Figure 2.

Different oscillations that have been linked to memory reactivations. (a) The slow oscillation (SO) is caused by global on-off states during NREM sleep—which is visible on the surface EEG as a K-complex (0.5–1.5 Hz). (b) Slow wave activity (SWA, or delta waves, 1–4 Hz), which are owing to local on-off states occurring mainly during deep or slow wave sleep. (c) The sleep spindle (12–16 Hz). This is present throughout all NREM sleep. (d) The sharp-wave ripple (SWR) of the hippocampus. The SWR is comprised two different components—the ripple and the sharp wave—that are seen on different electrode sites. The ripple occurs in the pyramidal layer while the sharp wave occurs in the input layer.