Spectral characteristics of the hippocampal EEG in the freely moving rat.

Abstract Rats were implanted chronically with electrodes dorsal and ventral to the pyramidal cell layer of hippocampal CA1 region. Multiple EEG signal with reference to a frontal screw were recorded and analyzed by means of the power, coherence and phase spectra during different gross behaviors which include awake-immobility, grooming, walking and rearing, slow wave sleep (SWS), rapid eye movement (REM) sleep and awakening from sleep. At least 3 types of EEG patterns, usually intermingled during one behavioral state, were detected: (1) Rhythmical slow activity (RSA) or theta: it possesses a behaviorally correlated fundamental peak frequency of 6–9 c/sec, small bandwidth, high dorsoventral coherence (across the pyramidal cell layer), and a number of harmonics (up to the third detected). RSA decreases in the order of REM or walking, grooming, awakening, awake-immobility and SWS. (2) Irregular slow activity (ISA): it has a broad-band (0.5–25 c/sec) spectrum resembling a low pass filter, low dorsoventral coherence. ISA decreases in the order of SWS, awake-immobility, grooming, REM or walking or desynchronization. (3) Fast waves of 20–60 c/sec, which are larger in walking or REM than awake-immobility or SWS. A model of the hippocampal circuitry suggests that these 3 types of EEG may require different afferent inputs and different bias on the hippocampal circuit. The phase of the EEG between dorsal and ventral RSA maxima does not show significant variation between behavior with one exception: this phase at the theta frequency is smaller during SWS and awakening than during other waking behavioral states or REM. The dorsoventral theta coherence increases in the order of SWS, awake-immobility, awakening, grooming, walking and REM, and is a sensitive indicator of the relative amount of theta in the EEG.