The site of initiation for evoked SPWs (red triangles) (A) varied less than that of spontaneous SPWs (blue triangles) (B). Overlay (grey) and averages of 100 initiated (red) and 100 spontaneous SPWs recorded over the same period from electrodes E1–E8 in the CA3 stratum pyramidale. SPWs were aligned at their initiation site defined from spikes and field. The stimulated pyramidal cell was situated between E6 and E7. C, the spatial distribution of extracellular unit activity at the start of initiated SPWs (red) was more restricted than that preceding spontaneous events (blue). Data from 10 slices, with distance 0 μm corresponding to the extracellular electrode closest to the initiating cell. D, cumulative variability of SPW fields was less for initiated than spontaneous events (P = 0.016, bootstrap). Above: overlays of 50 evoked SPWs (grey, mean shown in red) and 50 spontaneous SPWs (grey, mean in blue). E, time course of cumulative variability for initiated (red, n = 10 slices) and spontaneous SPWs (blue; n = 10), with mean ± SE (bold lines). A lower variability at initiation was maintained through SPW time course.