While the time lags between assemblies in the hippocampus depend on theta-nested gamma waves during exploration, assembly sequences can occur both in the absence of theta (or other) oscillations and environmental inputs. During consummatory behaviors, immobility and non-REM sleep, the hippocampal theta rhythm is replaced by irregular sharp waves (Buzsáki et al., 1983). Self-organized population bursts of the hippocampal CA3 pyramidal cells induce a strong depolarization in the apical dendrites of CA1 pyramidal cells, reflected by an LFP sharp wave of negative polarity, accompanied by a transient fast-field oscillation (140–200 Hz) or “ripple” confined to the cell body layer of CA1 pyramidal cells (Buzsáki et al., 1992; O’Keefe and Nadel, 1978). SPW-Rs are the most synchronous assembly pattern in the mammalian brain (Chrobak and Buzsáki, 1994), characterized by a three- to five-fold gain of network excitability (Csicsvari et al., 1999). SPW-Rs have been hypothesized to play a critical role in transferring transient memories from the hippocampus to the neocortex for permanent storage (Buzsáki, 1989; McClelland et al., 1995). In line with this postulated role, both place cell sequences and the
