Figure 1. Diagram of the placement of recording electrodes on the cortical surface and in the hippocampus. EEG signals were recorded differentially with impedance matched electrodes for both cortex and hippocampus. For cortical recordings, electrodes were implanted in frontal and occipital loci. Hippocampal microelectrodes were placed across the CA 1 and dentate regions to maximize response amplitudes of theta activity by differentially recording oriens and molecular layer synaptic currents. Cortical and hippocampal formation EEG signals were simultaneously recorded and representative 4-s traces are shown on the right. Note the higher frequency components in the cortical trace and higher amplitude, slower (theta) frequencies in the hippocampal trace for an awake unanesthetized rat. The calibration bar = 100 micro Volt for hippocampus and 50 micro Volt for cortex EEG signals.