Figure 2. Cortical and hippocampal xi EEG rhythms were associated with behavioral arousal. Fast Fourier transform analysis of signals revealed that most of the energy in the EEG of awake rats was confined to a narrow frequency range between 1 and 10 Hz. Fast Fourier transforms were averaged from the ten 4-s epochs shown below each graph. The xi rhythm (3.5–7.5 Hz) provided a reliable indication of movement and was particularly prominent in the hippocampal EEG. During awake immobility, higher amplitude, lower frequency activity predominated and ‘sharp wave’ activity was evident in both the cortical (C) and hippocampal (H) signals. Note the distinct sinusoidal wave forms contributing to the high energy xi peak for hippocampal signals during movement. Cortical signals exhibited an increase in beta frequency (11.5–50 Hz) energies in addition to increased xi frequencies during movement.