Ripple (approximately 200-Hz) oscillations in temporal structures.

Spontaneous network oscillations near 200 Hz have been described in the hippocampus and parahippocampal regions of rodents and humans. During the last decade the characteristics and the mechanisms behind these field "ripples" have been studied extensively, mainly in rodents. They occur during rest or slow-wave sleep and provide a very fast, short-lasting (approximately 50 msec) rhythmic and synchronous activation of specific projection cells and interneurons. Ripples are frequently triggered by a massive synaptic activation from the hippocampal CA3 subfield, which is called a sharp wave. Recent evidence suggests that ripples have a specific task in memory processing-namely, that they convey information stored in the hippocampus to the cortex where it can be preserved permanently. Network mechanisms involved in ripple oscillations may be relevant for understanding pathologic synchronization processes in temporal lobe epilepsy.