theta rhythm amplitude provide any information remains unclear, primarily because traditional analytical methods cannot directly quantify such changes. Objective: This study was conducted to (1) examine the characteristics of signal addition or multiplication through Holo-Hilbert spectral analysis (HHSA), (2) determine whether hippocampal EEG signals recorded in rats during locomotion provide high-dimensional information, and (3) investigate the physiological importance of high-dimensional information obtained from hippocampal EEG recordings. Methods: We conducted HHSA to analyze hippocampal EEG signals recorded in rats. HHSA enabled us to determine carrier frequency and amplitude modulation (AM) characteristics. Results: HHSA revealed that hippocampal EEG signals recorded before and after treadmill running were concentrated in the delta band and were characterized by AM at 0.5–1 Hz (P < 0.05). Throughout the treadmill exercise, the hippocampal EEG signals were concentrated in the theta band and were characterized by AM at 0.5–4 Hz (P < 0.05). Conclusion: Hippocampal theta signals recorded in the rats during treadmill running exhibited low-frequency AM. The hippocampal theta rhythm can provide insights into functional brain connectivity. Significance: HHSA could be used to obtain high-dimensional EEG information; moreover, EEG signals recorded in rats during treadmill running exhibited low-frequency AM.