Fast network oscillations in the hippocampal CA1 region of the behaving rat

This study examined intermittent, high-frequency (100-200 Hz) oscillatory patterns in the CA1 region of the hippocampus in the absence of theta activity, i.e., during and in between sharp wave (SPW) bursts. Pyramidal and interneuronal activity was phase-locked not only to large amplitude (>7 SD from baseline) oscillatory events, which are present mainly during SPWs, but to smaller amplitude (<4 SD) patterns, as well. Large-amplitude events were in the 140-200 Hz, "ripple" frequency range. Lower-amplitude events, however, contained slower, 100-130 Hz ("slow") oscillatory patterns. Fast ripple waves reversed just below the CA1 pyramidal layer, whereas slow oscillatory potentials reversed in the stratum radiatum and/or in the stratum oriens. Parallel CA1-CA3 recordings revealed correlated CA3 field and unit activity to the slow CA1 waves but not to fast ripple waves. These findings suggest that fast ripples emerge in the CA1 region, whereas slow (100-130 Hz) oscillatory patterns are generated in the CA3 region and transferred to the CA1 field.