Regional distribution of currents associated with ripple and fast gamma oscillation. CSD maps (1 Hz to 10 kHz) in two different animals with the average LFP waveforms superimposed (gray traces). Maps were constructed by triggering on the peaks of ripple and fast gamma episodes. Asterisks indicate reference sites. Note strong SPWs (sinks) in the stratum radiatum of CA1 and CA3 and the inner molecular layer of the dentate gyrus during ripples and weaker sinks but with similar spatial distribution during fast gamma oscillations. Sinks in the inner molecular layer of the dentate gyrus possibly reflect activation of excitatory inputs from hilar mossy cells. The most lateral shank in the rat with CA3 sites (bottom) was in the fimbria (data not shown).