Spatial distribution of fast ripples and their properties before onset of ictal activity. A, Example of population burst simultaneously recorded from the CA3 and CA1 pyramidal cell layer in hippocampal slice of P22 rat. Population bursts were evoked by 8.5 mm [K⁺]_o bath application. Time delay between population burst onset indicates propagation of the population burst from area CA3 to CA1. Power spectrums of population bursts from the CA3 and CA1 (n = 27 population bursts) pyramidal cell layer reveal wider and higher FR frequency range in the CA3 area. B, Fast ripples before onset of ictal activity. Extracellular field potential recording (raw data, 0.1 Hz-4 kHz) was made in the CA3 pyramidal cell layer of P16 hippocampal slice. Extended parts show FR oscillations after onset of population bursts and before onset of ictal activity. FR amplitude increased after the appearance of population bursts. Before onset of ictal activity, FRs (200-400 Hz) followed by slower 100-150 Hz oscillations and 10-30 Hz secondary afterdischarges.