Chapter 14, p. 227-249 in N. Axmacher and B. Rasch (eds.),
Cognitive Neuroscience of Memory Consolidation,
Springer, 2017, ISNB 978-3-319-45064-3,
DOI: 10.1007/978-3-319-45066-7_14
N. Maier and R. Kempter.
Hippocampal sharp wave/ripple complexes (SWRs) are rhythmic electrographic activities that appear strictly coupled with resting behavioral waking states of the brain and slow-wave sleep. Several lines of evidence now support their involvement in the formation and long-term consolidation of hippocampus-dependent memories. During SWRs, the hippocampal networks experience a sharp and transient (~50-100 ms) increase in neuronal activity that is temporally coherent across both hippocampi; further, sharp wave-associated ripple oscillations express a remarkably high oscillation frequency of ~120 to 250 cycles per second; finally, SWRs are irregular in occurrence. Despite around three decades of research into the mechanistic underpinnings of this phenomenon, a coherent theory of various aspects - e.g. its initiation and termination, and the precise synchronization of thousands of neurons at millisecond precision - is still unavailable. Here, we will outline the current understanding of the implications and the mechanisms that govern SWRs, from both a physiological and a network-theoretical perspective. We will put special emphasis on the contributing neuronal populations and will discuss unresolved aspects.